Problem 2982. Get a Fibonacci number's index.

Created by Jared Metzel

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N.B. For the purpose of this exercise, the first Fibonacci number is 1, and the second is 2; that is, fib(1) = 1 and fib(2) = 2 .

Make a function isfib(x) so that:

if the value of the input x is not a Fibonacci number, the function returns a zero.
if the value of the input x is a Fibonacci number, the function returns its index in the Fibonacci sequence. That is, isfib(fib(n)) should return the value of n .

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Last Solution submitted on Dec 10, 2025

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Jan Orwat on 13 Mar 2015

5.731478440138172e+20 from 7th test is not a Fibonacci Number. 100th Fibonacci Number in proposed convention is 573147844013817084101 which has 21 digits and exceeds double precision.

Rafael S.T. Vieira on 23 Sep 2020

I agree with Jan. http://www.maths.surrey.ac.uk/hosted-sites/R.Knott/Fibonacci/fibtable.html (this list begins at 0). The 100th fibonnaci number is 573147844013817084101 (5.731478440138171e+20), and not 5.731478440138172e+20.

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