21331 has 4 divisors (see below), whose sum is σ = 21672. Its totient is φ = 20992.

The previous prime is 21323. The next prime is 21341. The reversal of 21331 is 13312.

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

It is a cyclic number.

It is not a de Polignac number, because 21331 - 2³ = 21323 is a prime.

It is a super-2 number, since 2×21331² = 910023122, which contains 22 as substring.

It is a Duffinian number.

21331 is an undulating number in base 16.

21331 is a lucky number.

It is not an unprimeable number, because it can be changed into a prime (21341) 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, 46 + ... + 211.

It is an arithmetic number, because the mean of its divisors is an integer number (5418).

2²¹³³¹ is an apocalyptic number.

21331 is a deficient number, since it is larger than the sum of its proper divisors (341).

21331 is an equidigital number, since it uses as much as digits as its factorization.

21331 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 340.

The product of its digits is 18, while the sum is 10.

The square root of 21331 is about 146.0513608290. The cubic root of 21331 is about 27.7334397853.

Subtracting from 21331 its sum of digits (10), we obtain a triangular number (21321 = T₂₀₆).

Adding to 21331 its reverse (13312), we get a palindrome (34643).

It can be divided in two parts, 2 and 1331, that multiplied together give a palindrome (2662).

The spelling of 21331 in words is "twenty-one thousand, three hundred thirty-one".