---
id: 5900f3f51000cf542c50ff08
title: 'Problem 137: Fibonacci golden nuggets'
challengeType: 5
forumTopicId: 301765
dashedName: problem-137-fibonacci-golden-nuggets
---

# --description--

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 + Fk−2, 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.

xAF(x) √2−11 1/22 (√13−2)/33 (√89−5)/84 (√34−3)/55

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.

# --hints--

`euler137()` should return 1120149658760.

```js
assert.strictEqual(euler137(), 1120149658760);
```

# --seed--

## --seed-contents--

```js
function euler137() {

  return true;
}

euler137();
```

# --solutions--

```js
// solution required
```