John H. Conway, The angel problem, in: Richard Nowakowski (editor) Games of No Chance, volume 29 of MSRI Publications, pages 3–12, 1996.
John H. Conway, The angel problem, in: Richard Nowakowski (editor) Games of No Chance, volume 29 of MSRI Publications, pages ۳–۱۲، ۱۹۹۶.