21222720 has 84 divisors (see below), whose sum is σ = 73007220. Its totient is φ = 5658624.

The previous prime is 21222697. The next prime is 21222739. The reversal of 21222720 is 2722212.

It is a happy number.

It can be written as a sum of positive squares in 2 ways, for example, as 14379264 + 6843456 = 3792^2 + 2616^2 .

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

It is a zygodrome in base 4.

It is a junction number, because it is equal to n+sod(n) for n = 21222693 and 21222702.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 805 + ... + 6564.

Almost surely, 2^21222720 is an apocalyptic number.

21222720 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21222720, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (36503610).

21222720 is an abundant number, since it is smaller than the sum of its proper divisors (51784500).

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

21222720 is a wasteful number, since it uses less digits than its factorization.

21222720 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7392 (or 7379 counting only the distinct ones).

The product of its (nonzero) digits is 224, while the sum is 18.

The square root of 21222720 is about 4606.8123469488. The cubic root of 21222720 is about 276.8643351187.

Adding to 21222720 its reverse (2722212), we get a palindrome (23944932).

The spelling of 21222720 in words is "twenty-one million, two hundred twenty-two thousand, seven hundred twenty".