## Graham's Number

The smallest dimension of a Hypercube such that if the lines joining all pairs of corners are two-colored, a Planar Complete Graph of one color will be forced. That an answer exists was proved by R. L. Graham and B. L. Rothschild. The actual answer is believed to be 6, but the best bound proved is

where is stacked Arrow Notation. It is less than , where Chained Arrow Notation has been used.

Gardner, M. Mathematical Games.'' Sci. Amer. 237, 18-28, Nov. 1977.