21807721 has 8 divisors (see below), whose sum is σ = 23817024. Its totient is φ = 19845840.

The previous prime is 21807707. The next prime is 21807727. The reversal of 21807721 is 12770812.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21807721 - 2⁷ = 21807593 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21807721.

It is not an unprimeable number, because it can be changed into a prime (21807727) 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, 10891 + ... + 12736.

It is an arithmetic number, because the mean of its divisors is an integer number (2977128).

Almost surely, 2^21807721 is an apocalyptic number.

It is an amenable number.

21807721 is a deficient number, since it is larger than the sum of its proper divisors (2009303).

21807721 is a wasteful number, since it uses less digits than its factorization.

21807721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 23711.

The product of its (nonzero) digits is 1568, while the sum is 28.

The square root of 21807721 is about 4669.8737670305. The cubic root of 21807721 is about 279.3852200283.

The spelling of 21807721 in words is "twenty-one million, eight hundred seven thousand, seven hundred twenty-one".