Numbers / 639

The Number 639

639 is composite: factorization 3^2 × 71. That gives 639 exactly 6 divisors: 1, 3, 9, 71, 213, 639. 639's proper divisors total just 297, 342 short of 639, the deficient case most of 639's neighbors share too. 639 has a digit sum of 18; for 639, the digital root works out to 9. 639: 3, 9 all divide 639 evenly, out of 2-12. 639 is 1001111111 in binary and 27F in hex; as a Roman numeral it's DCXXXIX.