Base 21 Repunits
Table
Format: small prime × proven prime<size> × (probable prime)<size> × [composite]<size> × {unknown}<size>
87: | \[{21^{87}-1\over20}\] | 59 × 463 × 1103704099 × 246763300513<12> × 687613372491653<15> × 7826081384442721874824416529<28> × 13477015991023291802765110016390772920553907009<47> |