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PROJECT EULER · #0130

Composites with Prime Repunit Property

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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 p1 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 which gcd(n,10)=1 and n1 is divisible by A(n).

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