olymmath

Putnam 2021 B5

  • Session B

Say that an nn-by-nn matrix A=(aij)1≤i,j≤nA = (a_{ij})_{1 \leq i,j \leq n} with integer entries is very odd if, for every nonempty subset SS of {1,2,…,n}\{1,2,\dots,n\}, the ∣S∣|S|-by-∣S∣|S| submatrix (aij)i,j∈S(a_{ij})_{i,j \in S} has odd determinant. Prove that if AA is very odd, then AkA^k is very odd for every k≥1k \geq 1.

Curated text Kedlaya Putnam archive. Quoted with source.

Solutions

Human solution Solution 1 by Manjul Bhargava, 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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