21720640 has 56 divisors (see below), whose sum is σ = 52303680. Its totient is φ = 8590848.

The previous prime is 21720631. The next prime is 21720649. The reversal of 21720640 is 4602712.

It is an interprime number because it is at equal distance from previous prime (21720631) and next prime (21720649).

It is a plaindrome in base 13.

It is not an unprimeable number, because it can be changed into a prime (21720649) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 32631 + ... + 33289.

Almost surely, 2^21720640 is an apocalyptic number.

21720640 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 21720640, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (26151840).

21720640 is an abundant number, since it is smaller than the sum of its proper divisors (30583040).

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

21720640 is a wasteful number, since it uses less digits than its factorization.

21720640 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 779 (or 769 counting only the distinct ones).

The product of its (nonzero) digits is 672, while the sum is 22.

The square root of 21720640 is about 4660.5407411587. The cubic root of 21720640 is about 279.0128503907.

The spelling of 21720640 in words is "twenty-one million, seven hundred twenty thousand, six hundred forty".