21056621881 has 3 divisors (see below), whose sum is σ = 21056766991. Its totient is φ = 21056476772.

The previous prime is 21056621867. The next prime is 21056621891. The reversal of 21056621881 is 18812665012.

It is a happy number.

The square root of 21056621881 is 145109.

It is a perfect power (a square), and thus also a powerful number.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in only one way, i.e., 533194281 + 20523427600 = 23091^2 + 143260^2 .

It is not a de Polignac number, because 21056621881 - 2¹⁹ = 21056097593 is a prime.

It is a super-2 number, since 2×21056621881² (a number of 21 digits) contains 22 as substring.

It is a Smith number, since the sum of its digits (40) coincides with the sum of the digits of its prime factors.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21056621891) 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 2 ways as a sum of consecutive naturals, for example, 72555 + ... + 217663.

Almost surely, 2^21056621881 is an apocalyptic number.

21056621881 is the 145109-th square number.

21056621881 is the 72555-th centered octagonal number.

It is an amenable number.

21056621881 is a deficient number, since it is larger than the sum of its proper divisors (145110).

21056621881 is an frugal number, since it uses more digits than its factorization.

21056621881 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 290218 (or 145109 counting only the distinct ones).

The product of its (nonzero) digits is 46080, while the sum is 40.

The spelling of 21056621881 in words is "twenty-one billion, fifty-six million, six hundred twenty-one thousand, eight hundred eighty-one".