Sums of Games
A demonstration that a certain class of two-person zero-sum perfect information games, without chance moves, form a group under a suitable composition law.
Milnor, John Willard, Sums of Games. Santa Monica, CA: RAND Corporation, 1951. https://www.rand.org/pubs/research_memoranda/RM654.html. Also available in print form.
Milnor, John Willard, Sums of Games, RAND Corporation, RM-654, 1951. As of November 30, 2022: https://www.rand.org/pubs/research_memoranda/RM654.html