freeCodeCamp/curriculum/challenges/en/08-coding-interview-prep/project-euler/problem-130-composites-with-prime-repunit-property.en.md

id, challengeType, title

id	challengeType	title
5900f3ee1000cf542c50ff00	5	Problem 130: Composites with prime repunit property

Description

A number consisting entirely of ones is called a repunit. We shall define R(k) to be a repunit of length k; for example, R(6) = 111111. Given that n is a positive integer and GCD(n, 10) = 1, it can be shown that there always exists a value, k, for which R(k) is divisible by n, and let A(n) be the least such value of k; for example, A(7) = 6 and A(41) = 5. You are given that for all primes, p > 5, that p − 1 is divisible by A(p). For example, when p = 41, A(41) = 5, and 40 is divisible by 5. However, there are rare composite values for which this is also true; the first five examples being 91, 259, 451, 481, and 703. Find the sum of the first twenty-five composite values of n for whichGCD(n, 10) = 1 and n − 1 is divisible by A(n).

Instructions

Tests

- text: <code>euler130()</code> should return 149253.
  testString: 'assert.strictEqual(euler130(), 149253, ''<code>euler130()</code> should return 149253.'');'

Challenge Seed

function euler130() {
  // Good luck!
  return true;
}

euler130();

Solution

// solution required