21320721 has 12 divisors (see below), whose sum is σ = 30889872. Its totient is φ = 14170776.

The previous prime is 21320683. The next prime is 21320743. The reversal of 21320721 is 12702312.

It is not a de Polignac number, because 21320721 - 2⁹ = 21320209 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 21320694 and 21320703.

It is not an unprimeable number, because it can be changed into a prime (21320021) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 291 + ... + 6536.

It is an arithmetic number, because the mean of its divisors is an integer number (2574156).

Almost surely, 2^21320721 is an apocalyptic number.

It is an amenable number.

21320721 is a deficient number, since it is larger than the sum of its proper divisors (9569151).

21320721 is a wasteful number, since it uses less digits than its factorization.

21320721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7180 (or 7177 counting only the distinct ones).

The product of its (nonzero) digits is 168, while the sum is 18.

The square root of 21320721 is about 4617.4366265278. The cubic root of 21320721 is about 277.2898433189.

The spelling of 21320721 in words is "twenty-one million, three hundred twenty thousand, seven hundred twenty-one".