21990 has 16 divisors (see below), whose sum is σ = 52848. Its totient is φ = 5856.

The previous prime is 21977. The next prime is 21991. The reversal of 21990 is 9912.

It is a happy number.

It is an Ulam number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21991) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 337 + ... + 396.

It is an arithmetic number, because the mean of its divisors is an integer number (3303).

2²¹⁹⁹⁰ is an apocalyptic number.

21990 is an abundant number, since it is smaller than the sum of its proper divisors (30858).

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

It is a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (26424).

21990 is a wasteful number, since it uses less digits than its factorization.

21990 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 743.

The product of its (nonzero) digits is 162, while the sum is 21.

The square root of 21990 is about 148.2902559172. The cubic root of 21990 is about 28.0161471490.

The spelling of 21990 in words is "twenty-one thousand, nine hundred ninety".