21972 has 12 divisors (see below), whose sum is σ = 51296. Its totient is φ = 7320.

The previous prime is 21961. The next prime is 21977. The reversal of 21972 is 27912.

It is a happy number.

It is a tau number, because it is divible by the number of its divisors (12).

It is a plaindrome in base 15.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21977) by changing a digit.

21972 is an untouchable number, because it is not equal to the sum of proper divisors of any number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 904 + ... + 927.

2²¹⁹⁷² is an apocalyptic number.

It is an amenable number.

21972 is an abundant number, since it is smaller than the sum of its proper divisors (29324).

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 (25648).

21972 is a wasteful number, since it uses less digits than its factorization.

21972 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1838 (or 1836 counting only the distinct ones).

The product of its digits is 252, while the sum is 21.

The square root of 21972 is about 148.2295517095. The cubic root of 21972 is about 28.0085008202.

The spelling of 21972 in words is "twenty-one thousand, nine hundred seventy-two".