Numbers / 561

The Number 561

561 is composite: factorization 3 × 11 × 17. That gives 561 exactly 8 divisors: 1, 3, 11, 17, 33, 51, 187, 561. 561's proper divisors total just 303, 258 short of 561, the deficient case most of 561's neighbors share too. 561 is also the 33rd triangular number, the sum of 1 through 33. 561 has a digit sum of 12; for 561, the digital root works out to 3. 561: 3, 11 all divide 561 evenly, out of 2-12. 561 is 1000110001 in binary and 231 in hex; as a Roman numeral it's DLXI.