21565 has 8 divisors (see below), whose sum is σ = 27360. Its totient is φ = 16272.

The previous prime is 21563. The next prime is 21569. The reversal of 21565 is 56512.

It is a happy number.

21565 is digitally balanced in base 6, because in such base it contains all the possibile digits an equal number of times.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21565 - 2¹ = 21563 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (19).

21565 is strictly pandigital in base 6.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21563) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 19 + ... + 208.

It is an arithmetic number, because the mean of its divisors is an integer number (3420).

2²¹⁵⁶⁵ is an apocalyptic number.

It is an amenable number.

21565 is a deficient number, since it is larger than the sum of its proper divisors (5795).

21565 is a wasteful number, since it uses less digits than its factorization.

21565 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 251.

The product of its digits is 300, while the sum is 19.

The square root of 21565 is about 146.8502638745. The cubic root of 21565 is about 27.8344826837.

The spelling of 21565 in words is "twenty-one thousand, five hundred sixty-five".