## 1 Introduction

Recently, several breakthrough results have been obtained that clarify the relationship between certain important algebraic and dynamical properties of discrete groups. Among the most prominent are the characterization of strongly amenable groups by Frisch, Tamuz and Vahidi Ferdowsi [Reference Frisch, Tamuz and Vahidi Ferdowsi8] (see also [Reference Glasner, Tsankov, Weiss and Zucker11]) and the characterization of Choquet–Deny groups by Frisch, Hartman, Tamuz and Vahidi Ferdowsi [Reference Frisch, Hartman, Tamuz and Ferdowsi7] (see also [Reference Erschler and Kaimanovich6]). Both classes were identified with the algebraic notion of FC-hypercentral groups, where FC refers to finite conjugacy classes.

In this paper, we continue this line of research from a slightly different perspective. A key technical idea is the use of a device that we view as a kind of ‘topological Furstenberg correspondence’ that allows us to give algebraic descriptions of the universal minimal proximal flow and the universal minimal strongly proximal flow associated to a group.

The descriptions are in terms of subsets of the group that we call completely syndetic and strongly complete syndetic, respectively, because they are higher-order variants of syndetic subsets. These subsets have very interesting algebraic, combinatorial and topological dynamical properties. In addition, it turns out that the existence of disjoint subsets with these properties completely characterizes both strong amenability and amenability.

Specifically, for a discrete group *G*, we say that a subset $A \subseteq G$ is *completely syndetic* if for every $n \in \mathbb {N}$ finitely many translates of the Cartesian product $A^{n}$ by elements in *G* cover $G^{n}$, where *G* acts diagonally on $G^{n}$. Equivalently and more succinctly, if there is a finite subset $F \subseteq G$ such that $FA^{n} = G^{n}$. The definition of a *strongly completely syndetic* subset is similar, and every strongly completely syndetic subset is, as the name suggests, completely syndetic.

The next result, which is Theorem 6.5, realizes the universal minimal proximal flow $\operatorname {{\partial _{\mathrm {p}}}} G$ as the Stone space of a translation-invariant Boolean algebra of completely syndetic subsets of *G*. This is an analogue of a result of Balcar and Franek [Reference Balcar and Franek3] that realizes the universal minimal *G*-flow $\operatorname {{\partial _{\mathrm {m}}}} G$ as the Stone space of a translation-invariant Boolean algebra of syndetic subsets of *G*. We note that $\operatorname {{\partial _{\mathrm {p}}}} G$ and $\operatorname {{\partial _{\mathrm {m}}}} G$ are sometimes denoted by $\Pi (G)$ and $M(G)$ in existing literature.

Theorem 1.1. Let *G* be a discrete group. Then the universal minimal proximal *G*-flow $\operatorname {{\partial _{\mathrm {p}}}} G$ is isomorphic to the Stone space of any translation-invariant Boolean subalgebra of $2^{G}$ that is maximal with respect to the property that every nonempty element is completely syndetic.

The main result in [Reference Frisch, Tamuz and Vahidi Ferdowsi8] is that a discrete group is strongly amenable if and only if it is FC-hypercentral, meaning that it has no nontrivial ICC quotient, i.e., no nontrivial quotient with the property that every nontrivial conjugacy class is infinite. We show directly in Section 7.1 that the main technical step in their paper, which consists of constructing a family of subshifts satisfying certain disjointedness conditions, is equivalent to the construction of completely syndetic subsets.

Since the strong amenability of *G* is equivalent to the triviality of $\operatorname {{\partial _{\mathrm {p}}}} G$, as a consequence of this observation and the previous theorem, we obtain, in Theorem 7.1, a new characterization of strong amenability (and hence of FC-hypercentrality).

Corollary 1.2. Let *G* be a discrete group. Then *G* is not strongly amenable if and only if there is a proper normal subgroup $H \unlhd G$ such that, for every finite subset $F \subseteq G \setminus H$, there is a completely syndetic subset $A \subseteq G$ such that $FA \cap A = \emptyset $.

The next result, which is Theorem 6.8, is a similar realization of the universal minimal strongly proximal flow $\operatorname {{\partial _{\mathrm {sp}}}} G$ as the Stone space of a Boolean algebra of strongly completely syndetic subsets of *G*. See Definition 3.1 for the latter notion. In existing literature $\operatorname {{\partial _{\mathrm {sp}}}} G$ is sometimes denoted by $\Pi _{s}(G)$.

Theorem 1.3. Let *G* be a discrete group. Then the universal minimal strongly proximal *G*-flow $\operatorname {{\partial _{\mathrm {sp}}}} G$ is isomorphic to the Stone space of any translation-invariant Boolean subalgebra of $2^{G}$ that is maximal with respect to the property that every nonempty element is strongly completely syndetic.

Since the amenability of *G* is equivalent to the triviality of $\operatorname {{\partial _{\mathrm {sp}}}} G$, we also obtain, in Theorem 7.5, a new characterization of amenability that seems to have a different flavor than existing characterizations in terms of paradoxical subsets and Følner sequences.

Corollary 1.4. Let *G* be a discrete group. Then *G* is not amenable if and only if there is a subset $A \subseteq G$ such that both *A* and $G \setminus A$ are strongly completely syndetic.

A key idea in our paper is the correspondence between between totally disconnected *G*-flows and translation-invariant Boolean subalgebras of $2^{G}$ that we view as a kind of ‘topological Furstenberg correspondence.’ Zucker [Reference Zucker26] showed that the *G*-flows $\operatorname {{\partial _{\mathrm {p}}}} G$ and $\operatorname {{\partial _{\mathrm {sp}}}} G$ are maximally highly proximal, implying that they are extremally disconnected (and therefore totally disconnected).

An important observation in [Reference Kalantar and Kennedy22] is that the C*-algebra $\mathrm {C}(\operatorname {{\partial _{\mathrm {sp}}}} G)$ of continuous functions on $\operatorname {{\partial _{\mathrm {sp}}}} G$ is *G*-injective in a certain strong sense. One consequence is the existence of relatively invariant measures in the sense of [Reference Glasner12], which proves to be particularly useful when used in combination with the above-mentioned correspondence.

More generally, we show that for a *G*-flow *X*, the C*-algebra $\mathrm {C}(X)$ is *G*-injective in the above sense if and only if *X* is what we call *maximally affinely highly proximal*, which is a strengthening of the property of being maximally highly proximal. We prove in Theorem 5.12 that in addition to the *G*-flow $\operatorname {{\partial _{\mathrm {sp}}}} G$, the *G*-flows $\operatorname {{\partial _{\mathrm {m}}}} G$ and $\operatorname {{\partial _{\mathrm {p}}}} G$ also have this property.

Theorem 1.5. Let *G* be a discrete group. Then the Stone–Čech compactification $\beta G$, the universal minimal *G*-flow $\operatorname {{\partial _{\mathrm {m}}}} G$, the universal minimal proximal *G*-flow $\operatorname {{\partial _{\mathrm {p}}}} G$ and the universal strongly proximal *G*-flow $\operatorname {{\partial _{\mathrm {sp}}}} G$ are all maximally affinely highly proximal.

The perspective we take in this paper allows us to answer a question of Glasner, Tsankov, Weiss and Zucker from [Reference Glasner, Tsankov, Weiss and Zucker11]. They asked for a characterization of dense orbit sets, which are subsets $A \subseteq G$ with the property that for every minimal *G*-flow *X* and every point $x \in X$, the set $Ax$ is dense in *X*. The following characterization of dense orbit sets, which is implied by Theorem 8.2 and the results in Section 8, provides an answer to this question in terms of syndetic sets.

Theorem 1.6. Let *G* be a discrete group. A subset $A \subseteq G$ is a dense orbit set if and only if there is no subset $B \subseteq A^{c}$ with the property that for every pair of finite subsets $F_{1} \subseteq B$ and $F_{2} \subseteq B^{c}$, the set $(\cap _{f_{1} \in F_{1}} f_{1}^{-1} B) \cap (\cap _{f_{2} \in F_{2}} f_{2}^{-1} B^{c})$ is syndetic.

Other results in Section 8 may also be of interest. For example, we establish a characterization of dense orbit sets in terms of the semigroup structure of the Stone–Čech compactification of *G*.

## 2 Preliminaries

In this section, we will briefly review some of the basic facts that we will require from the theory of topological dynamics and the theory of C*-algebras. For a reference on topological dynamics, we direct the reader to Glasner’s monograph [Reference Glasner13]. For a reference on C*-algebras, we direct the reader to Arveson’s monograph [Reference Arveson1].

### 2.1 Flows

Let *G* be a discrete group. A nonempty compact Hausdorff space *X* is a *G*-flow if it is endowed with an action of *G* by homeomorphisms. For $g \in G$ and $x \in X$, we will write $gx$ for the image of *x* under the homeomorphism corresponding to *g*. We will consider the category of *G*-flows with equivariant continuous maps as morphisms.

A *G*-flow *Y* is an *extension* of *X*, and *X* is a *factor* of *Y* if there is an equivariant continuous surjective map $\alpha : Y \to X$. We will refer to the map $\alpha $ as an *extension*.

Almost all of the flows that we will consider in this paper will be minimal. A *G*-flow *X* is *minimal* if for every point $x \in X$, the orbit $Gx = \{gx : g \in G\}$ is dense. There is a unique minimal *G*-flow $\operatorname {{\partial _{\mathrm {m}}}} G$ that is universal in the sense that every minimal *G*-flow is a factor of $\operatorname {{\partial _{\mathrm {m}}}} G$.

Proximal and strongly proximal flows will also play an important role. A *G*-flow *X* is *proximal* if, for any two points $x,y \in X$, there is a net $(g_{i})$ in *G* such that $\lim g_{i}x = \lim g_{i} y$. It is *strongly proximal* if, for any probability measure $\mu \in \mathrm {P}(X)$, there is a net $(g_{i})$ in *G* such that $\lim g_{i} \mu \in X$. Here we have identified points in *X* with the corresponding Dirac measures in $\mathrm {P}(X)$. It is not hard to show that this definition is equivalent to the proximality of the *G*-flow $\mathrm {P}(X)$. There is a unique minimal proximal *G*-flow $\operatorname {{\partial _{\mathrm {p}}}} G$ that is universal in the sense that every minimal proximal *G*-flow is a factor of $\operatorname {{\partial _{\mathrm {p}}}} G$. Similarly, there is a unique universal minimal strongly proximal *G*-flow.

A *G*-flow *X* is *point transitive* if there is a *transitive point* $x \in X$, i.e., a point such that the orbit $Gx = \{gx : g \in G\}$ is dense in *X*. The Stone–Čech compactification $\beta G$ is the unique universal point transitive *G*-flow, meaning that if *X* is a *G*-flow with a transitive point *x*, then there is an extension $\alpha : \beta G \to X$ such that $\alpha (e) = x$. Here $e \in G$ denotes the unit in *G*, identified in a canonical way with an element of $\beta G$.

For a subset $A \subseteq G$, we will let $\overline {A}$ denote the closure of *A* in $\beta G$. The set $\overline {A}$ is clopen, and every clopen subset of $\beta G$ is of this form. Moreover, the family of clopen subsets of $\beta G$ form a basis for the topology on $\beta G$. These facts follow from the fact that $\beta G$ can be realized as the Stone space of the Boolean algebra of subsets of *G*. We will discuss the relationship between *G*-flows and Boolean subalgebra of *G* in more detail in Section 6.

We will consider $\beta G$ as a compact right topological semigroup, meaning that for $y \in \beta G$ the map $\beta G \to \beta G : x \to xy$ is continuous. Many aspects of the structure of $\beta G$ are well understood (see, e.g., [Reference Hindman and Strauss20]). For example, $\beta G$ contains idempotents and has minimal left ideals.

Let *L* be a minimal left ideal in $\beta G$. Then *L* is closed. Let $E \subseteq L$ denote the set of idempotents in *L*. Then *L* decomposes as a disjoint union $L = \sqcup _{u \in E} uL$. In particular, *L* contains idempotents, which are said to be minimal. Note that *L* is a compact *G*-flow with respect to the left multiplication action of *G*. We will frequently use the fact that *L* is isomorphic as a *G*-flow to the universal minimal flow $\operatorname {{\partial _{\mathrm {m}}}} G$. In fact, whenever *X* is a *G*-flow there is an action of $\beta G$ on *X* obtained by extending the maps $G \to X : x \to gx$ to maps $\beta G \to X$.

For a *G*-flow *X*, a subset $S \subseteq X$ and a point $x \in X$, the corresponding *return set* $S_{x} \subseteq G$ is defined by $S_{x} = \{g \in G : gx \in S \}$. We will frequently make use of the following characterization of the closure of the return set of a clopen subset.

Lemma 2.1. Let *X* be a *G*-flow, and let $U \subseteq X$ be a clopen subset. Then for $x \in X$, $\overline {U_{x}} = \{y \in \beta G : yx \in U\}$.

Proof. Suppose that $y \in \overline {U_{x}}$. Then there is a net $(g_{i})$ in $U_{x}$ such that $\lim g_{i} = y$. Then $\lim g_{i} x = yx$. Since $g_{i} x \in U$ for each *i* and *U* is closed, it follows that $yx \in U$.

Conversely, suppose that $yx \in U$. Let $(g_{i})$ be a net in *G* such that $\lim g_{i} = y$. Then $\lim g_{i} x = yx \in U$. Since *U* is open, eventually $g_{i} x \in U$. Hence, eventually $g_{i} \in \overline {U_{x}}$ and therefore $y \in \overline {U_{x}}$.

### 2.2 Unital commutative C*-algebras

The category of compact Hausdorff spaces with continuous maps as morphisms is dually equivalent to the category of unital commutative C*-algebras with unital *-homomorphisms as morphisms. For a compact Hausdorff space *X*, the corresponding dual object is the C*-algebra $\mathrm {C}(X)$ of continuous functions on *X*.

If *Y* is another compact Hausdorff space, then a continuous map $\alpha : Y \to X$ corresponds to the *-homomorphism $\pi : \mathrm {C}(X) \to \mathrm {C}(Y)$ defined by $\pi (f)(y) = f(\alpha (y))$ for $f \in \mathrm {C}(X)$ and $y \in Y$. We will frequently use the fact that the map $\alpha $ is surjective if and only if the map $\pi $ is injective.

Let *G* be a discrete group. We will say that the unital commmutative C*-algebra $\mathrm {C}(X)$ is a *G*-C*-algebra if there is an action of *G* on $\mathrm {C}(X)$ by automorphisms. For $g \in G$ and $f \in \mathrm {C}(X)$, we will write $gf$ for the image of *f* under the automorphism corresponding to *f*.

The category of *G*-flows with equivariant continuous maps as morphisms is dually equivalent to the category of unital commutative *G*-C*-algebras. If *X* is a *G*-flow, then the C*-algebra $\mathrm {C}(X)$ is a *G*-C*-algebra with respect to the corresponding action of *G* defined by $gf(x) = f(g^{-1}x)$ for $g \in G$ and $f \in \mathrm {C}(X)$. We will discuss the relationship between *G*-flows and *G*-C*-algebras in more detail in Section 5.

## 3 Higher-order syndeticity

### 3.1 Higher-order syndeticity

Let *G* be a discrete group. Recall that a subset $A \subseteq G$ is *(left) syndetic* if there is a finite subset $F \subseteq G$ such that $FA = G$. In this section, we will introduce a ‘higher-order’ notion of syndeticity for subsets of *G* and consider several characterizations of sets with this property. In later sections, we will establish connections with the topological dynamics of *G*. In particular, we will see that the structure of the completely syndetic subsets of a group is closely related to the problem of the existence of minimal proximal flows for the group.

Definition 3.1. Let *G* be a discrete group. For $n \in \mathbb {N}$, a subset $A \subseteq G$ is *(left) n-syndetic* if there is a finite subset $F \subseteq G$ such that $F A^{n} = G^{n}$. We will say that *A* is *completely (left) syndetic* if it is *n*-syndetic for all $n \in \mathbb {N}$.

The next result follows immediately from Definition 3.1, and provides a useful reformulation of the definition of an *n*-syndetic subset.

Lemma 3.2. Let *G* be a discrete group and $n \in \mathbb {N}$. A subset $A \subseteq G$ is *n*-syndetic if and only if there is a finite subset $F \subseteq G$ such that for every finite subset $K \subseteq G$ with $|K| = n$, there is $f \in F$ such that $fK \subseteq A$.

The next proposition provides another characterization of *n*-syndetic subsets.

Proposition 3.3. Let *G* be a discrete group. A subset $A \subseteq G$ is *n*-syndetic if and only if there is a finite subset $F \subseteq G$ such that, whenever *F* is partitioned as $F = F_{1} \sqcup \cdots \sqcup F_{n}$, then $F_{i} A = G$ for some *i*.

Proof. For $n \in \mathbb {N}$ and a finite subset $F \subseteq G$,

where the union is taken over all partitions $F = F_{1} \sqcup \cdots \sqcup F_{n}$, with the convention that, if $F_{i} = \emptyset $, then $\cap _{f \in F_{i}} fA^{c} = G$ and $F_{i} A = \emptyset $. It follows that $FA^{n} = G^{n}$ if and only if whenever *F* is partitioned as $F = F_{1} \sqcup \cdots \sqcup F_{n}$, then $F_{i} A = G$ for some *i*.

Recall that a subset $B \subseteq G$ is *(right) thick* if for every finite subset $F \subseteq G$ there is $h \in G$ such that $Fh \subseteq B$. One can see right from the definitions that a subset $A \subseteq G$ is syndetic if and only if $A^{c}$ is not thick. This correspondence can be expressed in terms of dual families.

Let $\mathcal {F}$ be a family of subsets of *G*. The *dual family* $\mathcal {F}^{*}$ is the family of subsets of *G* having nonzero intersection with every element of $\mathcal {F}$, i.e.,

An important property of the family of thick subsets of *G* is that it is dual to the family of syndetic subsets of *G*. This fact will be generalised to *n*-syndetic subsets in Corollary 3.6 below.

We first identify the complements of *n*-syndetic subsets.

Definition 3.4. Let *G* be a discrete group. For $n \in \mathbb {N}$, we will say that a subset $B \subseteq G$ is *(right)* $1/n$*-thick* if for every finite subset $F \subseteq G$ there is $(h_{1},\ldots ,h_{n})\in G^{n}$ such that

We will say that *B* is *fractionally (right) thick* if it is $1/n$-thick for some $n \in \mathbb {N}$.

We observe that also $1/n$-thickness admits a characterization in terms of partitions of sets, in analogy with Proposition 3.3. Indeed, a subset $B \subseteq G$ is $1/n$-thick if for every finite subset $F \subseteq G$ there are elements $h_{1},\dotsc ,h_{n} \in G$ such that for every element $f \in F$ at least one of the elements $fh_{i}$, $i \in \{1,\dotsc , n\}$ lies in *B*. Phrased differently, there is a partition $F = F_{1} \sqcup \dotsm \sqcup F_{n}$ and element $h_{1}, \dotsc , h_{n} \in G$ such that $F_{i} h_{i} \subseteq B$ for all *i*.

Proposition 3.5. Let *G* be a discrete group. For $n \in \mathbb {N}$, a subset $A \subseteq G$ is *n*-syndetic if and only if $A^{c}$ is not $1/n$-thick. Hence, *A* is completely syndetic if and only if $A^{c}$ is not fractionally thick.

Proof. For a finite subset $F \subseteq G$,

Therefore, $FA^{n} \ne G^{n}$ if and only if the intersection on the right is nonempty. This is equivalent to the condition that there is $(h_{1},\ldots ,h_{n}) \in G^{n}$ such that

Corollary 3.6. Let *G* be a discrete group. For $n \in \mathbb {N}$, the dual to the family of *n*-syndetic subsets is the family of $1/n$-thick sets. Hence, the dual to the family of completely syndetic subsets is the family of fractionally thick subsets.

Proof. Suppose $B \subseteq G$ is a subset satisfying $A \cap B \ne \emptyset $ for all *n*-syndetic subsets $A \subseteq G$. Then since $B \cap B^{c} = \emptyset $, $B^{c}$ is not *n*-syndetic. Hence, by Proposition 3.5, *B* is $1/n$-thick.

Conversely, suppose that $A \cap B = \emptyset $ for some *n*-syndetic subset *A*. Then $A \subseteq B^{c}$, and since any set containing an *n*-syndetic subset is itself *n*-syndetic, $B^{c}$ is *n*-syndetic. Hence by Proposition 3.5, *B* is not $1/n$-thick.

Example 3.7. It is well known that a subset $A \subseteq \mathbb {Z}$ is syndetic if and only if it has ‘bounded gaps’, meaning that there is $k \in \mathbb {N}$ such that, for any $a \in \mathbb {Z}$, $\{a, a + 1, \ldots , a + k\} \cap A \ne \emptyset $.

For $n \in \mathbb {N}$, a subset $A \subseteq \mathbb {Z}$ is *n*-syndetic if and only if $A^{n}$ has ‘bounded diagonal gaps’, meaning that there is $k \in \mathbb {N}$ such that, for any *n*-tuple $(a_{1},\dots ,a_{n}) \in \mathbb {Z}^{n}$,

To see this, recall that *A* is *n*-syndetic if and only if $A^{c}$ is not $1/n$-thick, meaning that there is a finite subset $F \subseteq \mathbb Z$ such that, for every *n*-tuple $(a_{1},\dots ,a_{n}) \in \mathbb {Z}^{n}$, $(F+(a_{1},\dots ,a_{n})) \not \subseteq \bigcup _{i=1}^{n} \mathbb {Z}^{n-i} \times A^{c} \times \mathbb {Z}^{i-1}$. Equivalently, $(F+(a_{1},\dots ,a_{n})) \cap A^{n} \neq \emptyset $. Therefore, if *A* is *n*-syndetic and $F \subseteq \mathbb {Z}$ satisfies $F+A^{n} = \mathbb {Z}^{n}$, then equation (3.1) is satisfied for $k=\max (F)-\min (F)+1$. Conversely, if there is $k \in \mathbb {N}$ such that equation (3.1) is satisfied, then for $F = \{0,1,\ldots ,k\}$, $F+A^{n} = \mathbb {Z}^{n}$, showing that *A* is *n*-syndetic.

Since a subgroup is syndetic if and only if it is finite index and since every subgroup of $\mathbb {Z}$ is of the form $m\mathbb {Z}$ for $m \in \mathbb {N}$ and so is of finite index, every subgroup of $\mathbb {Z}$ is syndetic. However, no subgroup of $\mathbb {Z}$ is $2$-syndetic since, for any $m \in \mathbb {N} \setminus \{1\}$, there are arbitrarily long diagonal segments in $\mathbb {Z}^{2}$ that do not intersect $(m\mathbb {Z})^{2}$. This can be seen, for example, in Figure 1.

Now fix $n \in \mathbb {N}$ with $n> 2$. We will show that the set $A=\mathbb {Z} \setminus n\mathbb {Z}$ is $(n-1)$-syndetic but not *n*-syndetic. To see that *A* is $(n-1)$-syndetic, first note that, for $a \in \mathbb {Z}$, there is exactly one multiple of *n* in the set $\{a,a+1 \dots , a+(n-1)\}$. Hence, for an $(n-1)$-tuple $(a_{1},\dots , a_{n-1}) \in \mathbb {Z}^{n-1}$, at most $n-1$ elements of the set

have an entry that is a multiple of *n*. Therefore, at least one element in the set has no entries that are multiples of *n*, and this element belongs to $A^{n-1}$.

To see that *A* is not *n*-syndetic, note that, for $k\in \mathbb {N}$, every member of the set

has an entry that is a multiple of *n*. Therefore, the set does not intersect $A^{n}$.

Finally, we will construct an example of a nontrivial completely syndetic set. Let *B* denote the complement of the set of powers of $2$ in $\mathbb {Z}$. Fix $n\in \mathbb {N}$, and let $k=2^{n-1}+1$. Then, for $a \in \mathbb {Z}$, there are at most *n* powers of $2$ in the subset $\{a,a+1, \dots , a+k\}$. Hence, for every *n*-tuple $(a_{1},\dots , a_{n}) \in \mathbb {Z}^{n}$, there are at most $n^{2}$ elements of the set

with an entry that is a power of $2$. Since $n^{2}<2^{n}+2=k+1$, this implies that at least one element of the set has no entries that are a power of $2$, and this element belongs to $B^{n}$. Therefore, *B* is *n*-syndetic. Since *n* was arbitary, it follows that *B* is completely syndetic.

The next result is a generalization of the criterion that was used in Example 3.7.

Lemma 3.8. Let *G* be a discrete group. For $n \in \mathbb {N}$, a subset $A \subseteq G$ is *n*-syndetic if and only if there is a finite subset $F \subseteq G$ such that, for any finite subset $K \subseteq G$ with $|K| = n$, $F \cap (\cap _{k \in K} Ak) \ne \emptyset $.

Proof. If *A* is *n*-syndetic, then there is a finite subset $F \subseteq G$ such that $FA^{n} = G^{n}$. For finite $K \subseteq G$ with $|K| = n$, write $K=\{k_{1},\ldots , k_{n}\}$. Then there is $f \in F$ such that $(k_{1}^{-1},\ldots ,k_{n}^{-1}) \in fA^{n}$. Equivalently, $f^{-1} \in \cap _{i=1}^{n} Ak_{i}$. In particular, $F^{-1} \cap (\cap _{k \in K} Ak) \ne \emptyset $.

Conversely, if there is a finite subset $F \subseteq G$ such that for any finite subset $K \subseteq G$ with $|K| = n$, $F \cap (\cap _{k \in K} Ak) \ne \emptyset $, then it is easy to check that $F^{-1}A^{n} = G^{n}$. Hence, *A* is *n*-syndetic.

Example 3.9. Let *G* be a discrete group. Let $A,B \subseteq G$ be subsets with *A* syndetic and *B* thick. Then the set $AB$ satisfies the property that $\bigcap _{f \in F} fAB$ is syndetic for every finite subset $F \subseteq G$. Equivalently, $\overline {AB} \subseteq \beta G$ contains every minimal ideal. Such sets are called thickly syndetic in the literature. We claim that the product set $AB$ is also completely syndetic. To see this, choose a finite subset $F \subseteq G$ such that $FA = G$. Given $K \subseteq G$ finite, the assumption on *B* implies that there is $h \in \cap _{k \in K} Bk \ne \emptyset $. Then Lemma 3.8 applied for $n = 1$, says that $F^{-1} \cap Ah \ne \emptyset $. Therefore,

so Lemma 3.8 implies that $AB$ is completely syndetic.

In later sections we will be interested in subsets $A \subseteq G$ that “avoid” a given finite subset of $F \subseteq G$, in the sense that $FA \cap A = \emptyset $. For $n \in \mathbb {N}$, let $2^{G}_{n}$ denote the family of subsets of *G* of size *n*. Then *n*-syndetic subsets of *G* with this property correspond to certain “colorings” of $2^{G}_{n}$.

Definition 3.10. Let *G* be a countable discrete group. For a finite subset $F \subseteq G$ and $n \in \mathbb {N}$, an $(F,n)$-coloring of *G* is a pair $(K,k)$ consisting of a finite subset $K \subseteq G$ and a function $k : 2^{G}_{n} \to K$ such that, for any pair of subsets $E_{1},E_{2} \in 2^{G}_{n}$, we have $F k(E_{1})E_{1} \cap k(E_{2})E_{2} = \emptyset $.

Proposition 3.11. Let *G* be a discrete group and $F \subseteq G$ finite. There is a surjection from $(F,n)$-colorings of *G* onto *n*-syndetic subsets $A \subseteq G$ satisfying $FA \cap A = \emptyset $. For an $(F,n)$-coloring $(K,k)$, the corresponding *n*-syndetic subset is $A = \bigcup _{E \in 2^{G}_{n}} k(E)E$.

Proof. Let $(K,k)$ be an $(F,n)$-coloring of *G*, and let $A = \cup _{E \in 2^{G}_{n}} k(E) E$. Then by the definition of an $(F,n)$-coloring,

Furthermore, for $E \in 2^{G}_{n}$, we have the inclusion $k(E)E \subseteq A$, implying that *A* is *n*-syndetic.

Conversely, let $A \subseteq G$ be an *n*-syndetic subset such that $FA \cap A = \emptyset $. Then there is a finite subset $K \subseteq G$ such that $KA^{n} = G^{n}$. Write $K = \{k_{1},\ldots ,k_{m}\}$, and define $k : 2_{n}^{G} \to K^{-1}$ by $k(E) = k_{i}^{-1}$ for $E \subseteq 2_{n}^{G}$, where $i = \min \{i : E \subseteq k_{i} A \}$. Then, for $E_{1},E_{2} \subseteq 2^{G^{n}}$, the fact that $k(E_{1}) E_{1},k(E_{2}) E_{2} \subseteq A$ implies that $F k(E_{1}) E_{1} \cap k(E_{2}) E_{2} = \emptyset $. Therefore, the pair $(k,K^{-1})$ is an $(F,n)$-coloring of *G*.

Remark 3.12. Since the choice of the enumeration of *K* in the second part of the proof of Proposition 3.11 is arbitrary, this correspondence will not generally be a bijection.

### 3.2 Stone–Čech compactification

Let *G* be a discrete group. There is an important characterization of syndetic subsets in terms of the Stone–Čech compactification $\beta G$ of *G*. Namely, a subset $A \subseteq G$ is syndetic if and only if for every minimal left ideal *L* of $\beta G$, $\overline {A} \cap L \ne \emptyset $. In this section, we will establish a characterization of higher-order syndetic subsets in terms of $\beta G$. This characterization will be important in later sections when we consider the topological dynamical structure of *G*.

For a subset $A \subseteq G$ and a point $x \in \beta G$, the corresponding *return set* is $\overline {A}_{x} = \{g \in G : gx \in \overline {A} \}$.

Proposition 3.13. Let *G* be a discrete group. A subset $A \subseteq G$ is *n*-syndetic if and only if, for every finite subset $K \subseteq \beta G$ with $|K| \leq n$, there is $g \in G$ such that $gK \subseteq \overline {A}$. Equivalently, $\cap _{x \in K} \overline {A}_{x} \ne \emptyset $.

Proof. Suppose that *A* is *n*-syndetic. Then there is a finite subset $F \subseteq G$ such that $FA^{n} = G^{n}$, and taking closures on both sides with respect to the product topology implies $F\overline {A}^{n} = (\beta G)^{n}$. Let $K \subseteq \beta G$ be a finite subset with $|K| \leq n$. Then from above, $K \subseteq f \overline {A}$ for some $f \in F$. Hence, $f^{-1} K \subseteq \overline {A}$.

Conversely, suppose that, for every finite subset $K \subseteq \beta G$ with $|K| \leq n$, there is $g \in G$ such that $gK \subseteq \overline {A}$. Then $\bigcup _{g \in G} g \overline {A}^{n} = (\beta G)^{n}$. Since $\overline {A}$ is clopen and $\beta G$ is compact, $\overline {A}^{n}$ is clopen and $(\beta G)^{n}$ is compact with respect to the product topology. Hence, there is a finite subset $F \subseteq G$ such that $F \overline {A}^{n} = (\beta G)^{n}$. It follows that $FA^{n} = G^{n}$, so *A* is *n*-syndetic.

Proposition 3.14. Let *G* be a discrete group. A subset $A \subseteq G$ is completely syndetic if and only if the closure $\overline {A} \subseteq \beta G$ contains a closed right ideal of $\beta G$.

Proof. Suppose that $\overline {A}$ contains a right ideal *R*. Then for $x \in R$ and a finite subset $K \subseteq \beta G$, $xK \subseteq R \subseteq \overline {A}$. Let $(g_{i})$ be a net in *G* such that $\lim g_{i} = x$. Then, since $\overline {A}$ is clopen, eventually $g_{i} K \subseteq \overline {A}$ since the product on $\beta G$ is continuous in the first variable. Hence, by Proposition 3.13, *A* is completely syndetic.

Conversely, suppose that *A* is completely syndetic. Then by Proposition 3.13, the family of sets $\{\overline {A}_{y}\}_{y \in \beta G}$ has the finite intersection property. Hence, the family $\{\overline {\overline {A}_{y}}\}_{y \in \beta G}$ of clopen subsets also has the finite intersection property. Therefore, by the compactness of $\beta G$, there is $x \in \cap _{y \in \beta G} \overline {\overline {A}_{y}}$. Then $xy \in \overline {A}$ for all $y \in \beta G$, implying that the right ideal $R = x\beta G$ satisfies $R \subseteq \overline {A}$.

If $\overline {A}$ contains a right ideal, then, since the closure of every right ideal in $\beta G$ is a right ideal (see, e.g., [Reference Hindman and Strauss20, Theorem 2.15]), $\overline {A}$ necessarily contains a closed right ideal.

If *G* is amenable, then syndetic subsets of *G* can be characterized in terms of left invariant means on *G*. Specifically, a subset $A \subseteq G$ is syndetic if and only if, for every left invariant mean *m* on *G*, $m(A)> 0$ (see, e.g., [Reference Bergelson, Hindman and McCutcheon4, Theorem 2.7]). The next result generalizes this characterization.

Lemma 3.15. Let *G* be an amenable discrete group. For $n \in \mathbb {N}$, a subset $A \subseteq G$ is *n*-syndetic if and only if for every mean *m* on $G^{n}$ that is invariant under left translation by *G* we have $m(A^{n})> 0$.

Proof. Suppose that *A* is *n*-syndetic, and let *m* be a mean on $G^{n}$ that is invariant under left translation by *G*. Since *A* is *n*-syndetic, there is a finite subset $F \subseteq G$ such that $FA^{n} = G^{n}$. Then $m(FA^{n}) = 1$. By the translation invariance of *m*, it follows that $m(A^{n})> 0$.

Conversely, suppose that $A \subseteq G$ is not *n*-syndetic. Then, by Proposition 3.5, for every finite subset $F \subseteq G$ there is $h_{F} = (h_{1},\ldots ,h_{n}) \in G^{n}$ such that $Fh_{F} \subseteq (A^{n})^{c}$. The net $(h_{F})_{F \subseteq G}$ indexed by finite subsets $F \subseteq G$ is directed by inclusion. Let $x \in \beta (G^{n})$ be a cluster point. Then $gx \in \overline {(A^{n})^{c}}$ for all $g \in G$.

From above, letting $\delta _{x}$ denote the Dirac measure on $\beta (G^{n})$ corresponding to *x*, $(g\delta _{x})(\overline {A^{n}}) = 0$ for all $g \in G$. Since *G* is amenable, there is a probability measure $\mu $ on $\beta (G^{n})$ contained in the closed convex hull of the set $\{g \delta _{x}\}_{g \in G}$ that is invariant under left translation by *G*. By construction, $\mu (\overline {A^{n}}) = 0$. Therefore, letting *m* denote the corresponding mean on $G^{n}$, *m* is invariant under left translation by *G* and satisfies $m(A^{n}) = 0$.

### 3.3 Subshift dynamics

Let *G* be a countable discrete group, and let $2^{G}$ denote the family of subsets of *G*. Fix an enumeration of *G* as $G = \{g_{k} : k \in \mathbb {N} \}$, and equip $2^{G}$ with the metric $d : 2^{G} \times 2^{G} \to \mathbb {R}$ defined by $d(A,B) = 1/n$ for $A,B \in 2^{G}$, where $n = \inf \{k \in \mathbb {N} : A \cap \{g_{k}\} \ne B \cap \{g_{k}\} \}$. Then $2^{G}$ is compact and the right translation action on $2^{G}$ defines a *G*-flow called the *Bernoulli shift*.

A subflow of the Bernoulli shift is called a *subshift*. In this section, we will establish a characterization of higher-order syndetic subsets in terms of a property of the subshifts that they generate. We will apply this result in later sections when we discuss strong amenability.

For a subset $A \subseteq G$, let $X \subseteq 2^{G}$ denote the subshift generated by *A*. Then it is well known that *A* is syndetic if and only if $\emptyset \notin X$ and $A^{c}$ is syndetic if and only if $G \notin X$. The next result is a generalization of this fact.

Proposition 3.16. Let *G* be a countable discrete group, let $A \subseteq G$ be a subset of *G* and let $X \subseteq 2^{G}$ denote the subshift generated by *A*. Then, for $n \in \mathbb {N}$, *A* is *n*-syndetic if and only if for any $B_{1},\ldots ,B_{n} \in X$, $B_{1} \cap \cdots \cap B_{n} \ne \emptyset $. Also, $A^{c}$ is *n*-syndetic if and only if for any $B_{1},\ldots ,B_{n} \in X$, $B_{1} \cup \cdots \cup B_{n} \ne G$.

Proof. Suppose that *A* is *n*-syndetic. Then by Lemma 3.8 there is a finite subset $F \subseteq G$ such that, for any finite subset $K \subseteq G$ with $|K| = n$, $F \cap (\cap _{k \in K} Ak) \ne \emptyset $. Choose $B_{1},\ldots ,B_{n} \in X$, and for each $i \in \{1, \dotsc , n\}$ let $(g_{k}^{i})$ be a sequence in *G* such that $\lim _{k} A g_{k}^{i} = B_{i}$. Then from above, $F \cap (\cap _{i=1}^{n} Ag_{k}^{i}) \ne \emptyset $ for each *k*. Since *F* is finite, by passing to subsequences we can assume that there is $f \in F$ such that $f \in A g_{k}^{i}$ for all *k* and *i*. Then $f \in B_{1} \cap \cdots \cap B_{n}$, and in particular $B_{1} \cap \cdots \cap B_{n} \ne \emptyset $.

Conversely, suppose there is $B_{1},\ldots ,B_{n} \in X$ such that $B_{1} \cap \cdots \cap B_{n} = \emptyset $. Let $(g_{k}^{i})$ be sequences in *G* such that $\lim _{k} A g_{k}^{i} = B_{i}$. Then for any finite subset $F \subseteq G$, eventually $F \cap (\cap _{i=1}^{n} A g_{k}^{i}) = \emptyset $. Indeed, otherwise we could argue as above that $B_{1} \cap \cdots \cap B_{n} \ne \emptyset $. Hence, by Lemma 3.8, *A* is not *n*-syndetic.

For subsets $B_{1},\ldots ,B_{n} \subseteq G$, $B_{1},\ldots ,B_{n}$ are in the subshift generated by *A* if and only if $B_{1}^{c},\ldots ,B_{n}^{c}$ are in the subshift generated by $A^{c}$. It follows from above that $A^{c}$ is *n*-syndetic if and only if for any $B_{1},\ldots ,B_{n} \in X$, $B_{1} \cup \cdots \cup B_{n} \ne G$.

### 3.4 Strongly complete syndeticity

In this section, we will introduce a slightly stronger notion of complete syndeticity for subsets of a discrete group inspired by the proof of [Reference Glasner13, Chapter VII, Proposition 2.1]. We will see later that, just as the structure of the completely syndetic subsets of a group is closely related to the problem of the existence of minimal proximal flows for the group, the structure of the subsets satisfying this stronger property is closely related to the problem of the existence of minimal strongly proximal flows.

Recall that a multiset is a set with multiplicity. Formally, a multiset is a pair $X = (A,m)$ consisting of a set *A* and a multiplicity function $m : A \to \mathbb {N}_{\geq 1}$. The cardinality of *X* is $|X| = \sum _{a \in A} m(a)$. For an ordinary set *Y*, we will write $X \subseteq Y$ if $A \subseteq Y$, and we will write $X \cap Y$ for the multiset $(A \cap Y, m|_{A \cap Y})$. Finally, for a discrete group *G*, a multiset $X = (A,m) \subseteq G$ and $g \in G$, we will write $gX = (gA, gm)$, where $gm : gA \to \mathbb {N}_{\geq 1}$ is defined by $gm(ga) = m(a)$ for $a \in A$. Note that, if *X* is an ordinary set, identified with a multiset in the obvious way, then these definitions will coincide with the usual definitions.

Definition 3.17. Let *G* be a discrete group. We will say that a subset $A \subseteq G$ is *strongly completely (left) syndetic* if for every $\epsilon> 0$ there is a finite subset $F \subseteq G$ such that, for every finite multiset $K \subseteq G$, there is $f \in F$ such that $|fK \cap A| \geq (1-\epsilon )|K|$.

Remark 3.18. We will see in Section 6.3 that it is essential that the set *K* in the above definition is allowed to be a multiset.

Remark 3.19. Recall from Definition 3.1 and Lemma 3.2 that a subset $A \subseteq G$ is completely syndetic if for every $n \in \mathbb {N}$ there are finitely many translates of *A* such that any sequence $g_{1}, \dotsc , g_{n}$ in *G* belongs all together to at least one of the translates. One possible attempt to strengthen this notion is to drop the bound on the number of elements from *G*, but then $A = G$ is the only set satisfying this definition. Strong complete syndeticity provides a probabilistic version of this idea. Indeed, a subset $A \subseteq G$ is strongly completely syndetic if and only if for every $\epsilon> 0$ there are finitely many translates of *A* such that of any sequence $g_{1}, \dotsc , g_{n}$ in *G* at least $(1 - \epsilon )n$ of its members belong to one of the translates.

Lemma 3.20. Let *G* be a discrete group. If a subset $A \subseteq G$ is strongly completely syndetic, then it is completely syndetic.

Proof. Suppose that *A* is strongly completely syndetic. For $n \in \mathbb {N}$, let $\epsilon = \frac {1}{2n}$. Then there is a finite subset $F \subseteq G$ such that for every finite subset $K \subseteq G$ with $|K| = n$ there is $f \in F$ such that $|fK \cap A| \geq (1 - \epsilon )|K| = n - \frac {1}{2}$. Hence, $|fK \cap A| = n$, implying $fK \subseteq A$. Therefore, *A* is left *n*-syndetic.

We will see later that not every completely syndetic subset is strongly completely syndetic.

Example 3.21. Consider the free group on two generators $\mathbb {F}_{2} = \langle a,b \rangle $, and let $A \subseteq \mathbb {F}_{2}$ denote the set of all elements in $\mathbb {F}_{2}$ with reduced form beginning with *a*. We will show that *A* is strongly completely syndetic. To this end, choose $\epsilon> 0$ and $n \in \mathbb {N}$ with $n \geq 1/(2\epsilon ) + 1$.

For a reduced word *w* in $\{a,b\}$, let $B_{w} \subseteq \mathbb {F}_{2}$ denote the subset of elements such that the corresponding reduced word begins with *w*. Then there are reduced words $w_{2},\ldots ,w_{2n}$ such that setting $w_{1} = a$, $\mathbb {F}_{2} \setminus \{e\}= B_{w_{1}} \sqcup B_{w_{2}} \sqcup \cdots \sqcup B_{w_{2n}}$ and there is a finite subset $F \subseteq \mathbb {F}_{2}$ with the property that, for each $2\leq i \leq 2n$, there is $f \in F$ satisfying $fB_{w_{j}} \subseteq B_{a}=B_{w_{1}}$ if $j \ne i$.

For example, if $n = 2$, we can take $w_{2} = a^{-1}$, $w_{3} = b$, $w_{4} = b^{-1}$ and $F = \{a,ab^{-1},ab\}$. Then, for all $j\neq 2$, $aB_{w_{j}}\subseteq B_{a}$; for all $j\neq 3$, $ab^{-1}B_{w_{j}}\subseteq B_{a}$ and for all $j\neq 4$, $abB_{w_{j}}\subseteq B_{a}$. Similiarly, for $n = 3$, we can take $w_{2} = a^{-2}$, $w_{3} = a^{-1}b$, $w_{4} = a^{-1}b^{-1}$, $w_{5} = b$, $B_{6} = b^{-1}$ and $F = \{a^{2},ab^{-1}a,aba,ab^{-1},ab\}$.

Then, for a finite multiset $K \subseteq G$, $|K \cap B_{w_{i}}| \leq |K|/(2n-1)$ for some $i \neq 1$. By construction there is $f \in F$ such that $fB_{w_{j}} \subseteq B_{a}$ for $j \ne i$. Then

Therefore, *A* is strongly completely syndetic.

## 4 Algebraically irreducible affine flows

Let *G* be a discrete group and let *X* be a *G*-flow. If *X* is minimal and strongly proximal, then the affine flow $\mathrm {P}(X)$ of probability measures on *X* is *irreducible*, meaning that it has no proper affine subflow. Indeed, an affine subflow of $\mathrm {P}(X)$ necessarily contains *X*, and hence, by the Krein–Milman theorem, contains all of $\mathrm {P}(X)$. Furstenberg proved (see, e.g., [Reference Glasner13, Theorem 2.3]) a kind of converse: If *K* is any irreducible affine *G*-flow, then the closure of the set of extreme points of *K* is a minimal strongly proximal *G*-flow.

Zorn’s lemma implies that every affine *G*-flow *L* contains an irreducible affine *G*-flow *K*. It follows from above that for a *G*-flow *X*, the affine flow $\mathrm {P}(X)$ always contains a minimal strongly proximal *G*-flow. Consequently, there is always a continuous equivariant map from the universal minimal strongly proximal *G*-flow $\operatorname {{\partial _{\mathrm {sp}}}} G$ to $\mathrm {P}(X)$. The existence of this map along with more general ‘boundary maps’ has played an important role in recent applications of topological dynamics to C*-algebras (see, e.g., [Reference Kalantar and Kennedy22] and [Reference Kennedy23]).

In this section, we will establish a similar result for minimal proximal flows. We will utilize this result in later sections. However, in general it does not seem to be as useful as the corresponding result for minimal strongly proximal flows. For the next definition, recall that $\partial K$ denotes the set of extreme points of a convex set *K*.

Definition 4.1. Let *G* be a discrete group, and let *K* be an affine *G*-flow. We will say that *K* is *algebraically irreducible* if whenever $L \subseteq K$ is an affine *G*-subflow with the property that $L \cap \operatorname {conv}(\overline {\partial K}) \ne \emptyset $, then $L = K$.

Proposition 4.2. Let *G* be a discrete group, let *K* be an algebraically irreducible affine *G*-flow and let $X = \overline {\partial K}$. Then *X* is a minimal proximal *G*-flow, and it is the unique minimal subflow of *K* with the property that $X \cap \operatorname {conv}(\overline {\partial K}) \ne \emptyset $.

Proof. We prove a statement from which both minimality and proximality of *X* can be deduced. Choose $x, y \in X$, and let $w = \frac {1}{2}(x + y) \in K$. Let $L \subseteq K$ denote the closed convex hull of the orbit $Gw$. Then *L* is an affine subflow with $L \cap \operatorname {conv}(\overline {\partial K}) \ne \emptyset $. Since *K* is algebraically irreducible, $L = K$. Hence, by Milman’s partial converse to the Krein–Milman theorem, $\partial K \subseteq \overline {Gw}$. It follows that $X \subseteq \overline {Gw}$.

Taking $x = y$ in the previous paragraph implies that *X* is minimal. To see that *X* is proximal, choose a point $z \in \partial K$. Then from above there is a net $(g_{i})$ in *G* such that $z = \lim g_{i} w = \lim \frac {1}{2}(g_{i} x + g_{i} y)$. Since *X* is compact, by passing to a subnet we can assume there are points $x^{\prime },y^{\prime } \in X$ such that $\lim g_{i} x = x^{\prime }$ and $\lim g_{i} y = y^{\prime }$. Then $z = \frac {1}{2}(x^{\prime } + y^{\prime })$. Since *z* is an extreme point, it follows that $x^{\prime } = y^{\prime } = z$. Hence, *X* proximal.

Finally, to see that *X* is unique, let $Y \subseteq K$ be a minimal subflow of *K* such that $Y \cap \operatorname {conv}(\overline {\partial K}) \ne \emptyset $. Then the closed convex hull *L* of *Y* is an affine *G*-flow with $L \cap \operatorname {conv}(\overline {\partial K}) \ne \emptyset $. Since *K* is algebraically irreducible, $L = K$. Hence, by Milman’s partial converse to the Krein–Milman theorem, $\partial K \subseteq Y$. Since $\overline {\partial K}$ is invariant, the minimality of *Y* implies that $Y = \overline {\partial K} = X$.

Theorem 4.3. Let *G* be a discrete group. A *G*-flow *X* is minimal and proximal if and only if the affine *G*-flow $\mathrm {P}(X)$ is algebraically irreducible.

Proof. Suppose that $\mathrm {P}(X)$ is algebraically irreducible. Then Proposition 4.2 implies $X = \partial \mathrm {P}(X)$ is minimal and proximal. Conversely, suppose that *X* is minimal and proximal. Let $K \subseteq \mathrm {P}(X)$ be an affine *G*-flow with $K \cap \mathrm {conv}(X) \ne \emptyset $. Then there is $\mu \in K$ of the form $\mu = \alpha _{1} \delta _{x_{1}} + \cdots + \alpha _{n} \delta _{x_{n}}$ for $\alpha _{1},\ldots ,\alpha _{n}> 0$ and $\alpha _{1} + \cdots + \alpha _{n} = 1$ and $x_{1},\ldots ,x_{n} \in X$. By the proximality of *x* there is a net $(s_{i})$ in *G* and $y \in X$ such that $\lim s_{i} x_{i} = y$ for all *i*. Hence, $\lim s_{i} \mu = \delta _{y}$. Thus, $X \subseteq K$ and $K = \mathrm {P}(X)$. Hence, $\mathrm {P}(X)$ is algebraically irreducible.

We wonder whether Theorem 4.3 could be used to establish the existence of nontrivial minimal proximal flows for non-FC-hypercentral groups, thereby giving a new proof of the main result in [Reference Frisch, Tamuz and Vahidi Ferdowsi8]. We will return to the question of the existence of nontrivial minimal proximal flows in later sections.

## 5 Highly proximal extensions and injectivity

Zucker recently showed in [Reference Zucker26] that, if *G* is a Polish group, then the universal minimal *G*-flow $\operatorname {{\partial _{\mathrm {m}}}} G$, the universal minimal proximal *G*-flow $\operatorname {{\partial _{\mathrm {p}}}} G$ and the universal strongly proximal *G*-flow $\operatorname {{\partial _{\mathrm {sp}}}} G$ are all maximally highly proximal (see Section 5.2). If *G* is discrete, then it turns out that a *G*-flow *X* is maximally highly proximal if and only if the C*-algebra $\mathrm {C}(X)$ is injective in the category of unital commutative C*-algebras.

Injectivity is an important and well-studied property of C*-algebras. However, a key observation in [Reference Kalantar and Kennedy22] is that the C*-algebra $\mathrm {C}(\operatorname {{\partial _{\mathrm {sp}}}} G)$ is actually injective in a much stronger sense. Specifically, it is injective in the category of function systems equipped with a *G*-action. Here, a function system refers to a closed unital self-adjoint subspace of a unital commutative C*-algebra.

In this section, we will consider a property of *G*-flows that corresponds to this stronger notion of injectivity. We will show that, in addition to the universal minimal strongly proximal *G*-flow $\operatorname {{\partial _{\mathrm {sp}}}} G$, the universal pointed *G*-flow (i.e., the Stone–Čech compactification) $\beta G$, the universal minimal *G*-flow $\operatorname {{\partial _{\mathrm {m}}}} G$ and the universal minimal proximal *G*-flow $\operatorname {{\partial _{\mathrm {p}}}} G$ also have this property.

### 5.1 Injectivity and essentiality

Since we will be considering injective objects in two different categories, it will be convenient to give a broad overview of the basic category theory that we will require.

Let $\mathcal {C}$ be a category with objects $\operatorname {\mathrm {Obj}}(\mathcal {C})$ and morphisms $\operatorname {\mathrm {Mor}}(\mathcal {C})$. We fix a class of morphisms $E \subseteq \operatorname {\mathrm {Mor}}(\mathcal {C})$. We will refer to the morphisms in *E* as *embeddings*. For the categories that we will consider, *E* will consist of morphisms that are ‘embeddings’ in an appropriate sense.

For example, in the next section, we will consider the category of unital commutative C*-algebras with unital *-homomorphisms as morphisms. In this setting, the class *E* will be the unital *-monomorphism.

An object $I \in \operatorname {\mathrm {Obj}}(\mathcal {C})$ is *injective* if, for any objects $A,B \in \operatorname {\mathrm {Obj}}(\mathcal {C})$, any embedding $\iota : A \to B$ and any morphism $\phi : A \to I$, there is a morphism $\psi : B \to I$ such that $\psi \circ \iota = \phi $, or equivalently, such that the following diagram commutes.

For an object $A \in \operatorname {\mathrm {Obj}}(\mathcal {C})$, we will be interested in finding an injective object $I \in \operatorname {\mathrm {Obj}}(\mathcal {C})$ along with an embedding $\iota : A \to I$ that is *minimal* in the sense that, if $J \in \operatorname {\mathrm {Obj}}(\mathcal {C})$ is injective and $\kappa : A \to J$ is an embedding, then there is a surjective morphism $\phi : J \to I$ such that $\phi \circ \kappa = \iota $. A pair $(A,\iota )$ with this property is an *injective hull* for *A*. For the categories that we will consider, it is a nontrivial result that every object has an injective hull that is unique up to isomorphism.

An embedding $\iota : A \to B$ is *essential* if for every object $C \in \operatorname {\mathrm {Obj}}(\mathcal {C})$ and every morphism $\phi : B \to C$, $\phi $ is an embedding whenever $\phi \circ \iota $ is an embedding.

There is typically a close relationship between injective objects and essential embeddings. For the categories that we will consider here, an object $I \in \operatorname {\mathrm {Obj}}(\mathcal {C})$ is injective if and only if it is *maximal essential*, meaning that for every object $A \in \operatorname {\mathrm {Obj}}(\mathcal {C})$, an essential embedding $\iota : I \to A$ is an isomorphism.

### 5.2 Maximally highly proximal flows

The notion of a maximally highly proximal flow was introduced by Auslander and Glasner [Reference Auslander and Glasner2] (see also [Reference Zucker26]). Let *G* be a discrete group, and let *X* and *Y* be *G*-flows. An extension $\phi : Y \to X$ is *highly proximal* if for every nonempty open subset $U \subseteq Y$ there is a point $x \in X$ such that $\phi ^{-1}(x) \subseteq U$. The extension $Y \to X$ is *universally highly proximal* if whenever *Z* is a *G*-flow and $\psi : Z \to X$ is a highly proximal extension, then there is an extension $\rho : Y \to Z$ such that $\phi = \psi \circ \rho $. The universal highly proximal extension is unique up to isomorphism. The *G*-flow *X* is called *maximally highly proximal* if the identity map $\operatorname {\mathrm {\operatorname {id}}}_{X}$ is the universal highly proximal extension.

Recall that the category with *G*-flows as objects and *G*-equivariant continuous maps as morphisms is dually equivalent to the category with unital commutative *G*-C*-algebras as objects and *G*-equivariant unital *-homomorphisms as morphisms. An embedding in this latter category is a unital *-monomorphism.

A pair of *G*-flows *X* and *Y* correspond to the *G*-C*-algebras $\mathrm {C}(X)$ and $\mathrm {C}(Y)$, and an extension $\alpha : Y \to X$ corresponds to an embedding $\pi : \mathrm {C}(X) \to \mathrm {C}(Y)$ via the formula $f \circ \alpha (y) = \pi (f)(y)$ for $f \in \mathrm {C}(X)$ and $y \in Y$. Vice versa, an embedding $\mathrm {C}(X) \to \mathrm {C}(Y)$ leads to an extension $Y \to X$ obtained by considering the spectra of $\mathrm {C}(Y)$ and $\mathrm {C}(X)$.

In order to identify the C*-algebras corresponding to maximally highly proximal flows, we will work within the category of unital commutative C*-algebras. Before stating the results, we briefly review some definitions.

For compact Hausdorff spaces *X* and *Y*, a *-homomorphism $\pi : \mathrm {C}(X) \to \mathrm {C}(Y)$ is an *embedding* if it is a *-monomorphism. It follows from Zorn’s lemma that every unital commutative C*-algebra $\mathrm {C}(X)$ admits an essential embedding $\pi : \mathrm {C}(X) \to \mathrm {C}(Y)$ which is maximal in the following sense: Whenever $\rho :\mathrm {C}(Y) \to \mathrm {C}(Z)$ is an embedding such that $\rho \circ \pi $ is essential, then $\rho $ is a $*$-isomorphism.

Gleason [Reference Gleason14] (see also [Reference Hadwin and Paulsen17]) showed that the unital commutative C*-algebra $\mathrm {C}(Y)$ is injective if and only if the space *Y* is extremally disconnected. Gonshor [Reference Gonshor15] showed that this is equivalent to $\mathrm {C}(Y)$ being maximal essential. Moreover, Gleason showed that every unital commutative C*-algebra $\mathrm {C}(X)$ has an injective hull $\mathrm {C}(Y)$ that is unique up to isomorphism. The space *Y* is often referred to as the *Gleason cover* of *X*.

The next result from [Reference Gonshor15] characterizes essential embeddings of unital commutative C*-algebras in terms of the corresponding compact Hausdorff spaces.

Theorem 5.1. Let *X* and *Y* be compact Hausdorff spaces, and let $\pi : \mathrm {C}(X) \to \mathrm {C}(Y)$ be an embedding. Let $\phi : Y \to X$ denote the corresponding continuous map. The following are equivalent:

1. The embedding $\pi $ is essential.

2. For every nonzero ideal

*J*in $\mathrm {C}(Y)$, $J \cap \pi (\mathrm {C}(X)) \ne 0$.3. For every proper closed subset $C \subseteq Y$, $\phi (C) \ne X$.

Gonshor [Reference Gonshor16] showed that the Gleason cover of a compact Hausdorff space *X* is isomorphic to the Stone space of the Boolean algebra of regular open subsets of *X*. Recall that an open subset $U \subseteq X$ is *regular* if it is equal to the interior of its closure. The regular open subsets of *X* form a Boolean algebra with respect to the following operations:

• $U \land V$ is $U \cap V$,

• $U \lor V$ is the interior of the closure of $U \cup V$ and

• $\neg U$ is the interior of $U^{c}$.

Theorem 5.2. Let *G* be a discrete group, and let *X* and *Y* be minimal *G*-flows. An extension $\phi : Y \to X$ is highly proximal if and only if the corresponding embedding $\pi : \mathrm {C}(X) \to \mathrm {C}(Y)$ is essential. Hence, *Y* is maximally highly proximal if and only if $\mathrm {C}(Y)$ is the injective hull of $\mathrm {C}(X)$. In this case, *Y* is isomorphic to the Stone space of the Boolean algebra of regular open subsets of *X*.

Proof. Suppose that $\phi $ is highly proximal. Let $C \subseteq Y$ be a proper closed subset. Then $V = Y \setminus C$ is a nonempty open subset, so there is $x \in X$ such that $\phi ^{-1}(x) \subseteq V$. Hence $x \notin \phi (C)$. Therefore, by (3) of Theorem 5.1, $\pi $ is essential.

Conversely, suppose that $\pi $ is essential. Fix a nonempty open subset $V \subseteq Y$. Let $C = Y \setminus V$ so that *C* is a proper closed subset. Then by (3) of Theorem 5.1, $\phi (C) \ne X$. Hence, there is $x \in X$ with $\phi ^{-1}(x) \subseteq V$. Therefore, $\phi $ is highly proximal.

The equivalence between *Y* being maximally highly proximal and $\mathrm {C}(Y)$ being the injective hull of $\mathrm {C}(X)$ is now implied by the characterization of the injective hull of $\mathrm {C}(X)$ as maximal essential. The final statement describing *Y* when $\mathrm {C}(Y)$ is injective is from [Reference Gonshor16].

Example 5.3. Let *G* be a discrete group and let *X* and *Y* be *G*-flows such that *Y* is maximally highly proximal and let $\alpha : Y \to X$ be an extension. By Theorem 5.2, the C*-algebra $\mathrm {C}(Y)$ is injective in the category of unital commutative C*-algebras. However, it is not necessarily injective in the category of unital commutative *G*-C*-algebras.

To see this, suppose that *G* is nontrivial and let *X* denote the trivial *G*-flow. Then *X* is maximally highly proximal, so by Theorem 5.2, $\mathbb {C} \mathrm {conv}ng \mathrm {C}(X)$ is injective (note that this also follows from the Hahn–Banach theorem). However, $\mathbb {C}$ is not *G*-injective in the category of unital commutative *G*-C*-algebras. Otherwise the identity map $\operatorname {\mathrm {\operatorname {id}}}_{\mathbb {C}}$ would extend to an equivariant *-homomorphism $\pi : \ell ^{\infty }(G) \to \mathbb {C}$. Since $\ell ^{\infty }(G) \mathrm {conv}ng \mathrm {C}(\beta G)$, such an extension would be of the form $\pi = \delta _{x}$ for some $x \in \beta G$, where $\delta _{x}$ denotes the Dirac measure corresponding to *x*. However, by [Reference Ellis5] $\beta G$ has no *G*-fixed point, so this is impossible. Therefore, $\mathbb {C}$ is not injective in the category of unital commutative *G*-C*-algebras.

### 5.3 Maximally affinely highly proximal flows

In this section, we will work with *function systems*, which are closed unital self-adjoint subspaces of unital commutative C*-algebras. For a reference on the theory of function systems, we refer the reader to the recent paper of Paulsen and Tomforde [Reference Paulsen and Tomforde25].

Let $\mathcal {R}$ and $\S $ be function systems. A unital linear map $\phi : \mathcal {R} \to \S $ is an *order homomorphism* if $\phi (f) \geq 0$ for all $f \in \mathcal {R}$ with $f \geq 0$. It is an *order isomorphism* if it has an inverse that is also an order homomorphism.

If *K* is a compact convex set, then the space $\mathrm {A}(K)$ of continuous affine functions on *K* is a function system. Moreover, Kadison’s [Reference Kadison21] representation theorem implies that every function system is isomorphic to a function system of this form. Specifically, let $\mathcal {R}$ be a function system, and let *K* denote the compact convex set of states on $\mathcal {R}$, i.e., the unital order homomorphisms from $\mathcal {R}$ to $\mathbb {C}$ equipped with the weak* topology. Then the map $\phi : \mathcal {R} \to \mathrm {A}(K)$ defined by $\phi (f)(\alpha ) = \alpha (f)$ for $f \in \mathcal {R}$ and $\alpha \in K$ is a unital order isomorphism.

The category with compact convex sets as objects and continuous affine maps as morphisms is dual to the category with function systems as objects and order homomorphisms as morphisms. An embedding in the latter category is an order monomorphism.

Let *G* be a discrete group. An action of *G* on a function system $\mathcal {R}$ is a group homomorphism from *G* into the group of order automorphisms of $\mathcal {R}$. We will refer to $\mathcal {R}$ as a *G-function system*.

It follows from the duality between the category of compact convex sets and the category of function systems that the category with affine *G*-flows as objects and equivariant continuous affine maps as morphisms is dual to the category with *G*-function systems as objects and equivariant order homomorphisms as morphisms. An embedding in the latter category is an equivariant order monomorphism.

Note that every unital *G*-C*-algebra is isomorphic (i.e., equivariantly order isomorphic to) a *G*-function system. If *X* and *Y* are *G*-flows and $\alpha : Y \to X$ is an extension, then the corresponding equivariant unital *-monomorphism $\pi : \mathrm {C}(X) \to \mathrm {C}(Y)$ is an embedding in the category of *G*-function systems.

A pair of affine *G*-flows *K* and *L* correspond to the *G*-function systems $\mathrm {A}(K)$ and $\mathrm {A}(L)$, and an affine extension $\alpha : L \to K$ corresponds to an embedding $\phi : \mathrm {A}(K) \to \mathrm {A}(L)$ via the formula $f \circ \alpha (y) = \phi (f)(y)$ for $f \in \mathrm {A}(K)$ and $y \in L$.

Hamana [Reference Hamana18] showed that a *G*-function system is injective if and only if it is maximal essential and used this to show that every *G*-function system has an injective envelope that is unique up to isomorphism. Hamana further observed that an injective *G*-function system is isomorphic (as a *G*-function system) to a *G*-C*-algebra. However, as the next exmaple shows, a unital commutative *G*-C*-algebra that is injective in the category of unital commutative C*-algebras may not be injective in the category of *G*-function systems.

Example 5.4. Let *G* be a discrete group and consider the complex numbers $\mathbb {C}$ equipped with the trivial *G*-action. Note that $\mathbb {C}$ is a unital commutative *G*-C*-algebra that is injective in the category of unital commutative C*-algebras. In particular, it is also a function system.

It was shown in [Reference Kalantar and Kennedy22] that the injective hull of $\mathbb {C}$ in the category of *G*-function systems is the C*-algebra $\mathrm {C}(\operatorname {{\partial _{\mathrm {sp}}}} G)$, where $\operatorname {{\partial _{\mathrm {sp}}}} G$ denotes the universal minimal strongly proximal *G*-flow. In particular, the embedding $\mathbb {C} \subseteq \mathrm {C}(\operatorname {{\partial _{\mathrm {sp}}}} G)$ is essential.

Furstenberg [Reference Furstenberg9] showed that $\operatorname {{\partial _{\mathrm {sp}}}} G$ is trivial if and only if *G* is amenable. This can also be seen using the ideas introduced in this section. Choose $x \in \operatorname {{\partial _{\mathrm {sp}}}} G$, and let $\pi _{x} : \mathrm {C}(\operatorname {{\partial _{\mathrm {sp}}}} G) \to \ell ^{\infty }(G)$ denote the equivariant unital *-homomorphism defined by $\pi _{x}(f)(g) = f(gx)$ for $f \in \mathrm {C}(\operatorname {{\partial _{\mathrm {sp}}}} G)$ and $g \in G$. Then, since $\operatorname {{\partial _{\mathrm {sp}}}} G$ is minimal, $\pi _{x}$ is an embedding.

Suppose that *G* is amenable. Then there is an invariant state on $\ell ^{\infty }(G)$. Equivalently, there is an equivariant order homomorphism $\phi : \ell ^{\infty }(G) \to \mathbb {C}$. Since the embedding $\mathbb {C} \subseteq \mathrm {C}(\operatorname {{\partial _{\mathrm {sp}}}} G)$ is essential, it follows that $\phi |_{\pi _{x}(\mathrm {C}(\operatorname {{\partial _{\mathrm {sp}}}} G))}$ is also an embedding. Hence, $\mathrm {C}(\operatorname {{\partial _{\mathrm {sp}}}} G) \mathrm {conv}ng \mathbb {C}$ and $\operatorname {{\partial _{\mathrm {sp}}}} G$ is trivial, so $\mathbb {C}$ is an injective *G*-function system.

Conversely, if *G* is nonamenable, then, since $\mathrm {C}(\operatorname {{\partial _{\mathrm {sp}}}} G)$ is injective in the category of *G*-function systems, there is an equivariant order homomorphism $\psi : \ell ^{\infty }(G) \to \mathrm {C}(\operatorname {{\partial _{\mathrm {sp}}}} G)$ (see Proposition 5.11 below). However, since *G* is nonamenable, there is no invariant state on $\ell ^{\infty }(G)$. Therefore, the range of $\psi $ cannot be $\mathbb {C}$, implying that $\mathrm {C}(\operatorname {{\partial _{\mathrm {sp}}}} G) \not \mathrm {conv}ng \mathbb {C}$. Hence, $\operatorname {{\partial _{\mathrm {sp}}}} G$ is nontrivial, so $\mathbb {C}$ is not an injective *G*-function system.

The next result characterizes essential embeddings of *G*-function systems in terms of the corresponding affine *G*-flows. This is a special case of a more general result from [Reference Kennedy and Schafhauser24, Theorem 7.4].

Proposition 5.5. Let *G* be a discrete group, and let *K* and *M* be affine *G*-flows. Let $\phi : \mathrm {A}(K) \to \mathrm {A}(M)$ be an embedding, and let $\alpha : M \to K$ denote the corresponding equivariant continuous affine map. The following are equivalent:

1. The embedding $\phi $ is essential.

2. The extension $\alpha : M \to K$ is

*K*-irreducible in the sense of [Reference Glasner12], i.e., for every proper affine subflow $L \subseteq M$, $\alpha (L) \ne K$.

Proof. (1) $\Rightarrow $ (2) Let $L \subseteq M$ be an affine subflow such that $\alpha (L) = K$. Let $\psi : \mathrm {A}(M) \to \mathrm {A}(L)$ denote the restriction map. Then $\psi $ is an equivariant order homomorphism and $\psi \circ \phi $ is isometric and therefore is an equivariant order isomorphism. The essentiality of $\phi $ implies that $\psi $ is an order isomorphism. Hence, $L = M$.

(2) $\Rightarrow $ (1) Let *N* be an affine *G*-flow, and let $\psi : \mathrm {A}(M) \to \mathrm {A}(N)$ be an equivariant order homomorphism such that $\psi \circ \phi $ is an isomorphism. Let $\beta : N \to M$ denote the corresponding equivariant continuous affine map and let $L = \beta (N)$. Then *L* is an affine subflow and $\alpha (L) = K$. Hence, $L = M$, so $\psi $ is an isomorphism. Therefore, $\phi $ is essential.

Definition 5.6. Let *G* be a discrete group.

1. Let

*K*and*M*be affine*G*-flows. We will say that an extension $\alpha : M \to K$ is*affinely highly proximal*if for every proper affine*G*-subflow $L \subseteq M$, there is a point $x \in K$ such that $\alpha ^{-1}(Gx) \subseteq M \setminus L$. Equivalently, $\alpha |_{L}$ is not surjective.2. Let

*X*and*Y*be*G*-flows. We will say that an extension $\alpha : Y \to X$ is*affinely highly proximal*if the corresponding affine extension $\tilde {\alpha } : \mathrm {P}(Y) \to \mathrm {P}(X)$ is affinely highly proximal.

*Universally affinely highly proximal* extensions and *maximally affinely highly proximal* affine *G*-flows are defined as they were for *G*-flows in Section 5.2. We will say that a *G*-flow *Y* is *maximally affinely highly proximal* if the corresponding affine *G*-flow $\mathrm {P}(Y)$ is maximally affinely highly proximal.

We observe directly that an affinely highly proximal extension of a minimal *G*-flow $\alpha : Y \to X$ automatically is minimal. Indeed, if $Z \subseteq Y$ is a subflow, then $\tilde {\alpha }(\mathrm {P}(Z)) = \mathrm {P}(X)$ by minimality so that $\mathrm {P}(Z) = \mathrm {P}(Y)$, and hence, $Z = Y$ follows from the assumption that $\alpha $ is affinely highly proximal.

Remark 5.7. Note that there is no ambiguity in Definition 5.6, since if *K* and *M* are affine *G*-flows and $\alpha : M \to K$ is an affinely highly proximal extension, then the corresponding extension $\tilde {\alpha } : \mathrm {P}(M) \to \mathrm {P}(K)$ is also affinely highly proximal. To see this, let $L \subseteq \mathrm {P}(M)$ be an affine *G*-subflow. Let $\beta _{K} : \mathrm {P}(K) \to K$ and $\beta _{M} : \mathrm {P}(M) \to M$ denote the barycenter maps. Then $\beta _{K}$ and $\beta _{M}$ are equivariant and continuous, and $\beta _{K} \circ \tilde {\alpha } = \alpha \circ \beta _{M}$.

It follows from above that $\beta (L) \subseteq M$ is an affine *G*-subflow, so by assumption there is $x \in K$ such that $\alpha ^{-1}(Gx) \subseteq M \setminus \beta _{M}(L)$. Applying $\beta _{M}^{-1}$ to both sides gives

Hence, $\tilde {\alpha }$ is affinely highly proximal.

Theorem 5.8. Let *G* be a discrete group, and let *K* and *M* be affine *G*-flows. Let $\alpha : M \to K$ be an extension, and let $\phi : \mathrm {A}(K) \to \mathrm {A}(M)$ denote the corresponding embedding. Then $\alpha $ is affinely highly proximal if and only if $\phi $ is essential. Hence, *M* is maximally affinely highly proximal if and only if $\mathrm {A}(M)$ is the injective hull of $\mathrm {A}(K)$.

Proof. Suppose that $\alpha $ is affinely highly proximal. Let $L \subseteq M$ be a proper affine subflow. Then there is $x \in K$ such that $\alpha ^{-1}(Gx) \subseteq M \setminus L$. In particular, $\alpha (L)$ is proper. Hence, by (2) of Proposition 5.5, $\phi $ is essential.

Conversely, suppose that $\phi $ is essential. Let $L \subseteq M$ be a proper affine subflow. Then by (2) of Proposition 5.5, $\alpha (L) \ne K$. Since $K \setminus \alpha (L)$ is invariant, there is $x \in K$ such that $Gx \in K \setminus \alpha (L)$. Hence, $\alpha ^{-1}(Gx) \subseteq M \setminus L$. Therefore, $\alpha $ is affinely highly proximal.

The final statement follows from the characterization of the injective hull of $\mathrm {A}(K)$ as maximal essential.

Since every unital commutative *G*-C*-algebra is also (equivariantly order isomorphic to) a *G*-function system, and since an equivariant *-monomorphism is an embedding in the category of *G*-function systems, Theorem 5.8 immediately implies the following result.

Corollary 5.9. Let *G* be a discrete group, and let *X* and *Y* be *G*-flows. Let $\alpha : Y \to X$ be an extension, and let $\pi : \mathrm {C}(X) \to \mathrm {C}(Y)$ denote the corresponding equivariant unital *-monomorphism. Then $\alpha $ is affinely highly proximal if and only if $\pi $ is an essential embedding in the category of *G*-function systems. Hence, *Y* is maximally affinely highly proximal if and only if $\mathrm {C}(Y)$ is the injective hull of $\mathrm {C}(X)$ in the category of *G*-function systems.

Remark 5.10. The notion of an affinely highly proximal extension is related to Glasner’s [Reference Glasner12] notion of a strongly proximal extension. Let *X* and *Y* be *G*-flows. An extension $\alpha : Y \to X$ is strongly proximal if the corresponding extension