21257664521 has 4 divisors (see below), whose sum is σ = 21259009044. Its totient is φ = 21256320000.

The previous prime is 21257664481. The next prime is 21257664523. The reversal of 21257664521 is 12546675212.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 18493008121 + 2764656400 = 135989^2 + 52580^2 .

It is a cyclic number.

It is not a de Polignac number, because 21257664521 - 2⁶ = 21257664457 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21257664523) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 648260 + ... + 680261.

It is an arithmetic number, because the mean of its divisors is an integer number (5314752261).

Almost surely, 2^21257664521 is an apocalyptic number.

It is an amenable number.

21257664521 is a deficient number, since it is larger than the sum of its proper divisors (1344523).

21257664521 is a wasteful number, since it uses less digits than its factorization.

21257664521 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1344522.

The product of its digits is 201600, while the sum is 41.

The spelling of 21257664521 in words is "twenty-one billion, two hundred fifty-seven million, six hundred sixty-four thousand, five hundred twenty-one".