81389813521 has 3 divisors (see below), whose sum is σ = 81390098811. Its totient is φ = 81389528232.

The previous prime is 81389813507. The next prime is 81389813537. The reversal of 81389813521 is 12531898318.

The square root of 81389813521 is 285289.

It is a perfect power (a square), and thus also a powerful number.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in only one way, i.e., 53286643921 + 28103169600 = 230839^2 + 167640^2 .

It is not a de Polignac number, because 81389813521 - 2²¹ = 81387716369 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (81389813591) by changing a digit.

It is a polite number, since it can be written in 2 ways as a sum of consecutive naturals, for example, 142645 + ... + 427933.

It is an arithmetic number, because the mean of its divisors is an integer number (27130032937).

Almost surely, 2^81389813521 is an apocalyptic number.

81389813521 is the 285289-th square number.

81389813521 is the 142645-th centered octagonal number.

It is an amenable number.

81389813521 is a deficient number, since it is larger than the sum of its proper divisors (285290).

81389813521 is an frugal number, since it uses more digits than its factorization.

81389813521 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 570578 (or 285289 counting only the distinct ones).

The product of its digits is 414720, while the sum is 49.

Multiplying 81389813521 by its sum of digits (49), we get a square (3988100862529 = 1997023²).

The spelling of 81389813521 in words is "eighty-one billion, three hundred eighty-nine million, eight hundred thirteen thousand, five hundred twenty-one".