21870 has 32 divisors (see below), whose sum is σ = 59040. Its totient is φ = 5832.

The previous prime is 21863. The next prime is 21871. The reversal of 21870 is 7812.

21870 is digitally balanced in base 6, because in such base it contains all the possibile digits an equal number of times.

It is a Harshad number since it is a multiple of its sum of digits (18).

It is an Ulam number.

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

21870 is strictly pandigital in base 6.

It is a plaindrome in base 16.

It is a nialpdrome in base 9.

It is a zygodrome in base 9.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21871) 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, 4372 + ... + 4376.

It is an arithmetic number, because the mean of its divisors is an integer number (1845).

21870 is a Friedman number, since it can be written as 270*81, using all its digits and the basic arithmetic operations.

2²¹⁸⁷⁰ is an apocalyptic number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21870, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (29520).

21870 is an abundant number, since it is smaller than the sum of its proper divisors (37170).

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

21870 is an frugal number, since it uses more digits than its factorization.

21870 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 28 (or 10 counting only the distinct ones).

The product of its (nonzero) digits is 112, while the sum is 18.

The square root of 21870 is about 147.8850905264. The cubic root of 21870 is about 27.9650925536.

The spelling of 21870 in words is "twenty-one thousand, eight hundred seventy".