MathDB
JBMO Shortlist 2021 N7

Source: JBMO Shortlist 2021

July 2, 2022
JuniorBalkanshortlist2021number theorygameexponential

Problem Statement

Alice chooses a prime number p>2p > 2 and then Bob chooses a positive integer n0n_0. Alice, in the first move, chooses an integer n1>n0n_1 > n_0 and calculates the expression s1=n0n1+n1n0s_1 = n_0^{n_1} + n_1^{n_0}; then Bob, in the second move, chooses an integer n2>n1n_2 > n_1 and calculates the expression s2=n1n2+n2n1s_2 = n_1^{n_2} + n_2^{n_1}; etc. one by one. (Each player knows the numbers chosen by the other in the previous moves.) The winner is the one who first chooses the number nkn_k such that pp divides sk(s1+2s2++ksk)s_k(s_1 + 2s_2 + · · · + ks_k). Who has a winning strategy?
Proposed by Borche Joshevski, Macedonia