---
id: 5900f4451000cf542c50ff57
challengeType: 5
title: 'Problem 216: Investigating the primality of numbers of the form 2n2-1'
forumTopicId: 301858
---

## Description
<section id='description'>
Consider numbers t(n) of the form t(n) = 2n2-1 with n > 1.
The first such numbers are 7, 17, 31, 49, 71, 97, 127 and 161.
It turns out that only 49 = 7*7 and 161 = 7*23 are not prime.
For n ≤ 10000 there are 2202 numbers t(n)  that are prime.

How many numbers t(n) are prime for n ≤ 50,000,000 ?
</section>

## Instructions
<section id='instructions'>

</section>

## Tests
<section id='tests'>

```yml
tests:
  - text: <code>euler216()</code> should return 5437849.
    testString: assert.strictEqual(euler216(), 5437849);

```

</section>

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

<div id='js-seed'>

```js
function euler216() {
  // Good luck!
  return true;
}

euler216();
```

</div>



</section>

## Solution
<section id='solution'>

```js
// solution required
```

</section>