Base 2 Repunits
Table
Format: small prime × proven prime<size> × (probable prime)<size> × [composite]<size> × {unknown}<size>
| 261: | \[2^{261}-1\] | 7 × 73 × 233 × 1103 × 2089 × 4177 × 9857737155463<13> × 328017025014102923449988663752960080886511412965881<51> |