21636720 has 280 divisors, whose sum is σ = 87824736. Its totient is φ = 4852224.

The previous prime is 21636709. The next prime is 21636743. The reversal of 21636720 is 2763612.

It is a happy number.

21636720 is a `hidden beast` number, since 2 + 1 + 636 + 7 + 20 = 666.

It is a tau number, because it is divible by the number of its divisors (280).

It is a Harshad number since it is a multiple of its sum of digits (27).

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 55 ways as a sum of consecutive naturals, for example, 408214 + ... + 408266.

Almost surely, 2^21636720 is an apocalyptic number.

21636720 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 21636720, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (43912368).

21636720 is an abundant number, since it is smaller than the sum of its proper divisors (66188016).

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

21636720 is an equidigital number, since it uses as much as digits as its factorization.

21636720 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 91 (or 70 counting only the distinct ones).

The product of its (nonzero) digits is 3024, while the sum is 27.

The square root of 21636720 is about 4651.5287809493. The cubic root of 21636720 is about 278.6530546935.

The spelling of 21636720 in words is "twenty-one million, six hundred thirty-six thousand, seven hundred twenty".