On the index of maximum density for irreducible Boolean matrices

The index of maximum density of a Boolean (or nonnegative) matrix A is defined as the least positive integer h=h(A) such that the number of ones (or positive entries) in A^h is maximized in all powers of A. Our main results are the following: (1) Let IB_n,p be the set of n × n irreducible Boolean matrices with period p. We give the largest value of h(A) for A ϵ IB_n,p. (2) Let H_n,p be the set of h(A) for A ϵ IB_n,p. We exhibit a system of gaps in H_n,p. (3) We completely determine the set of h(A) for all n × n symmetric irreducible Boolean matrices.     

The endomorphism spectrum of a monounary algebra

The endomorphism spectrum specA of an algebra A is defined as the set of all positive integers, which are equal to the number of elements in an endomorphic image of A, for all endomorphisms of A. In this paper we study finite monounary algebras and their endomorphism spectrum. If a finite set S of positive integers is given, one can look for a monounary algebra A with S = specA. We show that for countably many finite sets S, no such A exists. For some sets S, an appropriate A with spec A = S are described. For n ∈ ? it is easy to find a monounary algebra A with {1, 2, ..., n} = specA. It will be proved that if i ∈ ?, then there exists a monounary algebra A such that specA skips i consecutive (consecutive eleven, consecutive odd, respectively) numbers. Finally, for some types of finite monounary algebras (binary and at least binary trees) A, their spectrum is shown to be complete.     

Multiple cross-intersecting families of signed sets

A k-signed r-set on[n]={1,…,n} is an ordered pair (A,f), where A is an r-subset of [n] and f is a function from A to [k]. Families A₁,…,Ap are said to be cross-intersecting if any set in any family Ai intersects any set in any other family Aj. Hilton proved a sharp bound for the sum of sizes of cross-intersecting families of r-subsets of [n]. Our aim is to generalise Hilton's bound to one for families of k-signed r-sets on [n]. The main tool developed is an extension of Katona's cyclic permutation argument.     

Random A-permutations: Convergence to a Poisson process

Suppose that S _n is the permutation group of degree n, A is a subset of the set of natural numbers ?, and T _n(A) is the set of all permutations from S _n whose cycle lengths belong to the set A. Permutations from T _n are usually called A-permutations. We consider a wide class of sets A of positive asymptotic density. Suppose that ζ _mn is the number of cycles of length m of a random permutation uniformly distributed on T _n. It is shown in this paper that the finite-dimensional distributions of the random process {tz _mn, m ε A} weakly converge as n → ∞ to the finite-dimensional distributions of a Poisson process on A.     

Properties of P-sets and trapped compact convex sets

New properties of P-sets, which constitute a large class of convex compact sets in ? ⁿ that contains all convex polyhedra and strictly convex compact sets, are obtained. It is shown that the intersection of a P-set with an affine subspace is continuous in the Hausdorff metric. In this theorem, no assumption of interior nonemptiness is made, unlike in other known intersection continuity theorems for set-valued maps. It is also shown that if the graph of a set-valued map is a P-set, then this map is continuous on its entire effective set rather than only on the interior of this set. Properties of the so-called trapped sets are also studied; well-known Jung’s theorem on the existence of a minimal ball containing a given compact set in ? ⁿ is generalized. As is known, any compact set contains n + 1 (or fewer) points such that any translation by a nonzero vector takes at least one of them outside the minimal ball. This means that any compact set is trapped in the minimal ball. Compact sets trapped in any convex compact sets, rather than only in norm bodies, are considered. It is shown that, for any compact set A trapped in a P-set M ? ? ⁿ , there exists a set A ⁰ ? A trapped in M and containing at most 2n elements. An example of a convex compact set M ? ? ⁿ for which such a finite set A ⁰ ? A does not exist is given.     

Approximation of coincidence points and common fixed points of a collection of mappings of metric spaces

On a complete metric space X, we solve the problem of constructing an algorithm (in general, nonunique) of successive approximations from any point in space to a given closed subsetA. We give an estimate of the distance from an arbitrary initial point to the corresponding limit points. We consider three versions of the subset A: (1) A is the complete preimage of a closed subspace H under a mapping from X into the metric space Y; (2) A is the set of coincidence points of n (n > 1) mappings from X into Y; (3) A is the set of common fixed points of n mappings of X into itself (n = 1, 2, …). The problems under consideration are stated conveniently in terms of a multicascade, i.e., of a generalized discrete dynamical system with phase space X, translation semigroup equal to the additive semigroup of nonnegative integers, and the limit set A. In particular, in case (2) for n = 2, we obtain a generalization of Arutyunov’s theorem on the coincidences of two mappings. In case (3) for n = 1, we obtain a generalization of the contraction mapping principle.     

On the number of A-mappings

Suppose that $ \mathfrak{S} $ _n is the semigroup of mappings of the set of n elements into itself, A is a fixed subset of the set of natural numbers ?, and V _n (A) is the set of mappings from $ \mathfrak{S} $ _n whose contours are of sizes belonging to A. Mappings from V _n (A) are usually called A-mappings. Consider a random mapping σ _n , uniformly distributed on V n(A). Suppose that ν _n is the number of components and λ _n is the number of cyclic points of the random mapping σ _n . In this paper, for a particular class of sets A, we obtain the asymptotics of the number of elements of the set V _n (A) and prove limit theorems for the random variables ν _n and λ _n as n → ∞.     

A subgroup involvement of the Fibonacci length

For a non-Abelian 2-generated finite group G=〈a,b〉, the Fibonacci length of G with respect to A={a,b}, denoted by LEN _A (G), is defined to be the period of the sequence x ₁=a,x ₂=b,x ₃=x ₁ x ₂,…,x _n+1=x _n?1 x _n ,… of the elements of G. For a finite cyclic group C _n =〈a〉, LEN _A (C _n ) is defined in a similar way where A={1,a} and it is known that LEN _A (C _n )=k(n), the well-known Wall number of n. Over all of the interesting numerical results on the Fibonacci length of finite groups which have been obtained by many authors since 1990, an intrinsic property has been studied in this paper. Indeed, by studying the family of minimal non-Abelian p-groups it will be shown that for every group G of this family, there exists a suitable generating set A′ for the derived subgroup G′ such that LEN _A′(G′)|LEN _A (G) where, A is the original generating set of G.     

On the extendability of Bi-Cayley graphs of finite abelian groups

Let G be a finite group and A a nonempty subset (possibly containing the identity element) of G. The Bi-Cayley graph X=BC(G,A) of G with respect to A is defined as the bipartite graph with vertex set G×{0,1} and edge set {{(g,0),(sg,1)}∣g∈G,s∈A}. A graph Γ admitting a perfect matching is called n-extendable if ∣V(Γ)∣≥2n+2 and every matching of size n in Γ can be extended to a perfect matching of Γ. In this paper, the extendability of Bi-Cayley graphs of finite abelian groups is explored. In particular, 2-extendable and 3-extendable Bi-Cayley graphs of finite abelian groups are characterized.     

Polynomials with respect to a general basis. I. Theory

As n × n Hessenberg matrix A is defined whose characteristic polynomial is relative to an arbitrary basis. This generalizes the companion, colleague, and comrade matrices when the bases are, respectively, power, Chebyshev, and orthogonal, so the term “confederate” matrix is suggested. Some properties of A are derived, including an algorithm for computing powers of A. A scheme is given for inverting the transformation matrix between the arbitrary and power bases. A Vandermonde-type matrix associated with A and a block confederate matrix are defined.     

Preservers of eigenvalue inclusion sets of matrix products

For a square matrix A, let S(A) be an eigenvalue inclusion set such as the Gershgorin region, the union of Cassini ovals, and the Ostrowski’s set. Characterization is obtained for maps Φ on n×n matrices satisfying S(Φ(A)Φ(B))=S(AB) for all matrices A and B.     

Topologically inseparable functions II: Infinitary case

Given a set A and a function A: A → A, we study the set of all functions g: A → A that are continuous for all topologies for which f continuous. We prove that in a sense to be made precise in the text, for any essentially infinitary function f, any non-constant such g equals f ⁿ , for some n∈ ?. We also prove a similar result for the clone of n-ary functions from A ⁿ → A.     

A note on asymptotic values of quasiregular maps

In this note we give an example of a quasiregular map in ? ⁿ (n ≥ 3) of order of growth n ? 1 and whose set of asymptotic values is A ∪ {∞} for a given Suslin analytic set A ? ? ⁿ . Our example is a modification of Drasin’s construction in [4] of a quasiregular map with order of growth n ? 1 and set of asymptotic values ? ⁿ ∪ {∞}.     

Partitions of natural numbers with the same representation functions

For a set A of nonnegative integers the representation functions R₂(A,n), R₃(A,n) are defined as the number of solutions of the equation n=a+a^′,a,a^′∈A with a<a^′, a?a^′, respectively. Let D(0)=0 and let D(a) denote the number of ones in the binary representation of a. Let A₀ be the set of all nonnegative integers a with even D(a) and A₁ be the set of all nonnegative integers a with odd D(a). In this paper we show that (a) if R₂(A,n)=R₂(N?A,n) for all n?2N−1, then R₂(A,n)=R₂(N?A,n)?1 for all n?12N²−10N−2 except for A=A₀ or A=A₁; (b) if R₃(A,n)=R₃(N?A,n) for all n?2N−1, then R₃(A,n)=R₃(N?A,n)?1 for all n?12N²+2N. Several problems are posed in this paper.     

Invariant functions of the elements of the powers of a square matrix

A square matrix A is raised to any real power n, negative or fractional values being permitted when Aⁿ can be defined; C is a matrix that commute with A. Linear identities existing between the elements of Aⁿ or C are investigated, in such a way that the number of elements in each identity is minimized in general. Both this number and the method of investigation depend on the Jordan canonical form of A, but if A has a special property, some of these identities and their method of derivation are independent of the Jordan canonical form.     

Permutation groups generated by cycles of fixed length

Combinatorial conditions on a set of cycles of fixed degree inS _n are studied, so that they generateA _n orS _n . It is shown thatA _n orS _n is so generated if and only if a graph associated with the set of cycles is connected, provided two of the cycles satisfy certain, not too restrictive, criteria. As a corollary, the minimum number of cycles of degreem ≧ 2 that generateA _n or S_n is determined.     

The Lyapunov order for real matrices

The real Lyapunov order in the set of real n×n matrices is a relation defined as follows: A?B if, for every real symmetric matrix S, SB+B^tS is positive semidefinite whenever SA+A^tS is positive semidefinite. We describe the main properties of the Lyapunov order in terms of linear systems theory, Nevenlinna-Pick interpolation and convexity.     

On pointwise approximation of arbitrary functions by countable families of continuous functions

The following problem is considered. Given a real-valued function f defined on a topological space X, when can one find a countable familyf _n :n∈ω of continuous real-valued functions on X that approximates f on finite subsets of X? That is, for any finite set F?X and every real number ε>0 one can choosen∈ω such that ∥f(x)?f_n(x)∥<ε for everyx∈F. It will be shown that the problem has a positive solution if and only if X splits. A space X is said to split if, for any A?X, there exists a continuous mapf _A:X→R ^ω such that A=f _A ^?1 (A). Splitting spaces will be studied systematically.