21005506921 has 4 divisors (see below), whose sum is σ = 24006293632. Its totient is φ = 18004720212.

The previous prime is 21005506891. The next prime is 21005506927. The reversal of 21005506921 is 12960550012.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 21005506921 - 2⁷ = 21005506793 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21005506927) 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, 1500393345 + ... + 1500393358.

It is an arithmetic number, because the mean of its divisors is an integer number (6001573408).

Almost surely, 2^21005506921 is an apocalyptic number.

It is an amenable number.

21005506921 is a deficient number, since it is larger than the sum of its proper divisors (3000786711).

21005506921 is an equidigital number, since it uses as much as digits as its factorization.

21005506921 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3000786710.

The product of its (nonzero) digits is 5400, while the sum is 31.

The spelling of 21005506921 in words is "twenty-one billion, five million, five hundred six thousand, nine hundred twenty-one".