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Colourings of Uniform Hypergraphs with Large Girth and Applications

Published online by Cambridge University Press:  09 October 2017

ANDREY KUPAVSKII
Affiliation:
Laboratory of Advanced Combinatorics and Network Applications, Moscow Institute of Physics and Technology, Moscow Region, RussiaEcole Polytechnique Fédérale de Lausanne, Switzerland (e-mail: kupavskii@ya.ru)
DMITRY SHABANOV
Affiliation:
Laboratory of Advanced Combinatorics and Network Applications, Moscow Institute of Physics and Technology, 141700, Institutskiy per. 9, Dolgoprudny, Moscow Region, RussiaFaculty of Computer Science, National Research University Higher School of Economics (HSE), 101000, Myasnitskaya Str. 20, Moscow, Russia (e-mail: dm.shabanov.msu@gmail.com)
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Abstract

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This paper deals with a combinatorial problem concerning colourings of uniform hypergraphs with large girth. We prove that if H is an n-uniform non-r-colourable simple hypergraph then its maximum edge degree Δ(H) satisfies the inequality

$$\Delta(H)\geqslant c\cdot r^{n-1}\ffrac{n(\ln\ln n)^2}{\ln n}$$
for some absolute constant c > 0.

As an application of our probabilistic technique we establish a lower bound for the classical van der Waerden number W(n, r), the minimum natural N such that in an arbitrary colouring of the set of integers {1,. . .,N} with r colours there exists a monochromatic arithmetic progression of length n. We prove that

$$W(n,r)\geqslant c\cdot r^{n-1}\ffrac{(\ln\ln n)^2}{\ln n}.$$

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Paper
Copyright
Copyright © Cambridge University Press 2017