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Iran TST P6

Source: Iranian TST 2022 problem 6

April 2, 2022
number theorycombinatoricsGame Theory

Problem Statement

Let m,nm,n and a1,a2,,ama_1,a_2,\dots,a_m be arbitrary positive integers. Ali and Mohammad Play the following game. At each step, Ali chooses b1,b2,,bmNb_1,b_2,\dots,b_m \in \mathbb{N} and then Mohammad chosses a positive integers ss and obtains a new sequence {ci=ai+bi+s}i=1m\{c_i=a_i+b_{i+s}\}_{i=1}^m, where bm+1=b1, bm+2=b2,, bm+s=bsb_{m+1}=b_1,\ b_{m+2}=b_2, \dots,\ b_{m+s}=b_s The goal of Ali is to make all the numbers divisible by nn in a finite number of steps. FInd all positive integers mm and nn such that Ali has a winning strategy, no matter how the initial values a1,a2,,ama_1, a_2,\dots,a_m are. after we create the cic_i s, this sequence becomes the sequence that we continue playing on, as in it is our 'new' aia_i
Proposed by Shayan Gholami