47 lines
809 B
Markdown
47 lines
809 B
Markdown
---
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id: 5900f4451000cf542c50ff57
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title: 'Problem 216: Investigating the primality of numbers of the form 2n2-1'
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challengeType: 5
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forumTopicId: 301858
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dashedName: problem-216-investigating-the-primality-of-numbers-of-the-form-2n2-1
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---
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# --description--
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Consider numbers t(n) of the form t(n) = 2n2-1 with n > 1.
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The first such numbers are 7, 17, 31, 49, 71, 97, 127 and 161.
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It turns out that only 49 = 7\*7 and 161 = 7\*23 are not prime.
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For n ≤ 10000 there are 2202 numbers t(n) that are prime.
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How many numbers t(n) are prime for n ≤ 50,000,000 ?
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# --hints--
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`euler216()` should return 5437849.
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```js
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assert.strictEqual(euler216(), 5437849);
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```
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# --seed--
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## --seed-contents--
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```js
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function euler216() {
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return true;
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}
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euler216();
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```
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# --solutions--
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```js
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// solution required
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```
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