21090 has 32 divisors (see below), whose sum is σ = 54720. Its totient is φ = 5184.

The previous prime is 21089. The next prime is 21101. The reversal of 21090 is 9012.

It is a happy number.

21090 is digitally balanced in base 6, because in such base it contains all the possibile digits an equal number of times.

It is an alternating number because its digits alternate between even and odd.

21090 is strictly pandigital in base 6.

It is a Curzon number.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 552 + ... + 588.

It is an arithmetic number, because the mean of its divisors is an integer number (1710).

2²¹⁰⁹⁰ is an apocalyptic number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21090, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (27360).

21090 is an abundant number, since it is smaller than the sum of its proper divisors (33630).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21090 is a wasteful number, since it uses less digits than its factorization.

21090 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 66.

The product of its (nonzero) digits is 18, while the sum is 12.

The square root of 21090 is about 145.2239649645. The cubic root of 21090 is about 27.6285987954.

It can be divided in two parts, 210 and 90, that added together give a triangular number (300 = T₂₄).

The spelling of 21090 in words is "twenty-one thousand, ninety".