olymmath

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 2011×20112011 \times 2011 matrices, T=(Thk)T = (T_{hk}) and W=(Whk)W = (W_{hk}). Initially, T=W=0T=W=0. After every game, for every (h,k)(h,k) (including for h=kh=k), if players hh and kk tied (that is, both won or both lost), the entry ThkT_{hk} is increased by 1, while if player hh won and player kk lost, the entry WhkW_{hk} is increased by 1 and WkhW_{kh} is decreased by 1.

Prove that at the end of the tournament, det⁡(T+iW)\det(T+iW) is a non-negative integer divisible by 220102^{2010}.

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).

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