The iwasawa invariant μ in the composite of two Zl-extensions

Let k₀ be a finite extension field of the rational numbers, and assume k₀ has at least two Z_l-extensions. Assume that at least one Z_l-extension $\text{K}\text{k}{}_0$ has Iwasawa invariant μ = 0, and let L be the composite of K and some other Z_l-extension of k₀. In this paper we find an upper bound for the number of Z_l-extensions of k₀ contained in L with nonzero μ.     

Covering finite fields with cosets of subspaces

If V is a vector space over a finite field F, the minimum number of cosets of k-dimensional subspaces of V required to cover the nonzero points of V is established. This is done by first regarding V as a field extension of F and then associating with each coset L of a subspace of V a polynomial whose roots are the points of L. A covering with cosets is then equivalent to a product of such polynomials having the minimal polynomial satisfied by all nonzero points of V as a factor.     

Real-analytic,volume-preserving actions of lattices on 4-manifoldsActions analytiques réelles,conservant le volume,de réseaux sur les variétés de dimension 4

We prove that if Γ is a lattice of $\text{Q}$-rank at least 7 in a simple linear Lie group, then any real-analytic, volume-preserving action of Γ on a closed 4-manifold of nonzero Euler characteristic factors through a finite group action. To cite this article: B. Farb, P.B. Shalen, C. R. Acad. Sci. Paris, Ser. I 334 (2002) 1011–1014.     

Classification projective des espaces d’opérateurs différentiels agissant sur les densités

One classifies the representation D_λμ of sl(m + 1, R) obtained by letting its projective embedding in the Lie algebra of vector fields of R^m, m > 1, act by Lie derivatives on the space of differential operators between densities of weight λ and μ. For each μ - λ, there is only finitely many isomorphism classes, most frequently one, in which case D_λμ is isomorphic to its graded space relative to the order of differentiation.     

Martin boundaries of random walks on Fuchsian groups

Let Γ be a finitely generated non-elementary Fuchsian group, and let μ be a probability measure with finite support on Γ such that supp μ generates Γ as a semigroup. If Γ contains no parabolic elements we show that for all but a small number of co-compact Γ, the Martin boundaryM of the random walk on Γ with distribution μ can be identified with the limit set Λ of Γ. If Γ has cusps, we prove that Γ can be deformed into a group Γ', abstractly isomorphic to Γ, such thatM can be identified with Λ', the limit set of Γ'. Our method uses the identification of Λ with a certain set of infinite reduced words in the generators of Γ described in [15]. The harmonic measure ν (ν is the hitting distribution of random paths in Γ on Λ) is a Gibbs measure on this space of infinite words, and the Poisson boundary of Γ, μ can be identified with Λ, ν.     

An algorithmic approach for a special class of Markov chains

In this paper, we develop an algorithmic method for the evaluation of the steady state probability vector of a special class of finite state Markov chains. For the class of Markov chains considered here, it is assumed that the matrix associated with the set of linear equations for the steady state probabilities possess a special structure, such that it can be rearranged and decomposed as a sum of two matrices, one lower triangular with nonzero diagonal elements, and the other an upper triangular matrix with only very few nonzero columns. Almost all Markov chain models of queueing systems with finite source and/or finite capacity and first-come-first-served or head of the line nonpreemptive priority service discipline belongs to this special class.     

The probability of correcting errors by an antinoise coding method when the number of errors belongs to a random set

We consider n messages of N blocks each, where each block is encoded by some antinoise coding method. The method can correct no more than one error. We assume that the number of errors in the ith message belongs to some finite random subset of nonnegative integer numbers. Let A stand for the event that all errors are corrected; we study the probability P(A) and calculate it in terms of conditional probabilities. We prove that under certain moment conditions probabilities P(A) converge almost sure as n and N tend to infinity so that the value n/N has a finite limit. We calculate this limit explicitly.     

The Antipode of a Dual Quasi-Hopf Algebra with Nonzero Integrals is Bijective

For a Hopf algebra A of arbitrary dimension over a field K, it is well-known that if A has nonzero integrals, or, in other words, if the coalgebra A is co-Frobenius, then the space of integrals is one-dimensional and the antipode of A is bijective. Bulacu and Caenepeel recently showed that if H is a dual quasi-Hopf algebra with nonzero integrals, then the space of integrals is one-dimensional, and the antipode is injective. In this short note we show that the antipode is bijective.     

Solving stochastic convex feasibility problems in hilbert spaces

In a stochastic convex feasibility problem connected with a complete probability space (Ω,A,μ) and a family of closed convex sets (C_ω)_ωεΩ in a real Hilbert space H, one wants to find a point that belongs to C_ω for μ almost all ω ε Ω. We present a projection based method where the variable relaxation parameter is defined by a geometrical condition, leading to an iteration sequence that is always weakly convergent to a μ almost common point. We then give a general condition assuring norm convergence of this equation to that μ almost common point     

Finitely Related Clones and Algebras with Cube Terms

Aichinger et al. (2011) have proved that every finite algebra with a cube-term (equivalently, with a parallelogram-term; equivalently, having few subpowers) is finitely related. Thus finite algebras with cube terms are inherently finitely related??every expansion of the algebra by adding more operations is finitely related. In this paper, we show that conversely, if A is a finite idempotent algebra and every idempotent expansion of A is finitely related, then A has a cube-term. We present further characterizations of the class of finite idempotent algebras having cube-terms, one of which yields, for idempotent algebras with finitely many basic operations and a fixed finite universe A, a polynomial-time algorithm for determining if the algebra has a cube-term. We also determine the maximal non-finitely related idempotent clones over A. The number of these clones is finite.     

Eventual partition of conserved quantities in wave motion

Let u be a classical solution to the wave equation in an odd number n of space dimensions, with compact spatial support at each fixed time. Duffin (J. Math. Anal. Appl.32 (1970), 386–391) uses the Paley-Wiener theorem of Fourier analysis to show that, after a finite time, the (conserved) energy of u is partitioned into equal kinetic and potential parts. The wave equation actually has $\text{(n + 2)(n + 3)}\text{2}$ independent conserved quantities, one for each of the standard generators of the conformal group of (n + 1)-dimensional Minkowski space. Of concern in this paper is the “zeroth inversional quantity” I₀, which is commonly used to improve decay estimates which are obtained using conservation of energy. We use Duffin's method to partition I₀ into seven terms, each of which, after a finite time, is explicitly given as a constant-coefficient quadratic function of the time. Zachmanoglou has shown that under the above assumptions if n ? 3, the spatial L² norm of u is eventually constant. A consequence of the analysis here is a bound on this constant in terms of the energy and the radius of the support of the Cauchy data of u at a fixed time.     

On the number of idempotent linear transformations

Let M and N be two subspaces of a finite dimensional vector space V over a finite field F. We can count the number of all idempotent linear transformations T of V such that R(T) ?M and N?N(T), where R(T) and N(T) denote the range space and the null space of T, respectively.     

Quasi-injective dimension

Following our previous work about quasi-projective dimension [11], in this paper, we introduce quasi-injective dimension as a generalization of injective dimension. We recover several well-known results about injective and Gorenstein-injective dimensions in the context of quasi-injective dimension such as the following. (a) If the quasi-injective dimension of a finitely generated module M over a local ring R is finite, then it is equal to the depth of R. (b) If there exists a finitely generated module of finite quasi-injective dimension and maximal Krull dimension, then R is Cohen-Macaulay. (c) If there exists a nonzero finitely generated module with finite projective dimension and finite quasi-injective dimension, then R is Gorenstein. (d) Over a Gorenstein local ring, the quasi-injective dimension of a finitely generated module is finite if and only if its quasi-projective dimension is finite.     

On uniform local dispersion on a family of G-orbits

Consider a topological space T which is the union of a family of G-orbits, where G is a locally euclidean group G acting on T. On every G-orbit consider a probability which is absolutely continuous with respect to the image measure of the normalized restriction of the Haar measure on some compact neighborhood of the identity in G. Assume that the densities of the probabilities on the orbits have a common upper bound. Let μ be a probability on T which is the integral over the measures on the orbits with respect to some probability μ′ on T. It is shown that this specific kind of integral representation of μ does not depend on the size of the compact neighborhood of the identity in G.     

Approximation of measures by Markov processes and homogeneous affine iterated function systems

It is shown that under certain conditions, attractive invariant measures for iterated function systems (indeed for Markov processes on locally compact spaces) depend continuously on parameters of the system. We discuss a special class of iterated function systems, the homogeneous affine ones, for which an inverse problem is easily solved in principle by Fourier transform methods. We show that a probability measureμ onR ⁿ can be approximated by invariant measures for finite iterated function systems of this class if \(\hat \mu (t)/\hat \mu (a^T t)\) is positive definite for some nonzero contractive linear mapa:R ⁿ →R ⁿ . Moments and cumulants are also discussed.     

Pairwise compatibility for 2-simple minded collections

In τ-tilting theory, it is often difficult to determine when a set of bricks forms a 2-simple minded collection. The aim of this paper is to determine when a set of bricks is contained in a 2-simple minded collection for a τ-tilting finite algebra. We begin by extending the definition of mutation from 2-simple minded collections to more general sets of bricks (which we call semibrick pairs). This gives us an algorithm to check if a semibrick pair is contained in a 2-simple minded collection. We then use this algorithm to show that the 2-simple minded collections of a τ-tilting finite gentle algebra (whose quiver contains no loops or 2-cycles) are given by pairwise compatibility conditions if and only if every vertex in the corresponding quiver has degree at most 2. As an application, we show that the classifying space of the τ-cluster morphism category of a τ-tilting finite gentle algebra (whose quiver contains no loops or 2-cycles) is an Eilenberg-MacLane space if every vertex in the corresponding quiver has degree at most 2.     

Exceptional curves on rational surfaces having K2⩾0

We characterize the rational surfaces X which have a finite number of (?1)-curves under the assumption that ?K_X is nef, where K_X is a canonical divisor on X, and has self-intersection zero. We prove also that if ?K_X is not nef and has self-intersection zero, then X has a finite number of (?1)-curves. To cite this article: M. Lahyane, C. R. Acad. Sci. Paris, Ser. I 338 (2004).     

On a neutral functional differential equation in a fading memory space

The linear autonomous, neutral system of functional differential equations $\text{d}\text{dt}\text{(μ 1 x(t) + ?(t)) = v 1 s(t) + g(t) (t ? o)}$, (1) x(t) = ?(t) (t ? 0), in a fading memory space is studied. Here μ and ν are matrix-valued measures supported on [0, ∞), finite with respect to a weight function, and $\text{?, g,}\text{and}\text{?}$ are Cⁿ-valued, continuous or locaily integrable functions, bounded with respect to a fading memory norm. Conditions which imply that solutions of (1) can be decomposed into a stable part and an unstable part are given. These conditions are of frequency domain type. The usual assumption that the singular part of μ vanishes is not needed. The results can be used to decompose the semigroup generated by (1) into a stable part and an unstable part.     

Alternating sign matrices and descending plane partitions

An alternating sign matrix is a square matrix such that (i) all entries are 1, ?1, or 0, (ii) every row and column has sum 1, and (iii) in every row and column the nonzero entries alternate in sign. Striking numerical evidence of a connection between these matrices and the descending plane partitions introduced by Andrews (Invent. Math.53 (1979), 193–225) have been discovered, but attempts to prove the existence of such a connection have been unsuccessful. This evidence, however, did suggest a method of proving the Andrews conjecture on descending plane partitions, which in turn suggested a method of proving the Macdonald conjecture on cyclically symmetric plane partitions (Invent. Math.66 (1982), 73–87). In this paper is a discussion of alternating sign matrices and descending plane partitions, and several conjectures and theorems about them are presented.