Numbers / 9

The Number 9

9 is composite: factorization 3^2. That gives 9 exactly 3 divisors: 1, 3, 9. 9's proper divisors total just 4, 5 short of 9, the deficient case most of 9's neighbors share too. 9 is also a perfect square, since 3² = 9. 9 has a digit sum of 9; for 9, the digital root works out to 9. 9: 3 all divide 9 evenly, out of 2-12. 9 is 1001 in binary and 9 in hex; as a Roman numeral it's IX.