21561720 has 64 divisors (see below), whose sum is σ = 66078720. Its totient is φ = 5625984.

The previous prime is 21561697. The next prime is 21561733. The reversal of 21561720 is 2716512.

It is a Harshad number since it is a multiple of its sum of digits (24).

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 3729 + ... + 7551.

It is an arithmetic number, because the mean of its divisors is an integer number (1032480).

Almost surely, 2^21561720 is an apocalyptic number.

21561720 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 21561720, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (33039360).

21561720 is an abundant number, since it is smaller than the sum of its proper divisors (44517000).

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

21561720 is a wasteful number, since it uses less digits than its factorization.

21561720 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3884 (or 3880 counting only the distinct ones).

The product of its (nonzero) digits is 840, while the sum is 24.

The square root of 21561720 is about 4643.4599169154. The cubic root of 21561720 is about 278.3307141956.

The spelling of 21561720 in words is "twenty-one million, five hundred sixty-one thousand, seven hundred twenty".