81721521 has 16 divisors (see below), whose sum is σ = 138364800. Its totient is φ = 46697904.

The previous prime is 81721513. The next prime is 81721531. The reversal of 81721521 is 12512718.

It is not a de Polignac number, because 81721521 - 2³ = 81721513 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is not an unprimeable number, because it can be changed into a prime (81721501) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 216006 + ... + 216383.

It is an arithmetic number, because the mean of its divisors is an integer number (8647800).

Almost surely, 2^81721521 is an apocalyptic number.

It is an amenable number.

81721521 is a deficient number, since it is larger than the sum of its proper divisors (56643279).

81721521 is a wasteful number, since it uses less digits than its factorization.

81721521 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 432405 (or 432399 counting only the distinct ones).

The product of its digits is 1120, while the sum is 27.

The square root of 81721521 is about 9039.9956305299. The cubic root of 81721521 is about 433.9557831173.

The spelling of 81721521 in words is "eighty-one million, seven hundred twenty-one thousand, five hundred twenty-one".