21720565 has 16 divisors (see below), whose sum is σ = 27158400. Its totient is φ = 16656192.

The previous prime is 21720563. The next prime is 21720571. The reversal of 21720565 is 56502712.

It is a cyclic number.

It is not a de Polignac number, because 21720565 - 2¹ = 21720563 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21720563) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 23176 + ... + 24094.

It is an arithmetic number, because the mean of its divisors is an integer number (1697400).

Almost surely, 2^21720565 is an apocalyptic number.

It is an amenable number.

21720565 is a deficient number, since it is larger than the sum of its proper divisors (5437835).

21720565 is a wasteful number, since it uses less digits than its factorization.

21720565 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1116.

The product of its (nonzero) digits is 4200, while the sum is 28.

The square root of 21720565 is about 4660.5326948751. The cubic root of 21720565 is about 279.0125292524.

The spelling of 21720565 in words is "twenty-one million, seven hundred twenty thousand, five hundred sixty-five".