21665995921 has 4 divisors (see below), whose sum is σ = 22021176244. Its totient is φ = 21310815600.

The previous prime is 21665995913. The next prime is 21665995979. The reversal of 21665995921 is 12959956612.

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 324000000 + 21341995921 = 18000^2 + 146089^2 .

It is a cyclic number.

It is not a de Polignac number, because 21665995921 - 2³ = 21665995913 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21665995121) 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, 177590070 + ... + 177590191.

It is an arithmetic number, because the mean of its divisors is an integer number (5505294061).

Almost surely, 2^21665995921 is an apocalyptic number.

It is an amenable number.

21665995921 is a deficient number, since it is larger than the sum of its proper divisors (355180323).

21665995921 is an equidigital number, since it uses as much as digits as its factorization.

21665995921 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 355180322.

The product of its digits is 2624400, while the sum is 55.

The spelling of 21665995921 in words is "twenty-one billion, six hundred sixty-five million, nine hundred ninety-five thousand, nine hundred twenty-one".