IMO 1974 LL BUL2

Let {un} be the Fibonacci sequence, i.e., u0 = 0, u1 = 1,

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IMO 1974 LL BUL2

Origin: BUL

Problem

Let {un} be the Fibonacci sequence, i.e., u0 = 0, u1 = 1, un = un−1 + un−2 for n > 1. Prove that there exist infinitely many prime numbers p that divide up−1.