65,780 research outputs found
Partition and composition matrices
This paper introduces two matrix analogues for set partitions. A composition
matrix on a finite set X is an upper triangular matrix whose entries partition
X, and for which there are no rows or columns containing only empty sets. A
partition matrix is a composition matrix in which an order is placed on where
entries may appear relative to one-another. We show that partition matrices are
in one-to-one correspondence with inversion tables. Non-decreasing inversion
tables are shown to correspond to partition matrices with a row ordering
relation. Partition matrices which are s-diagonal are classified in terms of
inversion tables. Bidiagonal partition matrices are enumerated using the
transfer-matrix method and are equinumerous with permutations which are
sortable by two pop-stacks in parallel. We show that composition matrices on X
are in one-to-one correspondence with (2+2)-free posets on X. Also, composition
matrices whose rows satisfy a column-ordering relation are shown to be in
one-to-one correspondence with parking functions. Finally, we show that pairs
of ascent sequences and permutations are in one-to-one correspondence with
(2+2)-free posets whose elements are the cycles of a permutation, and use this
relation to give an expression for the number of (2+2)-free posets on
{1,...,n}.Comment: 14 page
Schur Polynomials and the Yang-Baxter equation
We show that within the six-vertex model there is a parametrized Yang-Baxter
equation with nonabelian parameter group GL(2)xGL(1) at the center of the
disordered regime. As an application we rederive deformations of the Weyl
character formule of Tokuyama and of Hamel and King.Comment: Revised introduction; slightly changed reference
Restricted -Stirling Numbers and their Combinatorial Applications
We study set partitions with distinguished elements and block sizes found
in an arbitrary index set . The enumeration of these -partitions
leads to the introduction of -Stirling numbers, an extremely
wide-ranging generalization of the classical Stirling numbers and the
-Stirling numbers. We also introduce the associated -Bell and
-factorial numbers. We study fundamental aspects of these numbers,
including recurrence relations and determinantal expressions. For with some
extra structure, we show that the inverse of the -Stirling matrix
encodes the M\"obius functions of two families of posets. Through several
examples, we demonstrate that for some the matrices and their inverses
involve the enumeration sequences of several combinatorial objects. Further, we
highlight how the -Stirling numbers naturally arise in the enumeration
of cliques and acyclic orientations of special graphs, underlining their
ubiquity and importance. Finally, we introduce related generalizations
of the poly-Bernoulli and poly-Cauchy numbers, uniting many past works on
generalized combinatorial sequences
Evaluating Matrix Functions by Resummations on Graphs: the Method of Path-Sums
We introduce the method of path-sums which is a tool for exactly evaluating a
function of a discrete matrix with possibly non-commuting entries, based on the
closed-form resummation of infinite families of terms in the corresponding
Taylor series. If the matrix is finite, our approach yields the exact result in
a finite number of steps. We achieve this by combining a mapping between matrix
powers and walks on a weighted directed graph with a universal graph-theoretic
result on the structure of such walks. We present path-sum expressions for a
matrix raised to a complex power, the matrix exponential, matrix inverse, and
matrix logarithm. We show that the quasideterminants of a matrix can be
naturally formulated in terms of a path-sum, and present examples of the
application of the path-sum method. We show that obtaining the inversion height
of a matrix inverse and of quasideterminants is an NP-complete problem.Comment: 23 pages, light version submitted to SIAM Journal on Matrix Analysis
and Applications (SIMAX). A separate paper with the graph theoretic results
is available at: arXiv:1202.5523v1. Results for matrices over division rings
will be published separately as wel
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