21606664021 has 4 divisors (see below), whose sum is σ = 21608027692. Its totient is φ = 21605300352.

The previous prime is 21606664019. The next prime is 21606664031. The reversal of 21606664021 is 12046660612.

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 10177984996 + 11428679025 = 100886^2 + 106905^2 .

It is a cyclic number.

It is not a de Polignac number, because 21606664021 - 2¹ = 21606664019 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21606664031) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 657786 + ... + 689851.

It is an arithmetic number, because the mean of its divisors is an integer number (5402006923).

Almost surely, 2^21606664021 is an apocalyptic number.

It is an amenable number.

21606664021 is a deficient number, since it is larger than the sum of its proper divisors (1363671).

21606664021 is a wasteful number, since it uses less digits than its factorization.

21606664021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1363670.

The product of its (nonzero) digits is 20736, while the sum is 34.

The spelling of 21606664021 in words is "twenty-one billion, six hundred six million, six hundred sixty-four thousand, twenty-one".