Base 21 Repunits
Table
Format: small prime × proven prime<size> × (probable prime)<size> × [composite]<size> × {unknown}<size>
81: | \[{21^{81}-1\over20}\] | 109 × 163 × 463 × 4779433 × 85775383 × 7429452749713<13> × 251097226443098886970664441840231990201201851332912023687544574398706423<72> |