Base 21 Repunits
Table
Format: small prime × proven prime<size> × (probable prime)<size> × [composite]<size> × {unknown}<size>
77: | \[{21^{77}-1\over20}\] | 43 × 631 × 3319 × 6225803431<10> × 17513875027111<14> × 39824892937303186001525100993399811<35> × 82721492925050166710438512180782661<35> |