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PROJECT EULER · #0129

Repunit Divisibility

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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.

The least value of n for which A(n) first exceeds ten is 17.

Find the least value of n for which A(n) first exceeds one-million.

Write-up coming later

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