A new proof of the density Hales-Jewett theorem

The Hales-Jewett theorem asserts that for every r and every k there exists n such that every r-colouring of the n-dimensional grid 1,. . . , k n contains a monochromatic combinatorial line. This result is a generalization of van der Waerden's theorem, and it is one of the fundamental results of Ramsey theory. The theorem of van der Waerden has a famous density version, conjectured by Erds and Turn in 1936, proved by Szemerdi in 1975, and given a different proof by Furstenberg in 1977. The Hales-Jewett theorem has a density version as well, proved by Furstenberg and Katznelson in 1991 by means of a significant extension of the ergodic techniques that had been pioneered by Furstenberg in his proof of Szemerdi's theorem. In this paper, we give the first elementary proof of the theorem of Furstenberg and Katznelson and the first to provide a quantitative bound on how large n needs to be. In particular, we show that a subset of 1, 2, 3 n of density contains a combinatorial line if n is at least as big as a tower of 2s of height O (1/ 2). Our proof is surprisingly simple: indeed, it gives arguably the simplest known proof of Szemerdi's theorem.