Numbers / 521

The Number 521

521 is prime: only 1 and 521 divide it evenly; the next prime after 521 is 523. 521's proper divisors total just 1, 520 short of 521, the deficient case most of 521's neighbors share too. 521 has a digit sum of 8; for 521, the digital root works out to 8. 521 shares no factor with 2-12; 521's own divisors start beyond that range. 521 is 1000001001 in binary and 209 in hex; as a Roman numeral it's DXXI.