81721520 has 40 divisors (see below), whose sum is σ = 190441680. Its totient is φ = 32613120.

The previous prime is 81721513. The next prime is 81721531. The reversal of 81721520 is 2512718.

It is a tau number, because it is divible by the number of its divisors (40).

It is a super-2 number, since 2×81721520² = 13356813662220800, which contains 22 as substring.

It is a self number, because there is not a number n which added to its sum of digits gives 81721520.

It is an unprimeable number.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 44786 + ... + 46574.

It is an arithmetic number, because the mean of its divisors is an integer number (4761042).

Almost surely, 2^81721520 is an apocalyptic number.

81721520 is a gapful number since it is divisible by the number (80) formed by its first and last digit.

It is an amenable number.

81721520 is an abundant number, since it is smaller than the sum of its proper divisors (108720160).

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

81721520 is a wasteful number, since it uses less digits than its factorization.

81721520 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2373 (or 2367 counting only the distinct ones).

The product of its (nonzero) digits is 1120, while the sum is 26.

The square root of 81721520 is about 9039.9955752202. The cubic root of 81721520 is about 433.9557813472.

The spelling of 81721520 in words is "eighty-one million, seven hundred twenty-one thousand, five hundred twenty".