21255421021 has 4 divisors (see below), whose sum is σ = 21268250332. Its totient is φ = 21242591712.

The previous prime is 21255421009. The next prime is 21255421027. The reversal of 21255421021 is 12012455212.

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 6953892100 + 14301528921 = 83390^2 + 119589^2 .

It is a cyclic number.

It is not a de Polignac number, because 21255421021 - 2⁹ = 21255420509 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 (21255421027) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (23) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 6412170 + ... + 6415483.

It is an arithmetic number, because the mean of its divisors is an integer number (5317062583).

Almost surely, 2^21255421021 is an apocalyptic number.

It is an amenable number.

21255421021 is a deficient number, since it is larger than the sum of its proper divisors (12829311).

21255421021 is a wasteful number, since it uses less digits than its factorization.

21255421021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 12829310.

The product of its (nonzero) digits is 1600, while the sum is 25.

Adding to 21255421021 its reverse (12012455212), we get a palindrome (33267876233).

The spelling of 21255421021 in words is "twenty-one billion, two hundred fifty-five million, four hundred twenty-one thousand, twenty-one".