21653720 has 64 divisors (see below), whose sum is σ = 55015200. Its totient is φ = 7598080.

The previous prime is 21653707. The next prime is 21653729. The reversal of 21653720 is 2735612.

It is a self number, because there is not a number n which added to its sum of digits gives 21653720.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21653729) 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 15 ways as a sum of consecutive naturals, for example, 11912 + ... + 13608.

Almost surely, 2^21653720 is an apocalyptic number.

21653720 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 21653720, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (27507600).

21653720 is an abundant number, since it is smaller than the sum of its proper divisors (33361480).

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

21653720 is a wasteful number, since it uses less digits than its factorization.

21653720 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1748 (or 1744 counting only the distinct ones).

The product of its (nonzero) digits is 2520, while the sum is 26.

The square root of 21653720 is about 4653.3557783604. The cubic root of 21653720 is about 278.7260149485.

It can be divided in two parts, 2165 and 3720, that added together give a palindrome (5885).

The spelling of 21653720 in words is "twenty-one million, six hundred fifty-three thousand, seven hundred twenty".