Erik D. Demaine, Martin L. Demaine, and David Eppstein, “Phutball Endgames are Hard”, in More Games of No Chance, edited by R. J. Nowakowski, 2002, pages 351–360, Cambridge University Press. Collection of papers from the MSRI Combinatorial Game Theory Research Workshop, Berkeley, California, July 24–28, 2000.
Abstract:
We show that, in John Conway's board game Phutball (or Philosopher's Football),
it is NP-complete to determine whether the current player has a move that
immediately wins the game. In contrast, the similar problems of determining
whether there is an immediately winning move in checkers, or a move that kings
a man, are both solvable in polynomial time.