Putnam 2011 B4
- Session B
In a tournament, 2011 players meet 2011 times to play a multiplayer game. Every game is played by all 2011 players together and ends with each of the players either winning or losing. The standings are kept in two matrices, and . Initially, . After every game, for every (including for ), if players and tied (that is, both won or both lost), the entry is increased by 1, while if player won and player lost, the entry is increased by 1 and is decreased by 1.
Prove that at the end of the tournament, is a non-negative integer divisible by .
Curated text Kedlaya Putnam archive. Quoted with source.
Solutions
Human solution Solution 1 by Kiran Kedlaya and Lenny Ng
The terms of the source let olymmath link to this solution but not copy it. Read the solution (Kedlaya Putnam archive).