Counting reachable positions in a simplified model of connect four

Alistair Hartley Folster1
1Columbus State Community College, Ohio, United States

Abstract

By eliminating the win condition in the game of Connect Four and extending the board to infinite height, a rich state space of positions is obtained. We investigate the number of positions reachable on an \(n\)-column board after \(k\) color-alternating moves. For fixed \(k\) we demonstrate polynomiality, derive a partial formula for the polynomial coefficients, and precisely characterize the asymptotic behavior as \(n \to \infty\). We then turn our attention to the fixed-\(n\) case and show that, under a natural addition operation, positions reachable in an even number of moves form a monoid with a highly symmetric finite generating set; by examining certain free submonoids, we bound the exponential growth rate as \(k \to \infty\).

Keywords: Connect Four, reachable positions, monoid growth