Problem 137
Fibonacci golden nuggets
Consider the infinite polynomial series AF(x) = xF1 + x2F2 + x3F3 + ..., where Fk is the kth term in the Fibonacci sequence: 1, 1, 2, 3, 5, 8, ... ; that is, Fk = Fk
1 + Fk2, F1 = 1 and F2 = 1.
For this problem we shall be interested in values of x for which AF(x) is a positive integer.
| Surprisingly AF(1/2) | = | (1/2).1 + (1/2)2.1 + (1/2)3.2 + (1/2)4.3 + (1/2)5.5 + ... |
| = | 1/2 + 1/4 + 2/8 + 3/16 + 5/32 + ... | |
| = | 2 |
The corresponding values of x for the first five natural numbers are shown below.
| x | AF(x) |
 21 |
1 |
| 1/2 | 2 |
| (132)/3 |
3 |
| (895)/8 |
4 |
| (343)/5 |
5 |
We shall call AF(x) a golden nugget if x is rational, because they become increasingly rarer; for example, the 10th golden nugget is 74049690.
Find the 15th golden nugget.
These problems are part of Project Euler and are licensed under CC BY-NC-SA 2.0 UK