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In the present paper we introduce a notion of $G-$decompositions of matrices. Main result of the paper is that a symmetric matrix $A_m$ has a $G-$decomposition in the class of stochastic (resp. substochastic) matrices if and only if $A_m$ belongs to the set ${\mathbf{U}}^m$ (resp. ${\mathbf{U}}_m$). To prove the main result, we study extremal points and geometrical structures of the sets ${\mathbf{U}}^m$, ${\mathbf{U}}_m$. Note that such kind of investigations enables to study Birkhoff's problem for quadratic $G-$doubly stochastic operators., 23 pages

As one of the serial papers on suborbits of point stabilizers in classical groups on the last subconstituent of dual polar graphs, the corresponding problem for orthogonal dual polar graphs over a finite field of odd characteristic is discussed in this paper. We determine all the suborbits of a point-stabilizer in the orthogonal group on the last subconstituent, and calculate the length of each suborbit. Moreover, we discuss the quasi-strongly regular graphs and the association schemes based on the last subconstituent, respectively.

For t a positive integer, the t-term rank of a (0,1)-matrix A is defined to be the largest number of 1s in A with at most one 1 in each column and at most t 1s in each row. Thus the 1-term rank is the ordinary term rank. We generalize some basic results for the term rank to the t-term rank, including a formula for the maximum term rank over a nonempty class of (0,1)-matrices with the the same row sum and column sum vectors. We also show the surprising result that in such a class there exists a matrix which realizes all of the maximum terms ranks between 1 and t., 18 pages

In this paper we introduce a class of regular bipartite graphs whose biadjacency matrices are circulant matrices and we describe some of their properties. Notably, we compute upper and lower bounds for the zero forcing number for such a graph based only on the parameters that describe its biadjacency matrix. The main results of the paper characterize the bipartite circulant graphs that achieve equality in the lower bound., Comment: 22 pages, 13 figures

This is a survey of the recent progress and open questions on the structure of the sets of 0-1 and non-negative integer matrices with prescribed row and column sums. We discuss cardinality estimates, the structure of a random matrix from the set, discrete versions of the Brunn-Minkowski inequality and the statistical dependence between row and column sums., Comment: 30 pages

In this paper, we classify the connected non-bipartite integral graphs with spectral radius three., Comment: 18 pages, 5 figures, 2 tables

In 1970s, Gutman introduced the concept of the energy $\En(G)$ for a simple graph $G$, which is defined as the sum of the absolute values of the eigenvalues of $G$. This graph invariant has attracted much attention, and many lower and upper bounds have been established for some classes of graphs among which bipartite graphs are of particular interest. But there are only a few graphs attaining the equalities of those bounds. We however obtain an exact estimate of the energy for almost all graphs by Wigner's semi-circle law, which generalizes a result of Nikiforov. We further investigate the energy of random multipartite graphs by considering a generalization of Wigner matrix, and obtain some estimates of the energy for random multipartite graphs., 16 pages

In this paper we show that certain almost distance-regular graphs, the so-called $h$-punctually walk-regular graphs, can be characterized through the cospectrality of their perturbed graphs. A graph $G$ with diameter $D$ is called $h$-punctually walk-regular, for a given $h\le D$, if the number of paths of length $\ell$ between a pair of vertices $u,v$ at distance $h$ depends only on $\ell$. The graph perturbations considered here are deleting a vertex, adding a loop, adding a pendant edge, adding/removing an edge, amalgamating vertices, and adding a bridging vertex. We show that for walk-regular graphs some of these operations are equivalent, in the sense that one perturbation produces cospectral graphs if and only if the others do. Our study is based on the theory of graph perturbations developed by Cvetkovi\'c, Godsil, McKay, Rowlinson, Schwenk, and others. As a consequence, some new characterizations of distance-regular graphs are obtained.

Let $F$ denote a field and let $V$ denote a vector space over $F$ with finite positive dimension. We consider a pair of linear transformations $A:V \to V$ and $A^*:V \to V$ that satisfy the following conditions: (i) each of $A,A^*$ is diagonalizable; (ii) there exists an ordering $\lbrace V_i\rbrace_{i=0}^d$ of the eigenspaces of $A$ such that $A^* V_i \subseteq V_{i-1} + V_{i} + V_{i+1}$ for $0 \leq i \leq d$, where $V_{-1}=0$ and $V_{d+1}=0$; (iii) there exists an ordering $\lbrace V^*_i\rbrace_{i=0}^\delta$ of the eigenspaces of $A^*$ such that $A V^*_i \subseteq V^*_{i-1} + V^*_{i} + V^*_{i+1}$ for $0 \leq i \leq \delta$, where $V^*_{-1}=0$ and $V^*_{\delta+1}=0$; (iv) there is no subspace $W$ of $V$ such that $AW \subseteq W$, $A^* W \subseteq W$, $W \neq 0$, $W \neq V$. We call such a pair a {\it tridiagonal pair} on $V$. It is known that $d=\delta$ and for $ 0 \leq i \leq d$ the dimensions of $V_i,V_{d-i},V^*_i, V^*_{d-i}$ coincide. The pair $A,A^*$ is called {\it sharp} whenever ${\rm dim} V_0=1$. It is known that if $F$ is algebraically closed then $A,A^*$ is sharp. In this paper we classify up to isomorphism the sharp tridiagonal pairs. As a corollary, we classify up to isomorphism the tridiagonal pairs over an algebraically closed field. We obtain these classifications by proving the $\mu$-conjecture., Comment: 36 pages

金沢大学理工研究域数物科学系, We introduce the notion of a mock tridiagonal system. This is a generalization of a tridiagonal system in which the irreducibility assumption is replaced by a certain nonvanishing condition. We show how mock tridiagonal systems can be used to construct tridiagonal systems that meet certain specifications. This paper is part of our ongoing project to classify the tridiagonal systems up to isomorphism. © 2011 Elsevier Inc. All rights reserved.

We introduce a new invariant, the coronal of a graph, and use it to compute the spectrum of the corona $G\circ H$ of two graphs $G$ and $H$. In particular, we show that this spectrum is completely determined by the spectra of $G$ and $H$ and the coronal of $H$. Previous work has computed the spectrum of a corona only in the case that $H$ is regular. We then explicitly compute the coronals for several families of graphs, including regular graphs, complete $n$-partite graphs, and paths. Finally, we use the corona construction to generate many infinite families of pairs of cospectral graphs., 9 pages

Let A be an association scheme on q\geq 3 vertices. We show that the Bose-Mesner algebra of the generalized Hamming scheme H(n,A), for n\geq 2, is not the Nomura algebra of a type II matrix. This result gives examples of formally self-dual Bose-Mesner algebras that are not the Nomura algebras of type II matrices., 15 pages, minor revision

In this paper we characterize the nonnegative irreducible tridiagonal matrices and their permutations, using certain entries in their primitive idempotents. Our main result is summarized as follows. Let $d$ denote a nonnegative integer. Let $A$ denote a matrix in $\matR$ and let $\{\th_i\}_{i=0}^d$ denote the roots of the characteristic polynomial of $A$. We say $A$ is multiplicity-free whenever these roots are mutually distinct and contained in $\R$. In this case $E_i$ will denote the primitive idempotent of $A$ associated with $\th_i$ $(0 \leq i \leq d)$. We say $A$ is symmetrizable whenever there exists an invertible diagonal matrix $\Delta \in \matR$ such that $\Delta A \Delta^{-1}$ is symmetric. Let $\Gamma(A)$ denote the directed graph with vertex set $\{0,1,...,d\}$, where $i \rightarrow j$ whenever $i \neq j$ and $A_{ij} \neq 0$. Theorem: Assume that each entry of $A$ is nonnegative. Then the following are equivalent for $0 \leq s,t \leq d$: (i) The graph $\Gamma(A)$ is a bidirected path with endpoints $s$, $t$: (ii) The matrix $A$ is symmetrizable and multiplicity-free. Moreover the $(s,t)$-entry of $E_i$ times $(\th_i-\th_0)...(\th_i-\th_{i-1})(\th_i-\th_{i+1})...(\th_i-\th_d)$ is independent of $i$ for $0 \leq i \leq d$, and this common value is nonzero. Recently Kurihara and Nozaki obtained a theorem that characterizes the $Q$-polynomial property for symmetric association schemes. We view the above result as a linear algebraic generalization of their theorem., Comment: 15 pages

The zero forcing number Z(G), which is the minimum number of vertices in a zero forcing set of a graph G, is used to study the maximum nullity / minimum rank of the family of symmetric matrices described by G. It is shown that for a connected graph of order at least two, no vertex is in every zero forcing set. The positive semidefinite zero forcing number Z_+(G) is introduced, and shown to be equal to |G|-OS(G), where OS(G) is the recently defined ordered set number that is a lower bound for minimum positive semidefinite rank. The positive semidefinite zero forcing number is applied to the computation of positive semidefinite minimum rank of certain graphs. An example of a graph for which the real positive symmetric semidefinite minimum rank is greater than the complex Hermitian positive semidefinite minimum rank is presented., Comment: 14 pages, 2 figures. To appear in Linear Algebra and its Applications.

Taking the d-th distance power of a graph, one adds edges between all pairs of vertices of that graph whose distance is at most d. It is shown that only the numbers -3, -2, -1, 0, 1, 2d can be integer eigenvalues of a circuit distance power. Moreover, their respective multiplicities are determined and explicit constructions for corresponding eigenspace bases containing only vectors with entries -1, 0, 1 are given., 14 pages

Let $K$ denote a field and let $V$ denote a vector space over $K$ with finite positive dimension. We consider a pair of linear transformations $A:V \to V$ and $A^*:V \to V$ that satisfy the following conditions: (i) each of $A,A^*$ is diagonalizable; (ii) there exists an ordering $\lbrace V_i\rbrace_{i=0}^d$ of the eigenspaces of $A$ such that $A^* V_i \subseteq V_{i-1} + V_{i} + V_{i+1}$ for $0 \leq i \leq d$, where $V_{-1}=0$ and $V_{d+1}=0$; (iii) there exists an ordering $\lbrace V^*_i\rbrace_{i=0}^\delta$ of the eigenspaces of $A^*$ such that $A V^*_i \subseteq V^*_{i-1} + V^*_{i} + V^*_{i+1}$ for $0 \leq i \leq \delta$, where $V^*_{-1}=0$ and $V^*_{\delta+1}=0$; (iv) there is no subspace $W$ of $V$ such that $AW \subseteq W$, $A^* W \subseteq W$, $W \neq 0$, $W \neq V$. We call such a pair a {\it tridiagonal pair} on $V$. It is known that $d=\delta$ and for $0 \leq i \leq d$ the dimensions of $V_i$, $V^*_i$, $V_{d-i}$, $V^*_{d-i}$ coincide; we denote this common dimension by $\rho_i$. In this paper we prove that $\rho_i \leq \rho_0 \binom{d}{i}$ for $0 \leq i \leq d$. It is already known that $\rho_0=1$ if $\K$ is algebraically closed., Comment: 30 pages

We generalize the results from [X.-D. Zhang, X.-P. Lv, Y.-H. Chen, \textit{Ordering trees by the Laplacian coefficients}, Linear Algebra Appl. (2009), doi:10.1016/j.laa.2009.04.018] on the partial ordering of trees with given diameter. For two $n$-vertex trees $T_1$ and $T_2$, if $c_k (T_1) \leqslant c_k (T_2)$ holds for all Laplacian coefficients $c_k$, $k = 0, 1, ..., n$, we say that $T_1$ is dominated by $T_2$ and write $T_1 \preceq_c T_2$. We proved that among $n$ vertex trees with fixed diameter $d$, the caterpillar $C_{n, d}$ has minimal Laplacian coefficients $c_k$, $k = 0, 1,..., n$. The number of incomparable pairs of trees on $\leqslant 18$ vertices is presented, as well as infinite families of examples for two other partial orderings of trees, recently proposed by Mohar. For every integer $n$, we construct a chain $\{T_i\}_{i = 0}^m$ of $n$-vertex trees of length $\frac{n^2}{4}$, such that $T_0 \cong S_n$, $T_m \cong P_n$ and $T_i \preceq_c T_{i + 1}$ for all $i = 0, 1,..., m - 1$. In addition, the characterization of the partial ordering of starlike trees is established by the majorization inequalities of the pendent path lengths. We determine the relations among the extremal trees with fixed maximum degree, and with perfect matching and further support the Laplacian coefficients as a measure of branching., Comment: 14 pages, 6 figures

Our main result is a sharp bound for the number of vertices in a minimal forbidden subgraph for the graphs having minimum rank at most 3 over the finite field of order 2. We also list all 62 such minimal forbidden subgraphs. We conclude by exploring how some of these results over the finite field of order 2 extend to arbitrary fields and demonstrate that at least one third of the 62 are minimal forbidden subgraphs over an arbitrary field for the class of graphs having minimum rank at most 3 in that field., Comment: 40 pages, added Sage program and improvements from referee process

In this work a composition-decomposition technique is presented that correlates tree eigenvectors with certain eigenvectors of an associated so-called skeleton forest. In particular, the matching properties of a skeleton determine the multiplicity of the corresponding tree eigenvalue. As an application a characterization of trees that admit eigenspace bases with entries only from the set {0, 1,-1} is presented. Moreover, a result due to Nylen concerned with partitioning eigenvectors of tree pattern matrices is generalized., Comment: 25 pages

The energy of a graph G, denoted by E(G), is defined as the sum of the absolute values of all eigenvalues of G. It is proved that E(G)>= 2(n-��(\bar{G}))>= 2(ch(G)-1) for every graph G of order n, and that E(G)>= 2ch(G) for all graphs G except for those in a few specified families, where \bar{G}, ��(G), and ch(G) are the complement, the chromatic number, and the choice number of G, respectively., to appear in Linear Algebra and its Applications

Let $K$ denote a field and let $V$ denote a vector space over $K$ with finite positive dimension. We consider a pair of linear transformations $A:V \to V$ and $A^*:V \to V$ that satisfy the following conditions: (i) each of $A,A^*$ is diagonalizable; (ii) there exists an ordering $\{V_i\}_{i=0}^d$ of the eigenspaces of $A$ such that $A^* V_i \subseteq V_{i-1} + V_i + V_{i+1}$ for $0 \leq i \leq d$, where $V_{-1}=0$ and $V_{d+1}=0$; (iii) there exists an ordering $\{V^*_i\}_{i=0}^\delta$ of the eigenspaces of $A^*$ such that $A V^*_i \subseteq V^*_{i-1} + V^*_i + V^*_{i+1}$ for $0 \leq i \leq \delta$, where $V^*_{-1}=0$ and $V^*_{\delta+1}=0$; (iv)there is no subspace $W$ of $V$ such that $AW \subseteq W$, $A^* W \subseteq W$, $W \neq 0$, $W \neq V$. We call such a pair a tridiagonal pair on $V$. It is known that $d=\delta$ and for $0 \leq i \leq d$ the dimensions of $V_i, V_{d-i}, V^*_i, V^*_{d-i}$ coincide. In this paper we show that the following (i)--(iv) hold provided that $K$ is algebraically closed: (i) Each of $V_0$, $V^*_0$, $V_d$, $V^*_d$ has dimension 1. (ii) There exists a nondegenerate symmetric bilinear form $(,)$ on $V$ such that $(Au,v)=(u,Av)$ and $(A^*u,v)=(u,A^*v)$ for all $u,v \in V$. (iii) There exists a unique anti-automorphism of $End(V)$ that fixes each of $A,A^*$. (iv) The pair $A,A^*$ is determined up to isomorphism by the data $(\{\th_i\}_{i=0}^d; \{\th^*_i\}_{i=0}^d; \{\zeta_i\}_{i=0}^d)$, where $\th_i$ (resp. $\th^*_i$) is the eigenvalue of $A$ (resp. $A^*$) on $V_i$ (resp. $V^*_i$), and $\{\zeta_i\}_{i=0}^d$ is the split sequence of $A,A^*$ corresponding to $\{\th_i\}_{i=0}^d$ and $\{\th^*_i\}_{i=0}^d$., Comment: 18 pages

Let $K$ denote a field and let $V$ denote a vector space over $K$ with finite positive dimension. Let $End(V)$ denote the $K$-algebra consisting of all $K$-linear transformations from $V$ to $V$. We consider a pair $A,A^* \in End(V)$ that satisfy (i)--(iv) below: (i) Each of $A,A^*$ is diagonalizable. (ii) There exists an ordering $\{V_i\}_{i=0}^d$ of the eigenspaces of $A$ such that $A^* V_i \subseteq V_{i-1} + V_{i} + V_{i+1}$ for $0 \leq i \leq d$, where $V_{-1}=0$ and $V_{d+1}=0$. (iii) There exists an ordering $\{V^*_i\}_{i=0}^\delta$ of the eigenspaces of $A^*$ such that $A V^*_i \subseteq V^*_{i-1} + V^*_{i} + V^*_{i+1}$ for $0 \leq i \leq \delta$, where $V^*_{-1}=0$ and $V^*_{\delta+1}=0$. (iv) There is no subspace $W$ of $V$ such that $AW \subseteq W$, $A^* W \subseteq W$, $W \neq 0$, $W \neq V$. We call such a pair a {\em tridiagonal pair} on $V$. Let $E^*_0$ denote the element of $End(V)$ such that $(E^*_0-I)V^*_0=0$ and $E^*_0V^*_i=0$ for $1 \leq i \leq d$. Let $D$ (resp. $D^*$) denote the $K$-subalgebra of $End(V)$ generated by $A$ (resp. $A^*$). In this paper we prove that the span of $E^*_0 D D^*DE^*_0$ equals the span of $E^*_0D E^*_0DE^*_0$, and that the elements of $E^*_0 D E^*_0$ mutually commute. We relate these results to some conjectures of Tatsuro Ito and the second author that are expected to play a role in the classification of tridiagonal pairs., Comment: 18 pages

We consider a class of graphs subject to certain restrictions, including the finiteness of diameters. Any surjective mapping φ : Γ → Γ ′ between graphs from this class is shown to be an isomorphism provided that the following holds: Any two points of Γ are at a distance equal to the diameter of Γ if, and only if, their images are at a distance equal to the diameter of Γ ′ . This result is then applied to the graphs arising from the adjacency relations of spaces of rectangular matrices, spaces of Hermitian matrices, and Grassmann spaces (projective spaces of rectangular matrices).

Answering some questions of Gutman, we show that, except for four specific trees, every connected graph G of order n, with no cycle of order 4 and with maximum degree at most 3, has energy greater that its order. Here, the energy of a graph is the sum of the moduli of its eigenvalues. We give more general theorems and state two conjectures., Some typos corrected

The spectrum of the normalized graph Laplacian yields a very comprehensive set of invariants of a graph. In order to understand the information contained in those invariants better, we systematically investigate the behavior of this spectrum under local and global operations like motif doubling, graph joining or splitting. The eigenvalue 1 plays a particular role, and we therefore emphasize those constructions that change its multiplicity in a controlled manner, like the iterated duplication of nodes., 9 pages, no figures

Write μ ( A ) = μ 1 ( A ) ⩾ ⋯ ⩾ μ min ( A ) for the eigenvalues of a Hermitian matrix A. Our main result is: Let A be a Hermitian matrix partitioned into r × r blocks so that all diagonal blocks are zero. Then for every real diagonal matrix B of the same size as A μ ( B - A ) ⩾ μ B + 1 r - 1 A . Let G be a nonempty graph, χ ( G ) be its chromatic number, A be its adjacency matrix, and L be its Laplacian. The above inequality implies the well-known result of Hoffman χ ( G ) ⩾ 1 + μ ( A ) - μ min ( A ) , and also, χ ( G ) ⩾ 1 + μ ( A ) μ ( L ) - μ ( A ) . Equality holds in the latter inequality if and only if every two color classes of G induce a ∣ μ min ( A ) ∣ -regular subgraph.

We determine the extreme points and facets of the convex hull of all dual degree partitions of simple graphs on $n$ vertices., 5 pages, 1 figure

We present sharp inequalities relating the number of vertices, edges, and triangles of a graph to the smallest eigenvalue of its adjacency matrix and the largest eigenvalue of its Laplacian., Comment: Some calculation errors in the first version are corrected

We improve recent results relating graph eigenvalues to other graph parameters like girth, domination number, and minimum degree., Lihua Feng drew my attention to some shortcomings in the first version of the paper. General tightening and corrections

Let G be a graph with n vertices and m edges and let μ(G) = μ1(G) ⩾ ⋯ ⩾ μn(G) be the eigenvalues of its adjacency matrix. Set s ( G ) = ∑ u ∈ V ( G ) ∣ d ( u ) - 2 m / n ∣ . We prove that s 2 ( G ) 2 n 2 2 m ⩽ μ ( G ) - 2 m n ⩽ s ( G ) . In addition we derive similar inequalities for bipartite G. We also prove that the inequality μ k ( G ) + μ n - k + 2 ( G ¯ ) ⩾ - 1 - 2 2 s ( G ) holds for every k = 2, … , n. Finally we prove that for every graph G of order n, μ n ( G ) + μ n ( G ¯ ) ⩽ - 1 - s 2 ( G ) 2 n 3 . We show that these inequalities are tight up to a constant factor.

Let $K$ denote a field, and let $V$ denote a vector space over $K$ with finite positive dimension. We consider a pair of linear transformations $A:V \to V$ and $A^*:V \to V$ that satisfy (i), (ii) below: (i) There exists a basis for $V$ with respect to which the matrix representing $A$ is irreducible tridiagonal and the matrix representing $A^*$ is diagonal. (ii) There exists a basis for $V$ with respect to which the matrix representing $A^*$ is irreducible tridiagonal and the matrix representing $A$ is diagonal. We call such a pair a {\em Leonard pair} on $V$. In the literature on Leonard pairs there exist two parameter sequences called the first split sequence and the second split sequence. We display some attractive formulae for the first and second split sequence that involve the trace function., 13 pages

We consider the matrix ${\frak Z}_P=Z_P+Z_P^t$, where the entries of $Z_P$ are the values of the zeta function of the finite poset $P$. We give a combinatorial interpretation of the determinant of ${\frak Z}_P$ and establish a recursive formula for this determinant in the case in which $P$ is a boolean algebra., Comment: 14 pages, AMS-TeX

The rank of a skew partition $\lambda/\mu$, denoted $rank(\lambda/\mu)$, is the smallest number $r$ such that $\lambda/\mu$ is a disjoint union of $r$ border strips. Let $s_{\lambda/\mu}(1^t)$ denote the skew Schur function $s_{\lambda/\mu}$ evaluated at $x_1=...=x_t=1, x_i=0$ for $i>t$. The zrank of $\lambda/\mu$, denoted $zrank(\lambda/\mu)$, is the exponent of the largest power of $t$ dividing $s_{\lambda/\mu}(1^t)$. Stanley conjectured that $rank(\lambda/\mu)=zrank(\lambda/\mu)$. We show the equivalence between the validity of the zrank conjecture and the nonsingularity of restricted Cauchy matrices. In support of Stanley's conjecture we give affirmative answers for some special cases., Comment: 17pages, 6 figures

Let (A,B) and (C,D) denote Leonard pairs on V. We say these pairs are adjacent whenever each basis for V which is standard for (A,B) (resp. (C,D)) is split for (C,D) (resp. (A,B)). Our main results are as follows: Theorem 1. There exists at most 3 mutually adjacent Leonard pairs on V provided the dimension of V is at least 2. Theorem 2. Let (A,B), (C,D), and (E,F) denote three mutually adjacent Leonard pairs on V. There for each of these pairs, the eigenvalue sequence and dual eigenvalue sequence are in arithmetic progression. Theorem 3. Let (A,B) denote a Leonard pair on V whose eigenvalue sequence and dual eigenvalue sequence are in arithmetic progression. Then there exist Leonard pairs (C,D) and (E,F) on V such that (A,B), (C,D), and (E,F) are mutually adjacent., Comment: 19 pages. To be published in Linear Algebra and it Applications

Two-uniform frames and their use for the coding of vectors are the main subject of this paper. These frames are known to be optimal for handling up to two erasures, in the sense that they minimize the largest possible error when up to two frame coefficients are set to zero. Here, we consider various numerical measures for the reconstruction error associated with a frame when an arbitrary number of the frame coefficients of a vector are lost. We derive general error bounds for two-uniform frames when more than two erasures occur and apply these to concrete examples. We show that among the 227 known equivalence classes of two-uniform (36,15)-frames arising from Hadamard matrices, there are 5 that give smallest error bounds for up to 8 erasures., Comment: 28 pages LaTeX, with AMS macros; v.3: fixed Thm 3.6, added comment, Lemma 3.7 and Proposition 3.8, to appear in Lin. Alg. Appl

Let $G=(V(G),E(G))$ be a planar digraph embedded in the plane in which all inner faces are equilateral triangles (with three edges in each), and let the union $\Rscr$ of these faces forms a convex polygon. The question is: given a function $\sigma$ on the boundary edges of $G$, does there exist a concave function $f$ on $\Rscr$ which is affinely linear within each bounded face and satisfies $f(v)-f(u)=\sigma(e)$ for each boundary edge $e=(u,v)$? The functions $\sigma$ admitting such an $f$ form a polyhedral cone $C$, and when the region $\Rscr$ is a triangle, $C$ turns out to be exactly the cone of boundary data of honeycombs. Studing honeycombs in connection with a problem on spectra of triples of zero-sum Hermitian matrices, Knutson, Tao, and Woodward \cite{KTW} showed that $C$ is described by linear inequalities of Horn's type with respect to so-called {\em puzzles}, along with obvious linear constraints. The purpose of this paper is to give an alternative proof of that result, working in terms of discrete concave finctions, rather than honeycombs, and using only linear programming and combinatorial tools. Moreover, we extend the result to an arbitrary convex polygon $\Rscr$., Comment: 21 pages

Let $K$ denote a field and let $V$ denote a vector space over $K$ with finite positive dimension. We consider an ordered pair of linear transformations $A:V\to V$ and $A^*:V\to V$ that satisfy conditions (i), (ii) below. (i) There exists a basis for $V$ with respect to which the matrix representing $A$ is irreducible tridiagonal and the matrix representing $A^*$ is diagonal. (ii) There exists a basis for $V$ with respect to which the matrix representing $A$ is diagonal and the matrix representing $A^*$ is irreducible tridiagonal. We call such a pair a {\it Leonard pair} on $V$. We discuss a correspondence between Leonard pairs and a class of orthogonal polynomials consisting of the $q$-Racah polynomials and some related polynomials of the Askey scheme. For the polynomials in this class we obtain the 3-term recurrence, difference equation, Askey-Wilson duality, and orthogonality in a uniform manner using the corresponding Leonard pair., Comment: 44 pages. Revised version with organization adjusted

Extending earlier work of R. Donaghey and P. J. Cameron, we investigate some canonical "eigen-sequences" associated with transformations of integer sequences. Several known sequences appear in a new setting: for instance the sequences (such as 1, 3, 11, 49, 257, 1531, ...) studied by T. Tsuzuku, H. O. Foulkes and A. Kerber in connection with multiply transitive groups are eigen-sequences for the binomial transform. Many interesting new sequences also arise, such as 1, 1, 2, 26, 152, 1144, ..., which shifts one place left when transformed by the Stirling numbers of the second kind, and whose exponential generating function satisfies A'(x) = A(e^x -1) + 1., 18 pages, 2 figures; Dedicated to Professor J. J. Seidel

We explore the connections between the linear algebra of symmetric matrices over GF(2) and the circuit theory of 4-regular graphs. In particular, we show that the equivalence relation on simple graphs generated by local complementation can also be generated by an operation defined using inverse matrices., Comment: 22 pages, 5 figures. v2 includes two new results and several minor corrections. v3 includes minor corrections and updated references. v4 has 24 pages and 6 figures; it includes new examples and many modest corrections and revisions. Further revisions may be made before publication in Linear Algebra and its Applications

In this paper we investigate the class of invariant positive definite kernels on the free semigroup on N generators. We provide a combinatorial description of the positivity of the kernel in terms of Dyck paths and then we find a displacement equation that encodes the invariance property of the kernel., 19 pages, 5 figures

Let Γ denote a distance-regular graph with diameter D ⩾ 3 . Assume Γ has classical parameters ( D , b , α , β ) with b - 1 . Let X denote the vertex set of Γ and let A ∈ Mat X ( C ) denote the adjacency matrix of Γ . Fix x ∈ X and let A ∗ ∈ Mat X ( C ) denote the corresponding dual adjacency matrix. Let T denote the subalgebra of Mat X ( C ) generated by A , A ∗ . We call T the Terwilliger algebra of Γ with respect to x . We show that up to isomorphism there exist exactly two irreducible T -modules with endpoint 1; their dimensions are D and 2 D - 2 . For these T -modules we display a basis consisting of eigenvectors for A ∗ , and for each basis we give the action of A .

We show that the effect of principal pivot transform on the nullity values of the principal submatrices of a given (square) matrix is described by the symmetric difference operator (for sets). We consider its consequences for graphs, and in particular generalize the recursive relation of the interlace polynomial and simplify its proof., small revision of Section 8 w.r.t. v2, 14 pages, 6 figures

Using linear algebra methods we study certain algebraic properties of monomial rings and matroids. Let I be a monomial ideal in a polynomial ring over an arbitrary field. If the Rees cone of I is quasi-ideal, we express the normalization of the Rees algebra of I in terms of an Ehrhart ring. We introduce the basis Rees cone of a matroid (or a polymatroid) and study their facets. Some applications to Rees algebras are presented. It is shown that the basis monomial ideal of a matroid (or a polymatroid) is normal., 9 pages

Immanants are homogeneous polynomials of degree n in n 2 variables associated to the irreducible representations of the symmetric group S n of n elements. We describe immanants as trivial S n modules and show that any homogeneous polynomial of degree n on the space of n × n matrices preserved up to scalar by left and right action by diagonal matrices and conjugation by permutation matrices is a linear combination of immanants. Building on works of Duffner [5] and Purificacao [3] , we prove that for n ⩾ 6 the identity component of the stabilizer of any immanant (except determinant, permanent, and π = ( 4 , 1 , 1 , 1 ) ) is Δ ( S n ) ⋉ T ( GL n × GL n ) ⋉ Z 2 , where T ( GL n × GL n ) is the group consisting of pairs of n × n diagonal matrices with the product of determinants 1, acting by left and right matrix multiplication, Δ ( S n ) is the diagonal of S n × S n , acting by conjugation ( S n is the group of symmetric group) and Z 2 acts by sending a matrix to its transpose. Based on the work of Purificacao and Duffner [4] , we also prove that for n ⩾ 5 the stabilizer of the immanant of any non-symmetric partition (except determinant and permanent) is Δ ( S n ) ⋉ T ( GL n × GL n ) ⋉ Z 2 .

Let $K$ denote a field and let $V$ denote a vector space over $K$ with finite positive dimension. We consider a pair of linear transformations $A : V \to V$ and $A^* : V \to V$ that satisfy (i) and (ii) below: (i) There exists a basis for $V$ with respect to which the matrix representing $A$ is irreducible tridiagonal and the matrix representing $A^*$ is diagonal. (ii) There exists a basis for $V$ with respect to which the matrix representing $A^*$ is irreducible tridiagonal and the matrix representing $A$ is diagonal. We call such a pair a {\it Leonard pair} on $V$. Let $\cal X$ denote the set of linear transformations $X:V \to V$ such that the matrix representing $X$ with respect to the basis (i) is tridiagonal and the matrix representing $X$ with respect to the basis (ii) is tridiagonal. We show that $\cal X$ is spanned by $I$, $A$, $A^*$, $AA^*$, $A^*A$, and these elements form a basis for $\cal X$ provided the dimension of $V$ is at least 3., 12 pages

A product formula for the parity generating function of the number of 1’s in invertible matrices over Z 2 is given. The computation is based on algebraic tools such as the Bruhat decomposition. It is somewhat surprising that the number of such matrices with odd number of 1’s is greater than the number of those with even number of 1’s. The same technique can be used to obtain a parity generating function also for symplectic matrices over Z 2 . We present also a generating function for the sum of entries of matrices over an arbitrary finite field F q calculated in F q . The Mahonian distribution appears in these formulas.

For a distance-regular graph with second largest eigenvalue (resp. smallest eigenvalue) \mu1 (resp. \muD) we show that (\mu1+1)(\muD+1), 15 pages, this is submitted to Linear Algebra and Applications.

Suppose $C$ is a subset of non-zero vectors from the vector space $\mathbb{Z}_2^d$. The cubelike graph $X(C)$ has $\mathbb{Z}_2^d$ as its vertex set, and two elements of $\mathbb{Z}_2^d$ are adjacent if their difference is in $C$. If $M$ is the $d\times |C|$ matrix with the elements of $C$ as its columns, we call the row space of $M$ the code of $X$. We use this code to study perfect state transfer on cubelike graphs. Bernasconi et al have shown that perfect state transfer occurs on $X(C)$ at time $\pi/2$ if and only if the sum of the elements of $C$ is not zero. Here we consider what happens when this sum is zero. We prove that if perfect state transfer occurs on a cubelike graph, then it must take place at time $\tau=\pi/2D$, where $D$ is the greatest common divisor of the weights of the code words. We show that perfect state transfer occurs at time $\pi/4$ if and only if D=2 and the code is self-orthogonal., Comment: 10 pages, minor revisions

The energy of a graph is the sum of the moduli of the eigenvalues of its adjacency matrix. We study the energy of integral circulant graphs, also called gcd graphs, which can be characterized by their vertex count n and a set D of divisors of n in such a way that they have vertex set Zn and edge set {{a, b} : a, b in Zn; gcd(a - b, n) in D}. Using tools from convex optimization, we study the maximal energy among all integral circulant graphs of prime power order ps and varying divisor sets D. Our main result states that this maximal energy approximately lies between s(p - 1)p^(s-1) and twice this value. We construct suitable divisor sets for which the energy lies in this interval. We also characterize hyperenergetic integral circulant graphs of prime power order and exhibit an interesting topological property of their divisor sets., 25 pages

We study some specializations and extensions of the quantum version of the MacMahon Master Theorem derived by Garoufalidis, Le and Zeilberger. In particular, we obtain a (t,q)-analogue for the Cartier-Foata noncommutative version and a semi-strong (t,q)-analogue for the contextual algebra., Comment: 12 pages