21256979557 has 18 divisors (see below), whose sum is σ = 24007616934. Its totient is φ = 18793613520.

The previous prime is 21256979509. The next prime is 21256979621. The reversal of 21256979557 is 75597965212.

It can be written as a sum of positive squares in only one way, i.e., 20682466596 + 574512961 = 143814^2 + 23969^2 .

It is not a de Polignac number, because 21256979557 - 2²³ = 21248590949 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 (21256979257) by changing a digit.

It is a polite number, since it can be written in 17 ways as a sum of consecutive naturals, for example, 9754294 + ... + 9756472.

Almost surely, 2^21256979557 is an apocalyptic number.

It is an amenable number.

21256979557 is a deficient number, since it is larger than the sum of its proper divisors (2750637377).

21256979557 is an frugal number, since it uses more digits than its factorization.

21256979557 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4417 (or 2227 counting only the distinct ones).

The product of its digits is 11907000, while the sum is 58.

The spelling of 21256979557 in words is "twenty-one billion, two hundred fifty-six million, nine hundred seventy-nine thousand, five hundred fifty-seven".