21230721 has 16 divisors (see below), whose sum is σ = 31684800. Its totient is φ = 14049504.

The previous prime is 21230719. The next prime is 21230749. The reversal of 21230721 is 12703212.

It is not a de Polignac number, because 21230721 - 2¹ = 21230719 is a prime.

It is an alternating number because its digits alternate between even and odd.

It is a junction number, because it is equal to n+sod(n) for n = 21230694 and 21230703.

It is not an unprimeable number, because it can be changed into a prime (21230761) 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 15 ways as a sum of consecutive naturals, for example, 925 + ... + 6581.

It is an arithmetic number, because the mean of its divisors is an integer number (1980300).

Almost surely, 2^21230721 is an apocalyptic number.

It is an amenable number.

21230721 is a deficient number, since it is larger than the sum of its proper divisors (10454079).

21230721 is a wasteful number, since it uses less digits than its factorization.

21230721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 5805 (or 5799 counting only the distinct ones).

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

The square root of 21230721 is about 4607.6806529967. The cubic root of 21230721 is about 276.8991235157.

Adding to 21230721 its reverse (12703212), we get a palindrome (33933933).

The spelling of 21230721 in words is "twenty-one million, two hundred thirty thousand, seven hundred twenty-one".