21181 has 4 divisors (see below), whose sum is σ = 21600. Its totient is φ = 20764.

The previous prime is 21179. The next prime is 21187. The reversal of 21181 is 18112.

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 a cyclic number.

It is not a de Polignac number, because 21181 - 2¹ = 21179 is a prime.

It is a super-2 number, since 2×21181² = 897269522, which contains 22 as substring.

It is a Duffinian number.

It is a nialpdrome in base 13 and base 15.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21187) 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, 121 + ... + 238.

It is an arithmetic number, because the mean of its divisors is an integer number (5400).

2²¹¹⁸¹ is an apocalyptic number.

It is an amenable number.

21181 is a deficient number, since it is larger than the sum of its proper divisors (419).

21181 is an equidigital number, since it uses as much as digits as its factorization.

21181 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 418.

The product of its digits is 16, while the sum is 13.

The square root of 21181 is about 145.5369368923. The cubic root of 21181 is about 27.6682794499.

Adding to 21181 its reverse (18112), we get a palindrome (39293).

It can be divided in two parts, 211 and 81, that added together give a palindrome (292).

The spelling of 21181 in words is "twenty-one thousand, one hundred eighty-one".