Programming F(n) for benchmarking purposes is interesting. However, the fastest way of all is probably: 
F(n)= integer nearest (1/sqrt 5)[(1+sqrt 5)/2]^n

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This isn't perl, but it would only take a line. (If n is extremely large, the exponentiation might become inaccurate, though...)
chas

In reply to Re^3: OT: Finding Factor Closest To Square Root by chas
in thread OT: Finding Factor Closest To Square Root by QM

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