21978 has 32 divisors (see below), whose sum is σ = 54720. Its totient is φ = 6480.

The previous prime is 21977. The next prime is 21991. The reversal of 21978 is 87912.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is not an unprimeable number, because it can be changed into a prime (21977) 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, 576 + ... + 612.

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

2²¹⁹⁷⁸ is an apocalyptic number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21978, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (27360).

21978 is an abundant number, since it is smaller than the sum of its proper divisors (32742).

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

21978 is a wasteful number, since it uses less digits than its factorization.

21978 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 59 (or 53 counting only the distinct ones).

The product of its digits is 1008, while the sum is 27.

The square root of 21978 is about 148.2497892073. The cubic root of 21978 is about 28.0110500603.

Multiplying 21978 by its reverse (87912), we get a square (1932129936 = 43956²).

It can be divided in two parts, 2197 and 8, that multiplied together give a cube (17576 = 26³).

The spelling of 21978 in words is "twenty-one thousand, nine hundred seventy-eight".