Base 21 Repunits
Table
Format: small prime × proven prime<size> × (probable prime)<size> × [composite]<size> × {unknown}<size>
55: | \[{21^{55}-1\over20}\] | 5 × 881 × 40841 × 5664205361<10> × 17513875027111<14> × 270573999780463721<18> × 54598616218616435702441<23> |