block it. The strategy can be proven to work because the time it takes the devil to convert a safe cube in the angel's path to an unsafe cube is longer than the time it takes the angel to get to that cube. This proof was published by Imre Leader and Béla Bollobás in 2006. A substantially similar proof was published by Martin Kutz in 2005. See also The homicidal chauffeur problem, another mathematical game which pits a powerful and maneuverable adversary against a highly resourceful but less powerful foe. References External links The Angel problem by John H Conway Kloster's Angel Problem site Combinatorial game theory 1996 introductions Pursuit–evasion John Horton Conway