Numbers / 391

The Number 391

391 is composite: factorization 17 × 23. That gives 391 exactly 4 divisors: 1, 17, 23, 391. 391's proper divisors total just 41, 350 short of 391, the deficient case most of 391's neighbors share too. 391 has a digit sum of 13; for 391, the digital root works out to 4. 391 shares no factor with 2-12; 391's own divisors start beyond that range. 391 is 110000111 in binary and 187 in hex; as a Roman numeral it's CCCXCI.