21621551565 has 32 divisors (see below), whose sum is σ = 35216424576. Its totient is φ = 11324770560.

The previous prime is 21621551563. The next prime is 21621551569. The reversal of 21621551565 is 56515512612.

It is not a de Polignac number, because 21621551565 - 2¹ = 21621551563 is a prime.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21621551561) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 579402 + ... + 615588.

It is an arithmetic number, because the mean of its divisors is an integer number (1100513268).

Almost surely, 2^21621551565 is an apocalyptic number.

It is an amenable number.

21621551565 is a deficient number, since it is larger than the sum of its proper divisors (13594873011).

21621551565 is a wasteful number, since it uses less digits than its factorization.

21621551565 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 36909.

The product of its digits is 90000, while the sum is 39.

The spelling of 21621551565 in words is "twenty-one billion, six hundred twenty-one million, five hundred fifty-one thousand, five hundred sixty-five".