21387 has 4 divisors (see below), whose sum is σ = 28520. Its totient is φ = 14256.

The previous prime is 21383. The next prime is 21391. The reversal of 21387 is 78312.

21387 is digitally balanced in base 4, because in such base it contains all the possibile digits an equal number of times.

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

It is an interprime number because it is at equal distance from previous prime (21383) and next prime (21391).

It is not a de Polignac number, because 21387 - 2² = 21383 is a prime.

It is a D-number.

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, 3562 + ... + 3567.

It is an arithmetic number, because the mean of its divisors is an integer number (7130).

2²¹³⁸⁷ is an apocalyptic number.

21387 is a deficient number, since it is larger than the sum of its proper divisors (7133).

21387 is an equidigital number, since it uses as much as digits as its factorization.

21387 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7132.

The product of its digits is 336, while the sum is 21.

The square root of 21387 is about 146.2429485480. The cubic root of 21387 is about 27.7576879924.

Adding to 21387 its reverse (78312), we get a palindrome (99699).

It can be divided in two parts, 2138 and 7, that added together give a triangular number (2145 = T₆₅).

The spelling of 21387 in words is "twenty-one thousand, three hundred eighty-seven".