21389 has 4 divisors (see below), whose sum is σ = 21756. Its totient is φ = 21024.

The previous prime is 21383. The next prime is 21391. The reversal of 21389 is 98312.

21389 is nontrivially palindromic in base 8.

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 16900 + 4489 = 130^2 + 67^2 .

It is not a de Polignac number, because 21389 - 2¹² = 17293 is a prime.

It is a Duffinian number.

It is a nialpdrome in base 13.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21383) 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, 74 + ... + 219.

It is an arithmetic number, because the mean of its divisors is an integer number (5439).

2²¹³⁸⁹ is an apocalyptic number.

It is an amenable number.

21389 is a deficient number, since it is larger than the sum of its proper divisors (367).

21389 is an equidigital number, since it uses as much as digits as its factorization.

21389 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 366.

The product of its digits is 432, while the sum is 23.

The square root of 21389 is about 146.2497863246. The cubic root of 21389 is about 27.7585532166.

Adding to 21389 its sum of digits (23), we get a palindrome (21412).

The spelling of 21389 in words is "twenty-one thousand, three hundred eighty-nine".