21890 has 16 divisors (see below), whose sum is σ = 43200. Its totient is φ = 7920.

The previous prime is 21881. The next prime is 21893. The reversal of 21890 is 9812.

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

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21893) 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, 11 + ... + 209.

It is an arithmetic number, because the mean of its divisors is an integer number (2700).

2²¹⁸⁹⁰ is an apocalyptic number.

21890 is a deficient number, since it is larger than the sum of its proper divisors (21310).

21890 is a wasteful number, since it uses less digits than its factorization.

21890 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 217.

The product of its (nonzero) digits is 144, while the sum is 20.

The square root of 21890 is about 147.9526951427. The cubic root of 21890 is about 27.9736145997.

The spelling of 21890 in words is "twenty-one thousand, eight hundred ninety".