Numbers / 645

The Number 645

645 is composite: factorization 3 × 5 × 43. That gives 645 exactly 8 divisors: 1, 3, 5, 15, 43, 129, 215, 645. 645's proper divisors total just 411, 234 short of 645, the deficient case most of 645's neighbors share too. 645 has a digit sum of 15; for 645, the digital root works out to 6. 645: 3, 5 all divide 645 evenly, out of 2-12. 645 is 1010000101 in binary and 285 in hex; as a Roman numeral it's DCXLV.