Results of Fowler and Sims show that every k-graph is completely determined by its k-coloured skeleton and collection of commuting squares. Here we give an explicit description of the k-graph associated with a given skeleton and collection of squares and show that two k-graphs are isomorphic if and only if there is an isomorphism of their skeletons which preserves commuting squares. We use this to prove directly that each k-graph Λ is isomorphic to the quotient of the path category of its skeleton by the equivalence relation determined by the commuting squares, and show that this extends to a homeomorphism of infinite-path spaces when the k-graph is row finite with no sources. We conclude with a short direct proof of the characterization, originally due to Robertson and Sims, of simplicity of the C*-algebra of a row-finite k-graph with no sources.

In this paper we study Lp−Lr estimates of both the extension operator and the averaging operator associated with the algebraic variety S = {x ∈ : Q(x) = 0}, where Q(x) is a non-degenerate quadratic form over the finite field with q elements. We show that the Fourier decay estimate on S is good enough to establish the sharp averaging estimates in odd dimensions. In addition, the Fourier decay estimate enables us to simply extend the sharp L2−L4 conical extension result in , due to Mockenhaupt and Tao, to the L2−L2(d+1)/(d−1) estimate in all odd dimensions d ≥ 3. We also establish a sharp estimate of the mapping properties of the average operators in the case when the variety S in even dimensions d ≥ 4 contains a d/2-dimensional subspace.

In this paper, we consider rational or multi-cut solutions to Hele-Shaw flow driven by suction. A lower bound of the distance between the moving boundary and the suction point is given and this lower bound enables us to find necessary conditions for rational or multi-cut solutions to blow up due to the boundary touching the sink.

Given two arbitrary sequences $({\lambda }_{j} )_{j\geq 1} $ and $({\mu }_{j} )_{j\geq 1} $ of real numbers satisfying

$$\begin{eqnarray*}\displaystyle \vert {\lambda }_{1} \vert \gt \vert {\mu }_{1} \vert \gt \vert {\lambda }_{2} \vert \gt \vert {\mu }_{2} \vert \gt \cdots \gt \vert {\lambda }_{j} \vert \gt \vert {\mu }_{j} \vert \rightarrow 0, &&\displaystyle\end{eqnarray*}$$

$c= ({c}_{n} )_{n\in { \mathbb{Z} }_{+ } } $

${\Gamma }_{c} $

${\Gamma }_{\tilde {c} } $

$c= ({c}_{n} )_{n\geq 0} $

$\tilde {c} = ({c}_{n+ 1} )_{n\geq 0} $

${\ell }^{2} ({ \mathbb{Z} }_{+ } )$

$({\lambda }_{j} )_{j\geq 1} $

$({\mu }_{j} )_{j\geq 1} $

$c$

${\Gamma }_{c} $

${\Gamma }_{\tilde {c} } $

$({\lambda }_{j} )_{j\geq 1} $

$({\mu }_{j} )_{j\geq 1} $

$({\rho }_{j} )_{j\geq 1} $

$({\sigma }_{j} )_{j\geq 1} $

$$\begin{eqnarray*}\displaystyle {\rho }_{1} \gt {\sigma }_{1} \gt {\rho }_{2} \gt {\sigma }_{2} \gt \cdots \gt {\rho }_{j} \gt {\sigma }_{j} \rightarrow 0, &&\displaystyle\end{eqnarray*}$$

$c= ({c}_{n} )_{n\in { \mathbb{Z} }_{+ } } $

${\Gamma }_{c} $

${\Gamma }_{\tilde {c} } $

${\ell }^{2} ({ \mathbb{Z} }_{+ } )$

$({\rho }_{j} )_{j\geq 1} $

$({\sigma }_{j} )_{j\geq 1} $

In this paper, we study a system of thermoelasticity with a degenerate second-order operator in the heat equation. We analyze the evolution of the energy density of a family of solutions. We consider two cases: when the set of points where the ellipticity of the heat operator fails is included in a hypersurface and when it is an open set. In the first case, and under special assumptions, we prove that the evolution of the energy density is that of a damped wave equation: propagation along the rays of the geometric optic and damping according to a microlocal process. In the second case, we show that the energy density propagates along rays which are distortions of the rays of the geometric optic.

We give two generalizations of Kac's Theorem on representations of quivers. One is to representations of equipped graphs by relations, in the sense of Gelfand and Ponomarev. The other is to representations of quivers in which certain of the linear maps are required to have maximal rank.

We compute the first three terms of the 1/d expansions for the growth constants and one-point functions of nearest-neighbour lattice trees and lattice (bond) animals on the integer lattice $\mathbb{Z}^d$, with rigorous error estimates. The proof uses the lace expansion, together with a new expansion for the one-point functions based on inclusion–exclusion.