21205605656 has 16 divisors (see below), whose sum is σ = 42099364440. Its totient is φ = 9979108480.

The previous prime is 21205605647. The next prime is 21205605679. The reversal of 21205605656 is 65650650212.

It is a happy number.

It is a super-2 number, since 2×21205605656² (a number of 21 digits) contains 22 as substring.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 77961650 + ... + 77961921.

Almost surely, 2^21205605656 is an apocalyptic number.

It is an amenable number.

21205605656 is a deficient number, since it is larger than the sum of its proper divisors (20893758784).

21205605656 is a wasteful number, since it uses less digits than its factorization.

21205605656 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 155923594 (or 155923590 counting only the distinct ones).

The product of its (nonzero) digits is 108000, while the sum is 38.

The spelling of 21205605656 in words is "twenty-one billion, two hundred five million, six hundred five thousand, six hundred fifty-six".