81720864 has 72 divisors (see below), whose sum is σ = 238705740. Its totient is φ = 26500608.

The previous prime is 81720829. The next prime is 81720883. The reversal of 81720864 is 46802718.

It can be written as a sum of positive squares in 2 ways, for example, as 62536464 + 19184400 = 7908^2 + 4380^2 .

It is a tau number, because it is divible by the number of its divisors (72).

It is a super-2 number, since 2×81720864² = 13356599225812992, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (36).

It is an unprimeable number.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 6822 + ... + 14490.

Almost surely, 2^81720864 is an apocalyptic number.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 81720864, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (119352870).

81720864 is an abundant number, since it is smaller than the sum of its proper divisors (156984876).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

81720864 is a wasteful number, since it uses less digits than its factorization.

81720864 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7722 (or 7711 counting only the distinct ones).

The product of its (nonzero) digits is 21504, while the sum is 36.

The square root of 81720864 is about 9039.9592919437. The cubic root of 81720864 is about 433.9546201853.

The spelling of 81720864 in words is "eighty-one million, seven hundred twenty thousand, eight hundred sixty-four".