21391025665 has 4 divisors (see below), whose sum is σ = 25669230804. Its totient is φ = 17112820528.

The previous prime is 21391025581. The next prime is 21391025687. The reversal of 21391025665 is 56652019312.

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 8810262769 + 12580762896 = 93863^2 + 112164^2 .

It is a cyclic number.

It is not a de Polignac number, because 21391025665 - 2²⁷ = 21256807937 is a prime.

It is a Smith number, since the sum of its digits (40) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Duffinian number.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2139102562 + ... + 2139102571.

It is an arithmetic number, because the mean of its divisors is an integer number (6417307701).

Almost surely, 2^21391025665 is an apocalyptic number.

It is an amenable number.

21391025665 is a deficient number, since it is larger than the sum of its proper divisors (4278205139).

21391025665 is an equidigital number, since it uses as much as digits as its factorization.

21391025665 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4278205138.

The product of its (nonzero) digits is 97200, while the sum is 40.

The spelling of 21391025665 in words is "twenty-one billion, three hundred ninety-one million, twenty-five thousand, six hundred sixty-five".