21781872 has 150 divisors (see below), whose sum is σ = 73549608. Its totient is φ = 6223392.

The previous prime is 21781843. The next prime is 21781873. The reversal of 21781872 is 27818712.

It is a happy number.

It is a powerful number, because all its prime factors have an exponent greater than 1 and also an Achilles number because it is not a perfect power.

It is a Harshad number since it is a multiple of its sum of digits (36).

It is a nude number because it is divisible by every one of its digits.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21781873) by changing a digit.

It is a polite number, since it can be written in 29 ways as a sum of consecutive naturals, for example, 3111693 + ... + 3111699.

Almost surely, 2^21781872 is an apocalyptic number.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21781872, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (36774804).

21781872 is an abundant number, since it is smaller than the sum of its proper divisors (51767736).

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

21781872 is an frugal number, since it uses more digits than its factorization.

21781872 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 55 (or 12 counting only the distinct ones).

The product of its digits is 12544, while the sum is 36.

The square root of 21781872 is about 4667.1053127179. The cubic root of 21781872 is about 279.2747899683.

The spelling of 21781872 in words is "twenty-one million, seven hundred eighty-one thousand, eight hundred seventy-two".