---
id: 5900f4451000cf542c50ff57
challengeType: 5
title: 'Problem 216: Investigating the primality of numbers of the form 2n2-1'
---
## 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 ?
## Instructions
## Tests
```yml
tests:
- text: euler216() should return 5437849.
testString: 'assert.strictEqual(euler216(), 5437849, "euler216() should return 5437849.");'
```
## Challenge Seed
```js
function euler216() {
// Good luck!
return true;
}
euler216();
```