60,628 research outputs found
Higher order matching polynomials and d-orthogonality
We show combinatorially that the higher-order matching polynomials of several
families of graphs are d-orthogonal polynomials. The matching polynomial of a
graph is a generating function for coverings of a graph by disjoint edges; the
higher-order matching polynomial corresponds to coverings by paths. Several
families of classical orthogonal polynomials -- the Chebyshev, Hermite, and
Laguerre polynomials -- can be interpreted as matching polynomials of paths,
cycles, complete graphs, and complete bipartite graphs. The notion of
d-orthogonality is a generalization of the usual idea of orthogonality for
polynomials and we use sign-reversing involutions to show that the higher-order
Chebyshev (first and second kinds), Hermite, and Laguerre polynomials are
d-orthogonal. We also investigate the moments and find generating functions of
those polynomials.Comment: 21 pages, many TikZ figures; v2: minor clarifications and addition
Nominal Unification of Higher Order Expressions with Recursive Let
A sound and complete algorithm for nominal unification of higher-order
expressions with a recursive let is described, and shown to run in
non-deterministic polynomial time. We also explore specializations like nominal
letrec-matching for plain expressions and for DAGs and determine the complexity
of corresponding unification problems.Comment: Pre-proceedings paper presented at the 26th International Symposium
on Logic-Based Program Synthesis and Transformation (LOPSTR 2016), Edinburgh,
Scotland UK, 6-8 September 2016 (arXiv:1608.02534
Computing Optimal Morse Matchings
Morse matchings capture the essential structural information of discrete
Morse functions. We show that computing optimal Morse matchings is NP-hard and
give an integer programming formulation for the problem. Then we present
polyhedral results for the corresponding polytope and report on computational
results
On Sparsification for Computing Treewidth
We investigate whether an n-vertex instance (G,k) of Treewidth, asking
whether the graph G has treewidth at most k, can efficiently be made sparse
without changing its answer. By giving a special form of OR-cross-composition,
we prove that this is unlikely: if there is an e > 0 and a polynomial-time
algorithm that reduces n-vertex Treewidth instances to equivalent instances, of
an arbitrary problem, with O(n^{2-e}) bits, then NP is in coNP/poly and the
polynomial hierarchy collapses to its third level.
Our sparsification lower bound has implications for structural
parameterizations of Treewidth: parameterizations by measures that do not
exceed the vertex count, cannot have kernels with O(k^{2-e}) bits for any e >
0, unless NP is in coNP/poly. Motivated by the question of determining the
optimal kernel size for Treewidth parameterized by vertex cover, we improve the
O(k^3)-vertex kernel from Bodlaender et al. (STACS 2011) to a kernel with
O(k^2) vertices. Our improved kernel is based on a novel form of
treewidth-invariant set. We use the q-expansion lemma of Fomin et al. (STACS
2011) to find such sets efficiently in graphs whose vertex count is
superquadratic in their vertex cover number.Comment: 21 pages. Full version of the extended abstract presented at IPEC
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