Two Dimensional Silver Dollar Game

Abstract: We define a variant of the two-dimensional Silver Dollar game. Two coins are placed on a chessboard of unbounded size, and two players take turns choosing one of the coins and moving it. Coins are to be moved to the left or upward vertically as far as desired. If a coin is dropped off the board, players cannot use this coin. Jumping a coin over another coin or on another coin is illegal. We add another operation: moving a coin and pushing another coin. For non-negative integers w,x,y,z, we denote the positions of the two coins by (w,x,y,z), where (w,x) is the position of one coin and (y,z) is the position of the other coin. Then, the set of P-positions (the previous player's winning positions) of this game is {(w,x,y,z):the nim-sum of (w-1),(x-1),(y-1), and (z-1) is 0} . Next, we make another game by omitting the rule of pushing another coin and permitting a jump over another coin. Then, there exist two relatively small sets A and B such that the set of P-positions of this game is ( {(w,x,y,z):the nim-sum of (w-1),(x-1),(y-1), and (z-1) is 0} cup A) -B.