Repunit

Repunit prime
No. of known terms	11
Conjectured no. of terms	Infinite
First terms	11, 1111111111111111111, 11111111111111111111111
Largest known term	(108177207−1)/9
OEIS index	A004022; Primes of the form (10^n − 1)/9;

In recreational mathematics, a repunit is a number like 11, 111, or 1111 that contains only the digit 1 — a more specific type of repdigit. The term stands for "repeated unit" and was coined in 1966 by Albert H. Beiler in his book Recreations in the Theory of Numbers.^{[note 1]}

A repunit prime is a repunit that is also a prime number. Primes that are repunits in base-2 are Mersenne primes. As of May 2023, the largest known prime number 2^82,589,933 − 1, the largest probable prime R_8177207 and the largest elliptic curve primality-proven prime R₈₆₄₅₃ are all repunits in various bases.

^ Beiler 2013, pp. 83

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[1] Beiler 2013, pp. 83

[note 1]