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Arash and Babak L-corners squares

Source: Iran MO Third Round 2021 F4

September 25, 2021
combinatorics

Problem Statement

Arash and Babak play the following game, taking turns alternatively, on a 1400×14011400\times 1401 table. Arash starts and in his turns he colors kk, LL-corners (any three cell of a square). Babak in his turn colors one 2×22\times 2 square. Neither player is allowed to recolor any cell. Find all positive integers kk for which Arash has a winning strategy.