41 lines
988 B
Markdown
41 lines
988 B
Markdown
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---
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id: 5900f4621000cf542c50ff74
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title: 'Problem 245: Coresilience'
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challengeType: 5
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forumTopicId: 301892
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dashedName: problem-245-coresilience
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---
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# --description--
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We shall call a fraction that cannot be cancelled down a resilient fraction. Furthermore we shall define the resilience of a denominator, R(d), to be the ratio of its proper fractions that are resilient; for example, R(12) = 4⁄11.
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The resilience of a number d > 1 is then φ(d)d − 1 , where φ is Euler's totient function. We further define the coresilience of a number n > 1 as C(n)= n − φ(n)n − 1. The coresilience of a prime p is C(p) = 1p − 1. Find the sum of all composite integers 1 < n ≤ 2×1011, for which C(n) is a unit fraction.
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# --hints--
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`euler245()` should return 288084712410001.
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```js
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assert.strictEqual(euler245(), 288084712410001);
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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 euler245() {
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return true;
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}
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euler245();
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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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