[FOM] Conway's Angel and Devil problem
Timothy Y. Chow
tchow at alum.mit.edu
Mon Jun 18 14:12:57 EDT 2007
Conway's angel/devil problem was first published, I believe, in Winning
Ways some 25 years ago. A nice description of the problem may be found in
The problem remained unsolved until recently, when four (!) independent
and almost simultaneous solutions appeared, showing that the angel wins.
I have a reverse mathematics question related to this problem. Say that
the devil wins if for some n, the devil wins in at most n steps. Say that
the angel wins if it has a strategy that allows it to survive infinitely
many steps. What is needed to prove the following?
(*) Either the angel or the devil has a winning strategy.
Now that the angel/devil problem has been solved, the solutions can
probably be used to prove (*) in RCA_0 or even something weaker, by
"cheating" and proving the appropriate disjunct directly. So I guess my
question is not a "pure" reverse mathematics question in the sense that we
don't expect (*) to reverse over an interesting base theory to an
interesting axiom. However, I think it's clear what my intended question
is---what do we need to give a simple proof of (*), or what do we need to
prove a suitable generalization of (*) to arbitrary games similar to this
My first thought was the weak Koenig lemma, but Jeff Hirst convinced me
that because the devil's possible moves are unbounded, WKL_0 may not
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