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BMO Round 1, 2023

Number theory Difficulty 7.0 National Olympiad, round 2 Find the answer United Kingdom

A sequence of positive integers ana_n begins with a1=aa_1 = a and a2=ba_2 = b for positive integers aa and bb. Subsequent terms in the sequence satisfy the following two rules for all positive integers nn:
a2n+1=a2na2n−1,a2n+2=a2n+1+4.a_{2n+1} = a_{2n}a_{2n-1}, \qquad a_{2n+2} = a_{2n+1} + 4.
Exactly mm of the numbers a1,a2,a3,…,a2022a_1, a_2, a_3, \ldots, a_{2022} are square numbers. What is the maximum possible value of mm? Note that mm depends on aa and bb, so the maximum is over all possible choices of aa and bb.

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