Base 21 Repunits
Table
Format: small prime × proven prime<size> × (probable prime)<size> × [composite]<size> × {unknown}<size>
57: | \[{21^{57}-1\over20}\] | 463 × 304609 × 12061389013<11> × 54921106624003<14> × 1244541772187848099210936765314595053007129<43> |