freeCodeCamp/curriculum/challenges/english/08-coding-interview-prep/project-euler/problem-245-coresilience.english.md

---
id: 5900f4621000cf542c50ff74
challengeType: 5
title: 'Problem 245: Coresilience'
---

## Description
<section id='description'>
We shall call a fraction that cannot be cancelled down a resilient fraction. Furthermore we shall define the resilience of a denominator, R(d), to be the ratio of its proper fractions that are resilient; for example, R(12) = 4⁄11.

The resilience of a number d > 1 is then
φ(d)d − 1
, where φ is Euler's totient function.
We further define the coresilience of a number n > 1 as C(n)= 
n − φ(n)n − 1.
The coresilience of a prime p is C(p)
= 
1p − 1.
Find the sum of all composite integers 1 < n ≤ 2×1011, for which C(n) is a unit fraction.
</section>

## Instructions
<section id='instructions'>

</section>

## Tests
<section id='tests'>

```yml
tests:
  - text: <code>euler245()</code> should return 288084712410001.
    testString: assert.strictEqual(euler245(), 288084712410001, '<code>euler245()</code> should return 288084712410001.');

```

</section>

## Challenge Seed
<section id='challengeSeed'>

<div id='js-seed'>

```js
function euler245() {
  // Good luck!
  return true;
}

euler245();
```

</div>



</section>

## Solution
<section id='solution'>

```js
// solution required
```
</section>