21865 has 4 divisors (see below), whose sum is σ = 26244. Its totient is φ = 17488.

The previous prime is 21863. The next prime is 21871. The reversal of 21865 is 56812.

It is a happy number.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 10201 + 11664 = 101^2 + 108^2 .

It is a cyclic number.

It is not a de Polignac number, because 21865 - 2¹ = 21863 is a prime.

It is a Smith number, since the sum of its digits (22) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is an Ulam number.

It is a Duffinian number.

It is a plaindrome in base 16.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21863) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2182 + ... + 2191.

It is an arithmetic number, because the mean of its divisors is an integer number (6561).

2²¹⁸⁶⁵ is an apocalyptic number.

It is an amenable number.

21865 is a deficient number, since it is larger than the sum of its proper divisors (4379).

21865 is an equidigital number, since it uses as much as digits as its factorization.

21865 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4378.

The product of its digits is 480, while the sum is 22.

The square root of 21865 is about 147.8681845429. The cubic root of 21865 is about 27.9629612303.

The spelling of 21865 in words is "twenty-one thousand, eight hundred sixty-five".