On the peripheral point spectrum and the asymptotic behavior of irreducible semigroups of Harris operators

Given a positive, irreducible and bounded $C_0$ -semigroup on a Banach lattice with order continuous norm, we prove that the peripheral point spectrum of its generator is trivial whenever one of its operators dominates a non-trivial compact or kernel operator. For a discrete semigroup, i.e. for powers of a single operator $T$ , we show that the point spectrum of some power $T^k$ intersects the unit circle at most in $1$ . As a consequence, we obtain a sufficient condition for strong convergence of the $C_0$ -semigroup and for a subsequence of the powers of $T$ , respectively.     

Spectral self-affine measures with prime determinant

The self-affine measure $\mu _{M,D}$ relating to an expanding matrix $M\in M_{n}(\mathbb Z )$ and a finite digit set $D\subset \mathbb Z ^n$ is a unique probability measure satisfying the self-affine identity with equal weight. In the present paper, we shall study the spectrality of $\mu _{M,D}$ in the case when $|\det (M)|=p$ is a prime. The main result shows that under certain mild conditions, if there are two points $s_{1}, s_{2}\in \mathbb R ^{n}, s_{1}-s_{2}\in \mathbb Z ^{n}$ such that the exponential functions $e_{s_{1}}(x), e_{s_{2}}(x)$ are orthogonal in $L^{2}(\mu _{M,D})$ , then the self-affine measure $\mu _{M,D}$ is a spectral measure with lattice spectrum. This gives some sufficient conditions for a self-affine measure to be a lattice spectral measure.     

Multiplicity of virtual levels at the lower edge of the continuous spectrum of a two-particle Hamiltonian on a lattice

We consider a system of two arbitrary quantum particles on a three-dimensional lattice with special dispersion functions (describing site-to-site particle transport), where the particles interact by a chosen attraction potential. We study how the number of eigenvalues of the family of the operators h(k) depends on the particle interaction energy and the total quasimomentum \(k \in \mathbb{T}^3\) (where \(\mathbb{T}^3\) is a three-dimensional torus). Depending on the particle interaction energy, we obtain conditions under which the left edge of the continuous spectrum is simultaneously a multiple virtual level and an eigenvalue of the operator h(0).     

Completely continuous and weakly completely continuous abstract Segal algebras

Let $\mathcal{A}$ be a Banach algebra. It is obtained a necessary and sufficient condition for the complete continuity and also weak complete continuity of symmetric abstract Segal algebras with respect to $\mathcal{A}$ , under the condition of the existence of an approximate identity for $\mathcal{B}$ , bounded in $\mathcal{A}$ . In addition, a necessary condition for the weak complete continuity of $\mathcal{A}$ is given. Moreover, the applications of these results about some group algebras on locally compact groups are obtained.     

Inductive topological Hausdorff dimensions and fibers of generic continuous functions

In an earlier paper Buczolich, Elekes and the author introduced a new concept of dimension for metric spaces, the so called topological Hausdorff dimension. They proved that it is precisely the right notion to describe the Hausdorff dimension of the level sets of the generic real-valued continuous function (in the sense of Baire category) defined on a compact metric space $K$ . The goal of this paper is to determine the Hausdorff dimension of the fibers of the generic continuous function from $K$ to $\mathbb {R}^n$ . In order to do so, we define the $n$ th inductive topological Hausdorff dimension, $\dim _{t^nH} K$ . Let $\dim _H K,\,\dim _t K$ and $C_n(K)$ denote the Hausdorff and topological dimension of $K$ and the Banach space of the continuous functions from $K$ to $\mathbb {R}^n$ . We show that $\sup _{y\in \mathbb {R}^n} \dim _{H}f^{-1}(y) = \dim _{t^nH} K -n$ for the generic $f \in C_n(K)$ , provided that $\dim _t K\ge n$ , otherwise every fiber is finite. In order to prove the above theorem we give some equivalent definitions for the inductive topological Hausdorff dimensions, which can be interesting in their own right. Here we use techniques coming from the theory of topological dimension. We show that the supremum is actually attained on the left hand side of the above equation. We characterize those compact metric spaces $K$ for which $\dim _{H} f^{-1}(y)=\dim _{t^nH}K-n$ for the generic $f\in C_n(K)$ and the generic $y\in f(K)$ . We also generalize a result of Kirchheim by showing that if $K$ is self-similar and $\dim _t K\ge n$ then $\dim _{H} f^{-1}(y)=\dim _{t^nH}K-n$ for the generic $f\in C_n(K)$ for every $y\in {{\mathrm{int}}}f(K)$ .     

Sharp regularity of linearization for C^{1,1} hyperbolic diffeomorphisms

$C^1$ linearization is of special significance because it preserves smooth dynamical behaviors and distinguishes qualitative properties in characteristic directions. However, $C^1$ smoothness is not enough to guarantee $C^1$ linearization. For $C^{1,1}$ hyperbolic diffeomorphisms on Banach spaces $C^1$ linearization was proved under a gap condition together with a band condition of the spectrum. In this paper, the result of $C^1$ linearization in Banach spaces is strengthened to $C^{1,\beta }$ linearization with a constant $\beta >0$ under a weaker band condition by a decomposition with invariant foliations. The weaker band condition allows the spectrum to be a union of more than two but finitely many bands but restricts those bands to be bounded by a number depending on the supremum of contractive spectrum and the infimum of expansive spectrum. Furthermore, we give an estimate for the exponent $\beta $ and prove that the estimate is sharp in the planar case.     

The subvariety of commutative residuated lattices represented by twist-products

Given an integral commutative residuated lattice L, the product L × L can be endowed with the structure of a commutative residuated lattice with involution that we call a twist-product. In the present paper, we study the subvariety ${\mathbb{K}}$ of commutative residuated lattices that can be represented by twist-products. We give an equational characterization of ${\mathbb{K}}$ , a categorical interpretation of the relation among the algebraic categories of commutative integral residuated lattices and the elements in ${\mathbb{K}}$ , and we analyze the subvariety of representable algebras in ${\mathbb{K}}$ . Finally, we consider some specific class of bounded integral commutative residuated lattices ${\mathbb{G}}$ , and for each fixed element ${{\bf L} \in \mathbb{G}}$ , we characterize the subalgebras of the twist-product whose negative cone is L in terms of some lattice filters of L, generalizing a result by Odintsov for generalized Heyting algebras.     

Semicontinuous limits of nets of continuous functions

In this paper we present a topology on the space of real-valued functions defined on a functionally Hausdorff space $X$ that is finer than the topology of pointwise convergence and for which (1) the closure of the set of continuous functions $\mathcal{C }(X)$ is the set of upper semicontinuous functions on $X$ , and (2) the pointwise convergence of a net in $\mathcal{C }(X)$ to an upper semicontinuous limit automatically ensures convergence in this finer topology.     

Non-coincidence of quenched and annealed connective constants on the supercritical planar percolation cluster

In this paper, we study the abundance of self-avoiding paths of a given length on a supercritical percolation cluster on \(\mathbb{Z }^d\) . More precisely, we count \(Z_N\) , the number of self-avoiding paths of length \(N\) on the infinite cluster starting from the origin (which we condition to be in the cluster). We are interested in estimating the upper growth rate of \(Z_N\) , \(\limsup _{N\rightarrow \infty } Z_N^{1/N}\) , which we call the connective constant of the dilute lattice. After proving that this connective constant is a.s. non-random, we focus on the two-dimensional case and show that for every percolation parameter \(p\in (1/2,1)\) , almost surely, \(Z_N\) grows exponentially slower than its expected value. In other words, we prove that \(\limsup _{N\rightarrow \infty } (Z_N)^{1/N}{<}\lim _{N\rightarrow \infty } \mathbb{E }[Z_N]^{1/N}\) , where the expectation is taken with respect to the percolation process. This result can be considered as a first mathematical attempt to understand the influence of disorder for self-avoiding walks on a (quenched) dilute lattice. Our method, which combines change of measure and coarse graining arguments, does not rely on the specifics of percolation on \(\mathbb{Z }^2\) , so our result can be extended to a large family of two-dimensional models including general self-avoiding walks in a random environment.     

Geometrical illustration of numerical semigroups and of some of their invariants

Let \(p,q\in \mathbb {N}\) be relatively prime integers with \(1 . In Knebl et al. (J Algebra 348:315–335, 2011) the numerical semigroups containing \(p\) and \(q\) have been illustrated by certain lattice paths in the plane. In this paper we derive some further information from this visualization, for example formulas to compute or estimate the number of certain semigroups by counting lattice paths, and formulas for their genus and Frobenius number.     

Solid Extensions of the Cesàro Operator on ℓ p and c 0

We introduce and study the largest Banach lattice (for the coordinate-wise order) which is a solid subspace of \({\mathbb{C}^\mathbb{N}}\) and to which the classical Cesàro operator \({\mathcal{C}\colon\ell^p \to \ell^p}\) (a positive operator) can be continuously extended while still maintaining its values in ? ^p . Properties of this optimal Banach lattice \({[\mathcal{C}, \ell^p]_s}\) are presented. In addition, all continuous convolution operators of \({[\mathcal{C}, \ell^p]_s}\) into itself are identified and the spectrum of \({\mathcal{C}\colon[\mathcal{C}, \ell^p]_s \to[\mathcal{C}, \ell^p]_s}\) is determined. A similar investigation is undertaken for the Cesàro operator \({\mathcal{C}\colon c_0\to c_0}\) .     

Spectral Measures Associated with the Factorization of the Lebesgue Measure on a Set via Convolution

Let \(Q\) be a fundamental domain of some full-rank lattice in \({\mathbb {R}}^d\) and let \(\mu \) and \(\nu \) be two positive Borel measures on \({\mathbb {R}}^d\) such that the convolution \(\mu *\nu \) is a multiple of \(\chi _Q\) . We consider the problem as to whether or not both measures must be spectral (i.e. each of their respective associated \(L^2\) space admits an orthogonal basis of exponentials) and we show that this is the case when \(Q = [0,1]^d\) . This theorem yields a large class of examples of spectral measures which are either absolutely continuous, singularly continuous or purely discrete spectral measures. In addition, we propose a generalized Fuglede’s Conjecture for spectral measures on \({\mathbb {R}}^1\) and we show that it implies the classical Fuglede’s Conjecture on \({\mathbb {R}}^1\) .     

Connections between Construction D and related constructions of lattices

Most practical constructions of lattice codes with high coding gains are multilevel constructions where each level corresponds to an underlying code component. Construction D, Construction \(\hbox {D}'\) , and Forney’s code formula are classical constructions that produce such lattices explicitly from a family of nested binary linear codes. In this paper, we investigate these three closely related constructions along with the recently developed Construction \(\hbox {A}'\) of lattices from codes over the polynomial ring \(\mathbb {F}_2[u]/u^a\) . We show that Construction by Code Formula produces a lattice packing if and only if the nested codes being used are closed under Schur product, thus proving the similarity of Construction D and Construction by Code Formula when applied to Reed–Muller codes. In addition, we relate Construction by Code Formula to Construction \(\hbox {A}'\) by finding a correspondence between nested binary codes and codes over \(\mathbb {F}_2[u]/u^a\) . This proves that any lattice constructible using Construction by Code Formula is also constructible using Construction \(\hbox {A}'\) . Finally, we show that Construction \(\hbox {A}'\) produces a lattice if and only if the corresponding code over \(\mathbb {F}_2[u]/u^a\) is closed under shifted Schur product.     

Kirkman frames having hole type h^{u} m^{1} for h \equiv 0 {\,\,\mathrm{mod}\, 12}\,

Constructing non-uniform designs has become a topic of considerable interest over the last two decades due to the vital role of such designs in the constructions for other types of designs. Ge, Rees and Shalaby started the investigation into the spectrum of non-uniform Kirkman frames of type \(h^{u}m^{1}\) . They determined the spectrum for the cases where \(h\in \{2,4,6,8,10,12\}\) , leaving 24 cases of \((h,u,m)\) . In this paper, we continue to investigate this spectrum problem by removing all of these 24 possible exceptions. Using this result, we show that for each value of \(h \equiv 0 {\,\,\mathrm{mod}\, 12}\,\) the obvious necessary conditions on \(u, m\) are also sufficient.     

Approximation of the Laplace and Stokes operators with Dirichlet boundary conditions through volume penalization: a spectral viewpoint

We report the results of a study on the spectral properties of Laplace and Stokes operators modified with a volume penalization term designed to approximate Dirichlet conditions in the limit when a penalization parameter, \(\eta \) , tends to zero. The eigenvalues and eigenfunctions are determined either analytically or numerically as functions of \(\eta \) , both in the continuous case and after applying Fourier or finite difference discretization schemes. For fixed \(\eta \) , we find that only the part of the spectrum corresponding to eigenvalues \(\lambda \lesssim \eta ^{-1}\) approaches Dirichlet boundary conditions, while the remainder of the spectrum is made of uncontrolled, spurious wall modes. The penalization error for the controlled eigenfunctions is estimated as a function of \(\eta \) and \(\lambda \) . Surprisingly, in the Stokes case, we show that the eigenfunctions approximately satisfy, with a precision \(O(\eta )\) , Navier slip boundary conditions with slip length equal to \(\sqrt{\eta }\) . Moreover, for a given discretization, we show that there exists a value of \(\eta \) , corresponding to a balance between penalization and discretization errors, below which no further gain in precision is achieved. These results shed light on the behavior of volume penalization schemes when solving the Navier–Stokes equations, outline the limitations of the method, and give indications on how to choose the penalization parameter in practical cases.     

Polytopal Affine Semigroups with Holes Deep Inside

Given a positive integer $k$ k , we construct a lattice $3$ 3 -simplex $P$ P with the following property: The affine semigroup $Q_P$ Q P associated to $P$ P is not normal, and every element $q \in \overline{Q}_P \setminus Q_P$ q ∈ Q ¯ P ? Q P has lattice distance at least $k$ k above every facet of $Q_P$ Q P .     

Logarithmic Expansions and the Stability of Periodic Patterns of Localized Spots for Reaction–Diffusion Systems in {\mathbb {R}}^2

The linear stability of steady-state periodic patterns of localized spots in \({\mathbb {R}}^2\) for the two-component Gierer–Meinhardt (GM) and Schnakenberg reaction–diffusion models is analyzed in the semi-strong interaction limit corresponding to an asymptotically small diffusion coefficient \({\displaystyle \varepsilon }^2\) of the activator concentration. In the limit \({\displaystyle \varepsilon }\rightarrow 0\) , localized spots in the activator are centered at the lattice points of a Bravais lattice with constant area \(|\Omega |\) . To leading order in \(\nu ={-1/\log {\displaystyle \varepsilon }}\) , the linearization of the steady-state periodic spot pattern has a zero eigenvalue when the inhibitor diffusivity satisfies \(D={D_0/\nu }\) for some \(D_0\) independent of the lattice and the Bloch wavevector \({\pmb k}\) . From a combination of the method of matched asymptotic expansions, Floquet–Bloch theory, and the rigorous study of certain nonlocal eigenvalue problems, an explicit analytical formula for the continuous band of spectrum that lies within an \({\mathcal O}(\nu )\) neighborhood of the origin in the spectral plane is derived when \(D={D_0/\nu } + D_1\) , where \(D_1={\mathcal O}(1)\) is a detuning parameter. The periodic pattern is linearly stable when \(D_1\) is chosen small enough so that this continuous band is in the stable left half-plane \(\text{ Re }(\lambda )<0\) for all \({\pmb k}\) . Moreover, for both the Schnakenberg and GM models, our analysis identifies a model-dependent objective function, involving the regular part of the Bloch Green’s function, that must be maximized in order to determine the specific periodic arrangement of localized spots that constitutes a linearly stable steady-state pattern for the largest value of \(D\) . From a numerical computation, based on an Ewald-type algorithm, of the regular part of the Bloch Green’s function that defines the objective function, it is shown within the class of oblique Bravais lattices that a regular hexagonal lattice arrangement of spots is optimal for maximizing the stability threshold in \(D\) .