Numbers / 991

The Number 991

991 is prime: only 1 and 991 divide it evenly; the next prime after 991 is 997. 991's proper divisors total just 1, 990 short of 991, the deficient case most of 991's neighbors share too. 991 has a digit sum of 19; for 991, the digital root works out to 1. 991 shares no factor with 2-12; 991's own divisors start beyond that range. 991 is 1111011111 in binary and 3DF in hex; as a Roman numeral it's CMXCI.