21555 has 12 divisors (see below), whose sum is σ = 37440. Its totient is φ = 11472.

The previous prime is 21529. The next prime is 21557. The reversal of 21555 is 55512.

It is not a de Polignac number, because 21555 - 2⁵ = 21523 is a prime.

It is a nialpdrome in base 16.

It is not an unprimeable number, because it can be changed into a prime (21557) 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 11 ways as a sum of consecutive naturals, for example, 195 + ... + 284.

It is an arithmetic number, because the mean of its divisors is an integer number (3120).

2²¹⁵⁵⁵ is an apocalyptic number.

21555 is a deficient number, since it is larger than the sum of its proper divisors (15885).

21555 is a wasteful number, since it uses less digits than its factorization.

21555 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 490 (or 487 counting only the distinct ones).

The product of its digits is 250, while the sum is 18.

The square root of 21555 is about 146.8162116389. The cubic root of 21555 is about 27.8301796021.

It can be divided in two parts, 21 and 555, that added together give a square (576 = 24²).

The spelling of 21555 in words is "twenty-one thousand, five hundred fifty-five".