21365 has 4 divisors (see below), whose sum is σ = 25644. Its totient is φ = 17088.

The previous prime is 21347. The next prime is 21377. The reversal of 21365 is 56312.

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 6724 + 14641 = 82^2 + 121^2 .

It is a cyclic number.

It is not a de Polignac number, because 21365 - 2¹⁰ = 20341 is a prime.

It is a Duffinian number.

It is a congruent number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 21365.

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, 2132 + ... + 2141.

It is an arithmetic number, because the mean of its divisors is an integer number (6411).

2²¹³⁶⁵ is an apocalyptic number.

It is an amenable number.

21365 is a deficient number, since it is larger than the sum of its proper divisors (4279).

21365 is an equidigital number, since it uses as much as digits as its factorization.

21365 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4278.

The product of its digits is 180, while the sum is 17.

The square root of 21365 is about 146.1677118929. The cubic root of 21365 is about 27.7481669649.

Adding to 21365 its reverse (56312), we get a palindrome (77677).

The spelling of 21365 in words is "twenty-one thousand, three hundred sixty-five".