A156989 Largest size of a subset of {1,2,3}^n that does not contain any combinatorial lines (i.e., strings formed by 1, 2, 3, and at least one instance of a wildcard x, with x then substituted for 1, 2, or 3, e.g. 12x3x gives the combinatorial line 12131, 12232, 12333.)

1, 2, 6, 18, 52, 150, 450

Offset

0,2

Comments

The density Hales-Jewett theorem implies that a(n) = o(3^n). a(n) is studied further in the polymath1 project, see link below.

Links

Table of n, a(n) for n=0..6.

Thomas Bloom, Problem 171, Erdős Problems.

Erdős problems database contributors, Erdős problem database, see no. 171.

Hillel H. Furstenberg and Yitzhak Katznelson, A density version of the Hales-Jewett theorem for k=3, Graph Theory and Combinatorics (Cambridge, 1988). Discr. Math. 75 (1989) no. 1-3, 227-241.

Hillel H. Furstenberg and Yitzhak Katznelson, A density version of the Hales-Jewett theorem, J. Anal. Math. 57 (1991), 64-119.

Kevin O'Bryant, Sets of natural numbers with proscribed subsets, arXiv:1410.4900 [math.NT], 2014-2015.

Kevin O'Bryant, Sets of Natural Numbers with Proscribed Subsets, J. Int. Seq. 18 (2015) # 15.7.7.

D. H. J. Polymath, Density Hales-Jewett and Moser numbers, arXiv:1002.0374 [math.CO], 2010.

Polymath1 Project, Wiki Main Page

Terence Tao, Bounds for the first few density Hales-Jewett numbers, and related quantities

Example

For n=2, one example that shows a(2) is at least 6 is { 11, 13, 22, 23, 31, 32 }.

Crossrefs

Bounded below by A003142. Cf. A000244, A090245.

Sequence in context: A318570 A027059 A078484 * A077935 A077835 A077984

Adjacent sequences: A156986 A156987 A156988 * A156990 A156991 A156992

Keyword

hard,more,nonn

Author

Terence Tao, Feb 20 2009

Status

approved