# Definition:Repunit Prime/Index

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## Definition

The index of a repunit prime is the number of digits it has.

Thus $R_n$ denotes a repunit prime with $n$ digits.

## Sequence

Expressing a repunit prime of $n$ digits by $R_n$, the complete sequence of known repunit primes (base $10$) can be expressed as:

$R_2, R_{19}, R_{23}, R_{317}, R_{1031}, R_{49 \, 081}, R_{86 \, 453}, R_{109 \, 297}, R_{270 \, 343}$

## Linguistic Note

The derivation of the term repunit is clear: it comes from repeated unit.