45 lines
1013 B
Markdown
45 lines
1013 B
Markdown
---
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id: 5900f45f1000cf542c50ff71
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title: 'Problem 242: Odd Triplets'
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challengeType: 5
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forumTopicId: 301889
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dashedName: problem-242-odd-triplets
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---
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# --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 an odd-triplet \[n,k,f(n,k)].
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There are exactly five odd-triplets with n ≤ 10, namely: \[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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# --hints--
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`euler242()` should return 997104142249036700.
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```js
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assert.strictEqual(euler242(), 997104142249036700);
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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 euler242() {
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return true;
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}
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euler242();
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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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