Numbers / 81

The Number 81

81 is composite: factorization 3^4. That gives 81 exactly 5 divisors: 1, 3, 9, 27, 81. 81's proper divisors total just 40, 41 short of 81, the deficient case most of 81's neighbors share too. 81 is also a perfect square, since 9² = 81. 81 has a digit sum of 9; for 81, the digital root works out to 9. 81: 3, 9 all divide 81 evenly, out of 2-12. 81 is 1010001 in binary and 51 in hex; as a Roman numeral it's LXXXI.