21001000021 has 4 divisors (see below), whose sum is σ = 21001505344. Its totient is φ = 21000494700.

The previous prime is 21001000019. The next prime is 21001000037. The reversal of 21001000021 is 12000010012.

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 21001000021 - 2¹ = 21001000019 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21000999968 and 21001000013.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21001000001) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 184125 + ... + 275506.

It is an arithmetic number, because the mean of its divisors is an integer number (5250376336).

Almost surely, 2^21001000021 is an apocalyptic number.

It is an amenable number.

21001000021 is a deficient number, since it is larger than the sum of its proper divisors (505323).

21001000021 is an equidigital number, since it uses as much as digits as its factorization.

21001000021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 505322.

The product of its (nonzero) digits is 4, while the sum is 7.

Adding to 21001000021 its reverse (12000010012), we get a palindrome (33001010033).

Subtracting from 21001000021 its reverse (12000010012), we obtain a palindrome (9000990009).

The spelling of 21001000021 in words is "twenty-one billion, one million, twenty-one", and thus it is an aban number and an uban number.