21381 has 4 divisors (see below), whose sum is σ = 28512. Its totient is φ = 14252.

The previous prime is 21379. The next prime is 21383. The reversal of 21381 is 18312.

It is a happy number.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is an interprime number because it is at equal distance from previous prime (21379) and next prime (21383).

It is a cyclic number.

It is not a de Polignac number, because 21381 - 2¹ = 21379 is a prime.

It is a super-2 number, since 2×21381² = 914294322, which contains 22 as substring.

It is a D-number.

21381 is a lucky number.

It is a self number, because there is not a number n which added to its sum of digits gives 21381.

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 pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3561 + ... + 3566.

It is an arithmetic number, because the mean of its divisors is an integer number (7128).

2²¹³⁸¹ is an apocalyptic number.

It is an amenable number.

21381 is a deficient number, since it is larger than the sum of its proper divisors (7131).

21381 is an equidigital number, since it uses as much as digits as its factorization.

21381 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7130.

The product of its digits is 48, while the sum is 15.

The square root of 21381 is about 146.2224332994. Note that the first 3 decimals coincide. The cubic root of 21381 is about 27.7550919964.

Adding to 21381 its reverse (18312), we get a palindrome (39693).

It can be divided in two parts, 21 and 381, that multiplied together give a triangular number (8001 = T₁₂₆).

The spelling of 21381 in words is "twenty-one thousand, three hundred eighty-one".