21312720 has 400 divisors, whose sum is σ = 90744192. Its totient is φ = 4561920.

The previous prime is 21312677. The next prime is 21312721. The reversal of 21312720 is 2721312.

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

It is a junction number, because it is equal to n+sod(n) for n = 21312693 and 21312702.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21312721) by changing a digit.

It is a polite number, since it can be written in 79 ways as a sum of consecutive naturals, for example, 926629 + ... + 926651.

Almost surely, 2^21312720 is an apocalyptic number.

21312720 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 21312720, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (45372096).

21312720 is an abundant number, since it is smaller than the sum of its proper divisors (69431472).

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

21312720 is a wasteful number, since it uses less digits than its factorization.

21312720 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 72 (or 57 counting only the distinct ones).

The product of its (nonzero) digits is 168, while the sum is 18.

The square root of 21312720 is about 4616.5701554292. The cubic root of 21312720 is about 277.2551529095.

It can be divided in two parts, 2131 and 2720, that added together give a triangular number (4851 = T₉₈).

The spelling of 21312720 in words is "twenty-one million, three hundred twelve thousand, seven hundred twenty".