19,862 research outputs found
Two combinatorial covering theorems
AbstractA theorem in convex bodies (in fact, measure theory) and a theorem about translates of sets of integers are generalized to coverings by subsets of a finite set. These theorems are then related to quasigroups and (0, 1)-matrices
A Combinatorial Analog of a Theorem of F.J.Dyson
Tucker's Lemma is a combinatorial analog of the Borsuk-Ulam theorem and the
case n=2 was proposed by Tucker in 1945. Numerous generalizations and
applications of the Lemma have appeared since then. In 2006 Meunier proved the
Lemma in its full generality in his Ph.D. thesis. There are generalizations and
extensions of the Borsuk-Ulam theorem that do not yet have combinatorial
analogs. In this note, we give a combinatorial analog of a result of Freeman J.
Dyson and show that our result is equivalent to Dyson's theorem. As with
Tucker's Lemma, we hope that this will lead to generalizations and applications
and ultimately a combinatorial analog of Yang's theorem of which both
Borsuk-Ulam and Dyson are special cases.Comment: Original version: 7 pages, 2 figures. Revised version: 12 pages, 4
figures, revised proofs. Final revised version: 9 pages, 2 figures, revised
proof
Covering of Subspaces by Subspaces
Lower and upper bounds on the size of a covering of subspaces in the
Grassmann graph \cG_q(n,r) by subspaces from the Grassmann graph
\cG_q(n,k), , are discussed. The problem is of interest from four
points of view: coding theory, combinatorial designs, -analogs, and
projective geometry. In particular we examine coverings based on lifted maximum
rank distance codes, combined with spreads and a recursive construction. New
constructions are given for with or . We discuss the density
for some of these coverings. Tables for the best known coverings, for and
, are presented. We present some questions concerning
possible constructions of new coverings of smaller size.Comment: arXiv admin note: text overlap with arXiv:0805.352
Residues and tame symbols on toroidal varieties
We introduce a new approach to the study of a system of algebraic equations
in the algebraic torus whose Newton polytopes have sufficiently general
relative positions. Our method is based on the theory of Parshin's residues and
tame symbols on toroidal varieties. It provides a uniform algebraic explanation
of the recent result of Khovanskii on the product of the roots of such systems
and the Gel'fond--Khovanskii result on the sum of the values of a Laurent
polynomial over the roots of such systems, and extends them to the case of an
algebraically closed field of arbitrary characteristic.Comment: 26 pages, minor changes, title changed, new introduction, references
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Problems on q-Analogs in Coding Theory
The interest in -analogs of codes and designs has been increased in the
last few years as a consequence of their new application in error-correction
for random network coding. There are many interesting theoretical, algebraic,
and combinatorial coding problems concerning these q-analogs which remained
unsolved. The first goal of this paper is to make a short summary of the large
amount of research which was done in the area mainly in the last few years and
to provide most of the relevant references. The second goal of this paper is to
present one hundred open questions and problems for future research, whose
solution will advance the knowledge in this area. The third goal of this paper
is to present and start some directions in solving some of these problems.Comment: arXiv admin note: text overlap with arXiv:0805.3528 by other author
Coxeter groups and random groups
For every dimension d, there is an infinite family of convex co-compact
reflection groups of isometries of hyperbolic d-space --- the superideal
(simplicial and cubical) reflection groups --- with the property that a random
group at any density less than a half (or in the few relators model) contains
quasiconvex subgroups commensurable with some member of the family, with
overwhelming probability.Comment: 18 pages, 14 figures; version 2 incorporates referee's correction
Generalizations of Tucker-Fan-Shashkin lemmas
Tucker and Ky Fan's lemma are combinatorial analogs of the Borsuk-Ulam
theorem (BUT). In 1996, Yu. A. Shashkin proved a version of Fan's lemma, which
is a combinatorial analog of the odd mapping theorem (OMT). We consider
generalizations of these lemmas for BUT-manifolds, i.e. for manifolds that
satisfy BUT. Proofs rely on a generalization of the OMT and on a lemma about
the doubling of manifolds with boundaries that are BUT-manifolds.Comment: 10 pages, 2 figure
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