---
id: 5900f4621000cf542c50ff74
challengeType: 5
title: 'Problem 245: Coresilience'
forumTopicId: 301892
---
## 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.
## Instructions
## Tests
```yml
tests:
- text: euler245() should return 288084712410001.
testString: assert.strictEqual(euler245(), 288084712410001);
```
## Challenge Seed
```js
function euler245() {
return true;
}
euler245();
```