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2024 Portuguese MO Day 2 P6

Source: 2024 Portuguese MO Day 2 P6

March 24, 2024
Game Theorycombinatorics

Problem Statement

Alexandre and Bernado are playing the following game. At the beginning, there are nn balls in a bag. At first turn, Alexandre can take one ball from the bag; at second turn, Bernado can take one or two balls from the bag, and so on. So they take turns and in kk turn, they can take a number of balls from 11 to kk. Wins the one who makes the bag empty.
For each value of nn, find who has the winning strategy.