A classical theorem due to Young states that the cosine polynomial

$$ C_n(x)=1+\sum_{k=1}^{n}\frac{\cos(kx)}{k} $$

is positive for all $n\geq1$ and $x\in(0,\pi)$. We prove the following refinement. For all $n\geq2$ and $x\in[0,\pi]$ we have

$$ \tfrac{1}{6}+c(\pi-x)^2\leq C_n(x), $$

with the best possible constant factor

$$ c=\min_{0\leq t\lt\pi}\frac{5+6\cos(t)+3\cos(2t)}{6(\pi-t)^2}=0.069\dots. $$

For a set $X$ and a variety $\mathcal{V}$ of bands, let $B_{\mathcal{V}}(X)$ be the relatively free band in $\mathcal{V}$ on $X$. For an arbitrary band variety $\mathcal{V}$ and an arbitrary set $X$, we determine the group of automorphisms of $\mathrm{End}(B_{\mathcal{V}}(X))$, the monoid of endomorphisms of $B_{\mathcal{V}}(X)$.

We study hypersurfaces of constant mean curvature immersed into warped product spaces of the form $\mathbb{R}\times_\varrho\mathbb{P}^n$, where $\mathbb{P}^n$ is a complete Riemannian manifold. In particular, our study includes that of constant mean curvature hypersurfaces in product ambient spaces, which have recently been extensively studied. It also includes constant mean curvature hypersurfaces in the so-called pseudo-hyperbolic spaces. If the hypersurface is compact, we show that the immersion must be a leaf of the trivial totally umbilical foliation $t\in\mathbb{R}\mapsto\{t\}\times\mathbb{P}^n$, generalizing previous results by Montiel. We also extend a result of Guan and Spruck from hyperbolic ambient space to the general situation of warped products. This extension allows us to give a slightly more general version of a result by Montiel and to derive height estimates for compact constant mean curvature hypersurfaces with boundary in a leaf.

A terrace for $\mathbb{Z}_m$ is a particular type of sequence formed from the $m$ elements of $\mathbb{Z}_m$. For $m$ odd, many procedures are available for constructing power-sequence terraces for $\mathbb{Z}_m$; each terrace of this sort may be partitioned into segments, of which one contains merely the zero element of $\mathbb{Z}_m$, whereas every other segment is either a sequence of successive powers of an element of $\mathbb{Z}_m$ or such a sequence multiplied throughout by a constant. We now refine this idea to show that, for $m=n-1$, where $n$ is an odd prime power, there are many ways in which power-sequences in $\mathbb{Z}_n$ can be used to arrange the elements of $\mathbb{Z}_n\setminus\{0\}$ in a sequence of distinct entries $i$, $1\le i\le m$, usually in two or more segments, which becomes a terrace for $\mathbb{Z}_m$ when interpreted modulo $m$ instead of modulo $n$. Our constructions provide terraces for $\mathbb{Z}_{n-1}$ for all prime powers $n$ satisfying $0\ltn\lt300$ except for $n=125$, $127$ and $257$.

In this paper we give Bessel- and Grüss-type inequalities in an inner product module over a proper $H^*$-algebra or a $C^*$-algebra.

We study sets of range uniqueness (SRUs) for analytic functions inside a disc of an algebraically closed field $K$ complete with respect to an ultrametric absolute value. The SRUs we obtain are converging sequences. We first obtain results that look like those known in $\mathbb{C}$ but involve a weaker hypothesis than in $\mathbb{C}$: let $(a_n)$ be a sequence of limit $a$ in a disc $d(a,r^-)$ such that $|a_n-a|$ is a strictly decreasing sequence. If the sequence $(a_n)$ does not make an SRU for the set $\mathcal{A}(d(a,r^-))$ of analytic functions inside $d(a,r^-)$, then, for a certain integer $k\in\mathbb{Z}$, the sequence

$$ \bigg(\frac{a_{n+k}-a}{a_n-a}\bigg) $$

has a finite limit in $K$ and the sequence

$$ \bigg(\frac{\log|a_{n+k}-a|}{\log|a_n-a|}\bigg) $$

has a finite rational limit. Next, we show that if the sequence

$$ \frac{\log(a_{n+1}-a)}{\log(a_n-a)} $$

converges to a limit $b\geq1$ in such a way that $-b\log|a_{n}-a|\lt-b\log|a_{n+1}-a|$ and if $\log|a_{n}-a|-b\log|a_{n+1}-a|$ has limit $0$ or $+\infty$ and if $b^k\notin\mathbb{Q}$ whenever $b>1$ and $k\in \mathbb{N}^*$, then the sequence $(a_n)$ is an SRU for $\mathcal{A}(d(a,r^-))$. In particular, for every $\gamma\in\;]0,1[\;\cup\;]1,+\infty[$, $L\in\mathbb{Q}\;\cap\;]0,+\infty[$ and $b\geq 1$, there exist SRUs for $\mathcal{A}(d(a,r^-))$ of the form $\{a_n\mid n\in\mathbb{N}\}$ such that

$$ \lim_{n\rightarrow+\infty}\frac{-\log|a_n-a|}{b^nn^\gamma}=L. $$

For example, if $\gamma\in\mathbb{N}$ with $\gamma\neq0,1$, there exist SRUs of the form $\{a_n\mid n\in\mathbb{N}\}$ such that $-\log |a_n-a|=Ln^\gamma$ for all $n\in\mathbb{N}^*$. The latter result ceases to hold when $\gamma=1$. Many examples and counterexamples are provided.

We define a partial Radon transform mapping functions on $\mathbb{R}^{n+l}$ to functions on $\mathbb{R}^{n}$ which intertwines the Laplace operator on the two spaces. As a consequence, transplantation formulae relating the radial eigenfunctions of the Laplacian on Euclidean spaces of different dimensions are obtained. Our formulae provide a geometric interpretation of integral formulae for Bessel functions of Abel type, which are found useful in potential theory. The formulae portray a view of Hadamard's method of descent within the realm of harmonic analysis, allowing the transplant of local problems from even dimensions to odd dimensions and unifying the techniques of several authors.

Let $R$ be a commutative ring with unity and let $G$ be a group. The group ring $RG$ has a natural involution that maps each element $g\in G$ to its inverse. We denote by $RG^-$ the set of skew symmetric elements under this involution. We study necessary and sufficient conditions for $RG^-$ to be commutative.

The symmetric inverse monoid $\mathcal{I}_{n}$ is the set of all partial permutations of an $n$-element set. The largest possible size of a $2$-generated subsemigroup of $\mathcal{I}_{n}$ is determined. Examples of semigroups with these sizes are given. Consequently, if $M(n)$ denotes this maximum, it is shown that $M(n)/|\mathcal{I}_{n}|\rightarrow1$ as $n\rightarrow\infty$. Furthermore, we deduce the known fact that $\mathcal{I}_{n}$ embeds as a local submonoid of an inverse $2$-generated subsemigroup of $\mathcal{I}_{n+1}$.

Based on the identification of four-dimensional Möbius transformations

$$ g(x)=(ax+b)(cx+d)^{-1} $$

by the matrix group $\mathrm{PS}_\triangle L(2,\mathbb{H})$ of quaternionic $2\times2$ matrices with Dieudonné determinant equal to $1$, we give an explicit expression for the classification of $g$ in terms of $a$, $b$, $c$ and $d$.

Let $\varphi(\cdot)$ be the Euler function and let $\sigma(\cdot)$ be the sum-of-divisors function. In this note, we bound the number of positive integers $n\le x$ with the property that $s(n)=\sigma(n)-n$ divides $\varphi(n)$.

The principal results in this paper are concerned with the description of domains of infinitesimal generators of strongly continuous groups of isometries in non-commutative operator spaces $E(\mathcal{M},\tau)$, which are induced by $\mathbb{R}$-flows on $\mathcal{M}$. In particular, we are concerned with the description of operator functions which leave the domain of such generators invariant in all symmetric operator spaces, associated with a semi-finite von Neumann algebra $\mathcal{M}$ and a separable function space $E$ on $(0,\infty)$. Furthermore, we apply our results to the study of operator functions for which $[D,x]\in E(\mathcal{M},\tau)$ implies that $[D,f(x)]\in E(\mathcal{M},\tau)$, where $D$ is an unbounded self-adjoint operator. Our methods are partly based on the recently developed theory of double operator integrals in symmetric operator spaces and the theory of adjoint $C_{0}$-semigroups.

CoGalois groups appear in a natural way in the study of covers. They generalize the well-known group of covering automorphisms associated with universal covering spaces. Recently, it has been proved that each quasi-coherent sheaf over the projective line $\bm{P}^1(R)$ ($R$ is a commutative ring) admits a flat cover and so we have the associated coGalois group of the cover. In general the problem of computing coGalois groups is difficult. We study a wide class of quasi-coherent sheaves whose associated coGalois groups admit a very accurate description in terms of topological properties. This class includes finitely generated and cogenerated sheaves and therefore, in particular, vector bundles.

This paper is concerned with the critical nonlinear Gross–Pitaevskii equation, which describes the attractive Bose–Einstein condensate under a magnetic trap. We derive a sharp threshold between the global existence and the blowing-up of the system. Furthermore, we answer the question: how small are the initial data, such that the system has global solutions for the nonlinear critical power $p=1+(4/N)$?

We study certain sequences of rational functions with poles outside the unit circle. Such kinds of sequences are recursively constructed based on sequences of complex numbers with norm less than one. In fact, such sequences are closely related to the Schur–Nevanlinna algorithm for Schur functions on the one hand, and to orthogonal rational functions on the unit circle on the other. We shall see that rational functions belonging to a Schur–Nevanlinna sequence can be used to parametrize the set of all solutions of an interpolation problem of Nevanlinna–Pick type for Schur functions.

We consider the quasilinear elliptic variational system

\begin{alignat*} {2} -\Delta_pu\amp=\lambda F_u(x,u,v)+\mu H_u(x,u,v) \quad\amp \amp\text{in }\varOmega, \\ -\Delta_qv\amp=\lambda F_v(x,u,v)+\mu H_v(x,u,v) \amp \amp \text{in }\varOmega, \\ u\amp=v=0 \amp \amp \text{on }\partial\varOmega, \end{alignat*}

where $\varOmega$ is a strip-like domain and $\lambda$ and $\mu$ are positive parameters. Using a recent two-local-minima theorem and the principle of symmetric criticality, existence and multiplicity are proved under suitable conditions on $F$.

An element $a$ in a ring $R$ with identity is called strongly clean if it is the sum of an idempotent and a unit that commute. And $a\in R$ is called strongly $\pi$-regular if both chains $aR\supseteq a^2R\supseteq\cdots$ and $Ra\supseteq Ra^2\supseteq\cdots$ terminate. A ring $R$ is called strongly clean (respectively, strongly $\pi$-regular) if every element of $R$ is strongly clean (respectively, strongly $\pi$-regular). Strongly $\pi$-regular elements of a ring are all strongly clean. Let $\sigma$ be an endomorphism of $R$. It is proved that for $\varSigma r_ix^i\in R[[x,\sigma]]$, if $r_0$ or $1-r_0$ is strongly $\pi$-regular in $R$, then $\varSigma r_ix^i$ is strongly clean in $R[[x,\sigma]]$. In particular, if $R$ is strongly $\pi$-regular, then $R[[x,\sigma]]$ is strongly clean. It is also proved that if $R$ is a strongly $\pi$-regular ring, then $R[x,\sigma]/(x^n)$ is strongly clean for all $n\ge1$ and that the group ring of a locally finite group over a strongly regular or commutative strongly $\pi$-regular ring is strongly clean.

If $A$ is a stable basis algebra of rank $n$, then the set $S_{n-1}$ of endomorphisms of rank at most $n-1$ is a subsemigroup of the endomorphism monoid of $A$. This paper gives a number of necessary and sufficient conditions for $S_{n-1}$ to be generated by idempotents. These conditions are satisfied by finitely generated free modules over Euclidean domains and by free left $T$-sets of finite rank, where $T$ is cancellative monoid in which every finitely generated left ideal is principal.

Under certain assumptions on $g(x)$, we obtain an asymptotic formula for computing integrals of the form

$$ F(x,\alpha)=\int_{-\infty}^\infty g(t)^\alpha\exp\biggl(-\bigg|\int_x^tg(\xi)\,\mathrm{d}\xi\bigg|\biggr)\,\mathrm{d}t,\quad\alpha\in\mathbb{R}, $$

as $|x|\to\infty$. We use this formula to study the properties (as $|x|\to\infty$) of the solutions of the correctly solvable equations in $L_p(\mathbb{R})$, $p\in[1,\infty]$,

\begin{equation} -y''(x)+q(x)y(x)=f(x),\quad x\in\mathbb{R}, \tag{1} \end{equation}

where $0\le q\in L_1^{\mathrm{loc}}(\mathbb{R})$, and $f\in L_p(\mathbb{R})$. (Equation (1) is called correctly solvable in a given space $L_p(\mathbb{R})$ if for any function $f\in L_p(\mathbb{R})$ it has a unique solution $y\in L_p(\mathbb{R})$ and if the following inequality holds with an absolute constraint $c_p\in(0,\infty)$: $\|y\|_{L_p(\mathbb{R})}\leq c(p)\|f\|_{L_p(\mathbb{R})}$, $\forall f\in L_p(\mathbb{R})$.)

We establish two separation theorems in which the classic interior is replaced by the quasi-relative interior.