Sparse Partition Regularity
Department of Pure Mathematics and Mathematical Statistics, Centre for Mathematical Sciences Wilberforce Road, Cambridge, CB3 0WB, United Kingdom I.Leader{at}dpmms.cam.ac.uk, P.A.Russell{at}dpmms.cam.ac.uk
Received 3 June 2005. Revision received 25 October 2005.
Our aim in this paper is to prove Deuber's conjecture on sparse partition regularity, that for every m, p and c there exists a subset of the natural numbers whose (m,p,c)-sets have high girth and chromatic number. More precisely, we show that for any mp, c, k and g there is a subset S of the natural numbers that is sufficiently rich in (m,p,c)-sets that whenever S is k-coloured there is a monochromatic (m,p,c)-set, yet is so sparse that its (m,p,c)-sets do not form any cycles of length less than g.
Our main tools are some extensions of Ne
et
ilRödl amalgamation and a Ramsey theorem of Bergelson, Hindman and Leader. As a sideline, we obtain a Ramsey theorem for products of trees that may be of independent interest. 2000 Mathematics Subject Classification 05D10.