Star, written as * or *1, is the value given to the combinatorial game {0 | 0}, where zero is the zero game. This game is an unconditional first-player win.

Star, as defined by John Conway in Winning Ways for your Mathematical Plays, is a value, but not a number in the traditional sense. Star is not zero, but neither positive nor negative, and is therefore said to be fuzzy and confused with (a fourth alternative that means neither "less than", "equal to", nor "greater than") 0. It is provably less than all positive rational numbers, and greater than all negative rationals. Since the rationals are dense in the reals, this also makes * greater than any negative real, and less than any positive real.

Why * 0

A combinatorial game has a positive and negative player; which player moves first is left ambiguous. The combinatorial game 0 , or { | }, leaves no options and is a second-player win. Likewise, a combinatorial game is won (assuming optimal play) by the second player if and only if its value is 0. Therefore, a game of value *, which is a first-player win, is neither positive nor negative. However, * is not the only possible value for a first-player win game (see nimbers).

Star does have the property that * + * = 0, because the sum of two value-* games is the zero game.

Example of a value-* game

Nim, with one pile and one piece, has value *. The first player will remove the piece, and the second player will lose. A single-pile Nim game with one pile of n pieces (also a first-player win) has value *n. The numbers *z for integers z form an infinite field of characteristic 2, when addition is defined in the context of combinatorial games and multiplication is given a more complex definition.

See Also

Nimbers