Let denote the Fibonacci number , where and for .
If the equation above holds true for positive integers and , with square-free, find .
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Relevant wiki: Fibonacci Sequence
If we write ϕ = 2 1 ( 5 + 1 ) , then (by BInet's Formula) F 2 n + 1 + 1 = 5 1 [ ϕ 2 n + 1 + 5 + ϕ − 2 n − 1 ] = 5 1 ( ϕ n + ϕ − n ) ( ϕ n + 1 + ϕ − n − 1 ) and hence F 2 n + 1 + 1 1 = = ( ϕ n + ϕ − n ) ( ϕ n + 1 + ϕ − n − 1 ) 5 = ( ϕ 2 n + 1 ) ( ϕ 2 n + 2 + 1 ) 5 ϕ 2 n + 1 5 ( ϕ 2 n + 1 1 − ϕ 2 n + 2 + 1 1 ) from which it is clear that n = 0 ∑ N F 2 n + 1 + 1 1 = 5 ( 2 1 − ϕ 2 N + 2 + 1 1 ) and hence the infinite sum is n = 0 ∑ ∞ F 2 n + 1 + 1 1 = 2 5 making the answer 5 + 2 = 7 .