Numbers / 91

The Number 91

91 is composite: factorization 7 × 13. That gives 91 exactly 4 divisors: 1, 7, 13, 91. 91's proper divisors total just 21, 70 short of 91, the deficient case most of 91's neighbors share too. 91 is also the 13th triangular number, the sum of 1 through 13. 91 has a digit sum of 10; for 91, the digital root works out to 1. 91: 7 all divide 91 evenly, out of 2-12. 91 is 1011011 in binary and 5B in hex; as a Roman numeral it's XCI.