MathDB
Game ensuring a base-2 representation is in S

Source: 2023 Thailand Online MO P10

February 12, 2023
combinatoricscombinatorial game

Problem Statement

Let nn be an even positive integer. Alice and Bob play the following game. Before the start of the game, Alice chooses a set SS containing mm integers and announces it to Bob. The players then alternate turns, with Bob going first, choosing i{1,2,,n}i\in\{1,2,\dots, n\} that has not been chosen and setting the value of viv_i to either 00 or 11. At the end of the game, when all of v1,v2,,vnv_1,v_2,\dots,v_n have been set, the expression E=v120+v221++vn2n1E=v_1\cdot 2^0 + v_2 \cdot 2^1 + \dots + v_n \cdot 2^{n-1} is calculated. Determine the minimum mm such that Alice can always ensure that ESE\in S regardless of how Bob plays.