On Even Fibonacci Numbers
Beginning with the integer s[0] = 1, construct a sequence of integers
such that the nth term s[n] is the sum of (k+1)*s[n-k] for k=1 to
n-1. Thus we have
n s[n]
--- -----
0 1 1
1 1*s[0] 1
2 1*s[1] + 2*s[0] 3
3 1*s[2] + 2*s[1] + 3*s[0] 8
4 1*s[3] + 2*s[2] + 3*s[1] + 4*s[0] 21
5 1*s[4] + 2*s[3] + 3*s[2] + 4*s[1] + 5*s[0] 55
Interestingly, s[n] turns out to be just the Fibonacci number F[2n].
(I've taken the liberty of setting F[0]=1 and F[1]=0, reversing the
customary order of the initial values.) So we have the identity
n
F[2n] = SUM k F[2(n-k)]
k=1
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