---
id: 5900f5311000cf542c510042
title: 'Problem 451: Modular inverses'
challengeType: 5
forumTopicId: 302124
dashedName: problem-451-modular-inverses
---

# --description--

Consider the number 15.

There are eight positive numbers less than 15 which are coprime to 15: 1, 2, 4, 7, 8, 11, 13, 14.

The modular inverses of these numbers modulo 15 are: 1, 8, 4, 13, 2, 11, 7, 14

because

1\*1 mod 15=1

2\*8=16 mod 15=1

4\*4=16 mod 15=1

7\*13=91 mod 15=1

11\*11=121 mod 15=1

14\*14=196 mod 15=1

Let I(n) be the largest positive number m smaller than n-1 such that the modular inverse of m modulo n equals m itself. So I(15)=11. Also I(100)=51 and I(7)=1.

Find ∑I(n) for 3≤n≤2·107

# --hints--

`euler451()` should return 153651073760956.

```js
assert.strictEqual(euler451(), 153651073760956);
```

# --seed--

## --seed-contents--

```js
function euler451() {

  return true;
}

euler451();
```

# --solutions--

```js
// solution required
```