63 research outputs found
On the van der Waerden numbers w(2;3,t)
We present results and conjectures on the van der Waerden numbers w(2;3,t)
and on the new palindromic van der Waerden numbers pdw(2;3,t). We have computed
the new number w(2;3,19) = 349, and we provide lower bounds for 20 <= t <= 39,
where for t <= 30 we conjecture these lower bounds to be exact. The lower
bounds for 24 <= t <= 30 refute the conjecture that w(2;3,t) <= t^2, and we
present an improved conjecture. We also investigate regularities in the good
partitions (certificates) to better understand the lower bounds.
Motivated by such reglarities, we introduce *palindromic van der Waerden
numbers* pdw(k; t_0,...,t_{k-1}), defined as ordinary van der Waerden numbers
w(k; t_0,...,t_{k-1}), however only allowing palindromic solutions (good
partitions), defined as reading the same from both ends. Different from the
situation for ordinary van der Waerden numbers, these "numbers" need actually
to be pairs of numbers. We compute pdw(2;3,t) for 3 <= t <= 27, and we provide
lower bounds, which we conjecture to be exact, for t <= 35.
All computations are based on SAT solving, and we discuss the various
relations between SAT solving and Ramsey theory. Especially we introduce a
novel (open-source) SAT solver, the tawSolver, which performs best on the SAT
instances studied here, and which is actually the original DLL-solver, but with
an efficient implementation and a modern heuristic typical for look-ahead
solvers (applying the theory developed in the SAT handbook article of the
second author).Comment: Second version 25 pages, updates of numerical data, improved
formulations, and extended discussions on SAT. Third version 42 pages, with
SAT solver data (especially for new SAT solver) and improved representation.
Fourth version 47 pages, with updates and added explanation
Erdos-Ko-Rado theorems for simplicial complexes
A recent framework for generalizing the Erdos-Ko-Rado Theorem, due to
Holroyd, Spencer, and Talbot, defines the Erdos-Ko-Rado property for a graph in
terms of the graph's independent sets. Since the family of all independent sets
of a graph forms a simplicial complex, it is natural to further generalize the
Erdos-Ko-Rado property to an arbitrary simplicial complex. An advantage of
working in simplicial complexes is the availability of algebraic shifting, a
powerful shifting (compression) technique, which we use to verify a conjecture
of Holroyd and Talbot in the case of sequentially Cohen-Macaulay near-cones.Comment: 14 pages; v2 has minor changes; v3 has further minor changes for
publicatio
Bounds on sets with few distances
We derive a new estimate of the size of finite sets of points in metric
spaces with few distances. The following applications are considered:
(1) we improve the Ray-Chaudhuri--Wilson bound of the size of uniform
intersecting families of subsets;
(2) we refine the bound of Delsarte-Goethals-Seidel on the maximum size of
spherical sets with few distances;
(3) we prove a new bound on codes with few distances in the Hamming space,
improving an earlier result of Delsarte.
We also find the size of maximal binary codes and maximal constant-weight
codes of small length with 2 and 3 distances.Comment: 11 page
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