18009721 has 4 divisors (see below), whose sum is σ = 18066852. Its totient is φ = 17952592.

The previous prime is 18009683. The next prime is 18009763. The reversal of 18009721 is 12790081.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 12790081 = 487 ⋅26263.

It can be written as a sum of positive squares in 2 ways, for example, as 10169721 + 7840000 = 3189^2 + 2800^2 .

It is a cyclic number.

It is not a de Polignac number, because 18009721 - 2²¹ = 15912569 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 18009721.

It is not an unprimeable number, because it can be changed into a prime (18009421) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 28090 + ... + 28723.

It is an arithmetic number, because the mean of its divisors is an integer number (4516713).

Almost surely, 2^18009721 is an apocalyptic number.

It is an amenable number.

18009721 is a deficient number, since it is larger than the sum of its proper divisors (57131).

18009721 is an equidigital number, since it uses as much as digits as its factorization.

18009721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 57130.

The product of its (nonzero) digits is 1008, while the sum is 28.

The square root of 18009721 is about 4243.7861633216. The cubic root of 18009721 is about 262.1213091288.

The spelling of 18009721 in words is "eighteen million, nine thousand, seven hundred twenty-one".