21155 has 4 divisors (see below), whose sum is σ = 25392. Its totient is φ = 16920.

The previous prime is 21149. The next prime is 21157. The reversal of 21155 is 55112.

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

It is not a de Polignac number, because 21155 - 2⁴ = 21139 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21157) 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, 2111 + ... + 2120.

It is an arithmetic number, because the mean of its divisors is an integer number (6348).

2²¹¹⁵⁵ is an apocalyptic number.

21155 is a deficient number, since it is larger than the sum of its proper divisors (4237).

21155 is an equidigital number, since it uses as much as digits as its factorization.

21155 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4236.

The product of its digits is 50, while the sum is 14.

The square root of 21155 is about 145.4475850607. The cubic root of 21155 is about 27.6569537363.

Adding to 21155 its reverse (55112), we get a palindrome (76267).

The spelling of 21155 in words is "twenty-one thousand, one hundred fifty-five".