21855 has 16 divisors (see below), whose sum is σ = 36864. Its totient is φ = 11040.

The previous prime is 21851. The next prime is 21859. The reversal of 21855 is 55812.

21855 = 10² + 11² + ... + 40².

21855 is digitally balanced in base 6, because in such base it contains all the possibile digits an equal number of times.

It is an interprime number because it is at equal distance from previous prime (21851) and next prime (21859).

It is not a de Polignac number, because 21855 - 2² = 21851 is a prime.

21855 is strictly pandigital in base 6.

It is a plaindrome in base 4 and base 16.

It is a zygodrome in base 4.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21851) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 442 + ... + 488.

It is an arithmetic number, because the mean of its divisors is an integer number (2304).

2²¹⁸⁵⁵ is an apocalyptic number.

21855 is a deficient number, since it is larger than the sum of its proper divisors (15009).

21855 is a wasteful number, since it uses less digits than its factorization.

21855 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 86.

The product of its digits is 400, while the sum is 21.

The square root of 21855 is about 147.8343667758. The cubic root of 21855 is about 27.9586976088.

It can be divided in two parts, 21 and 855, that multiplied together give a triangular number (17955 = T₁₈₉).

The spelling of 21855 in words is "twenty-one thousand, eight hundred fifty-five".