21311 has 4 divisors (see below), whose sum is σ = 21624. Its totient is φ = 21000.

The previous prime is 21283. The next prime is 21313. The reversal of 21311 is 11312.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 21311 - 2⁶ = 21247 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21313) 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, 5 + ... + 206.

It is an arithmetic number, because the mean of its divisors is an integer number (5406).

2²¹³¹¹ is an apocalyptic number.

21311 is a deficient number, since it is larger than the sum of its proper divisors (313).

21311 is a wasteful number, since it uses less digits than its factorization.

21311 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 312.

The product of its digits is 6, while the sum is 8.

The square root of 21311 is about 145.9828757081. The cubic root of 21311 is about 27.7247694270.

Adding to 21311 its reverse (11312), we get a palindrome (32623).

Subtracting from 21311 its reverse (11312), we obtain a palindrome (9999).

The spelling of 21311 in words is "twenty-one thousand, three hundred eleven", and thus it is an iban number.