21813 has 8 divisors (see below), whose sum is σ = 31776. Its totient is φ = 13200.

The previous prime is 21803. The next prime is 21817. The reversal of 21813 is 31812.

It is a happy number.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 21813 - 2⁸ = 21557 is a prime.

It is a Curzon number.

It is a junction number, because it is equal to n+sod(n) for n = 21792 and 21801.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21817) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 298 + ... + 363.

It is an arithmetic number, because the mean of its divisors is an integer number (3972).

2²¹⁸¹³ is an apocalyptic number.

It is an amenable number.

21813 is a deficient number, since it is larger than the sum of its proper divisors (9963).

21813 is a wasteful number, since it uses less digits than its factorization.

21813 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 675.

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

The square root of 21813 is about 147.6922475961. The cubic root of 21813 is about 27.9407761824.

Subtracting 21813 from its reverse (31812), we obtain a palindrome (9999).

It can be divided in two parts, 218 and 13, that added together give a triangular number (231 = T₂₁).

The spelling of 21813 in words is "twenty-one thousand, eight hundred thirteen".