Numbers / 21

The Number 21

21 is composite: factorization 3 × 7. That gives 21 exactly 4 divisors: 1, 3, 7, 21. 21's proper divisors total just 11, 10 short of 21, the deficient case most of 21's neighbors share too. 21 also carries: the 6th triangular number, the sum of 1 through 6; and a Fibonacci number, since 13 + 8 = 21. 21 has a digit sum of 3; for 21, the digital root works out to 3. 21: 3, 7 all divide 21 evenly, out of 2-12. 21 is 10101 in binary and 15 in hex; as a Roman numeral it's XXI.