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divisibility in arithmetic sequences

Source: Bulgaria NMO 2021 P4

May 16, 2021
arithmetic sequencenumber theory

Problem Statement

Two infinite arithmetic sequences with positive integers are given:a1<a2<a3<;b1<b2<b3<a_1<a_2<a_3<\cdots ; b_1<b_2<b_3<\cdots It is known that there are infinitely many pairs of positive integers (i,j)(i,j) for which iji+2021i\leq j\leq i+2021 and aia_i divides bjb_j. Prove that for every positive integer ii there exists a positive integer jj such that aia_i divides bjb_j.