21521611521 has 8 divisors (see below), whose sum is σ = 31883868960. Its totient is φ = 14347740996.

The previous prime is 21521611481. The next prime is 21521611571. The reversal of 21521611521 is 12511612512.

It is a happy number.

21521611521 is a `hidden beast` number, since 2 + 1 + 5 + 21 + 611 + 5 + 21 = 666.

It is not a de Polignac number, because 21521611521 - 2⁷ = 21521611393 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (27), and also a Moran number because the ratio is a prime number: 797096723 = 21521611521 / (2 + 1 + 5 + 2 + 1 + 6 + 1 + 1 + 5 + 2 + 1).

It is not an unprimeable number, because it can be changed into a prime (21521611571) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 398548335 + ... + 398548388.

It is an arithmetic number, because the mean of its divisors is an integer number (3985483620).

Almost surely, 2^21521611521 is an apocalyptic number.

It is an amenable number.

21521611521 is a deficient number, since it is larger than the sum of its proper divisors (10362257439).

21521611521 is an equidigital number, since it uses as much as digits as its factorization.

21521611521 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 797096732 (or 797096726 counting only the distinct ones).

The product of its digits is 1200, while the sum is 27.

Subtracting from 21521611521 its reverse (12511612512), we obtain a palindrome (9009999009).

The spelling of 21521611521 in words is "twenty-one billion, five hundred twenty-one million, six hundred eleven thousand, five hundred twenty-one".