IMO 1986 LL FRA22

Let (an)n\inN be the sequence of integers defined recursively by

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IMO 1986 LL FRA22

Origin: FRA

Problem

Let (an)n\inN be the sequence of integers defined recursively by a0 = 0, a1 = 1, an+2 = 4an+1 + an for n \geq0. Find the common divisors of a1986 and a6891.