21008720 has 40 divisors (see below), whose sum is σ = 49684320. Its totient is φ = 8259200.

The previous prime is 21008681. The next prime is 21008749. The reversal of 21008720 is 2780012.

It is a tau number, because it is divible by the number of its divisors (40).

It is a Harshad number since it is a multiple of its sum of digits (20).

It is a junction number, because it is equal to n+sod(n) for n = 21008692 and 21008701.

It is a congruent number.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 2495 + ... + 6945.

It is an arithmetic number, because the mean of its divisors is an integer number (1242108).

Almost surely, 2^21008720 is an apocalyptic number.

21008720 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 21008720, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (24842160).

21008720 is an abundant number, since it is smaller than the sum of its proper divisors (28675600).

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

21008720 is a wasteful number, since it uses less digits than its factorization.

21008720 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4523 (or 4517 counting only the distinct ones).

The product of its (nonzero) digits is 224, while the sum is 20.

The square root of 21008720 is about 4583.5270262103. The cubic root of 21008720 is about 275.9305993678.

Adding to 21008720 its reverse (2780012), we get a palindrome (23788732).

The spelling of 21008720 in words is "twenty-one million, eight thousand, seven hundred twenty".