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A number whose digits are all units; the rep-unit (repeated unit) with n digits is denoted Rn. For example, R1=1, R2=11, R3=111, and Rn=(10n - 1)/9. Rn divides Rm whenever n divides m. No repunit can be a square number, but it is not known if one can be a cube. Repunit primes are repunits that are prime numbers. The only known repunit primes are R2 (11), R23, R19, R317, and R1031, though R49081 and R86453 are suspected primes.

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