66 lines
1.2 KiB
Markdown
66 lines
1.2 KiB
Markdown
---
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id: 5900f45f1000cf542c50ff71
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challengeType: 5
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title: 'Problem 242: Odd Triplets'
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forumTopicId: 301889
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---
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## Description
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<section id='description'>
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Given the set {1,2,...,n}, we define f(n,k) as the number of its k-element subsets with an odd sum of elements. For example, f(5,3) = 4, since the set {1,2,3,4,5} has four 3-element subsets having an odd sum of elements, i.e.: {1,2,4}, {1,3,5}, {2,3,4} and {2,4,5}.
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When all three values n, k and f(n,k) are odd, we say that they make
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an odd-triplet [n,k,f(n,k)].
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There are exactly five odd-triplets with n ≤ 10, namely:
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[1,1,f(1,1) = 1], [5,1,f(5,1) = 3], [5,5,f(5,5) = 1], [9,1,f(9,1) = 5] and [9,9,f(9,9) = 1].
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How many odd-triplets are there with n ≤ 1012 ?
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</section>
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## Instructions
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<section id='instructions'>
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</section>
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## Tests
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<section id='tests'>
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```yml
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tests:
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- text: <code>euler242()</code> should return 997104142249036700.
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testString: assert.strictEqual(euler242(), 997104142249036700);
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```
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</section>
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## Challenge Seed
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<section id='challengeSeed'>
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<div id='js-seed'>
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```js
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function euler242() {
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return true;
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}
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euler242();
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```
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</div>
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</section>
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## Solution
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<section id='solution'>
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```js
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// solution required
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```
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</section>
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