18990721 has 2 divisors, whose sum is σ = 18990722. Its totient is φ = 18990720.

The previous prime is 18990707. The next prime is 18990761. The reversal of 18990721 is 12709981.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 14930496 + 4060225 = 3864^2 + 2015^2 .

It is a cyclic number.

It is not a de Polignac number, because 18990721 - 2⁵ = 18990689 is a prime.

It is not a weakly prime, because it can be changed into another prime (18990761) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 9495360 + 9495361.

It is an arithmetic number, because the mean of its divisors is an integer number (9495361).

Almost surely, 2^18990721 is an apocalyptic number.

It is an amenable number.

18990721 is a deficient number, since it is larger than the sum of its proper divisors (1).

18990721 is an equidigital number, since it uses as much as digits as its factorization.

18990721 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 9072, while the sum is 37.

The square root of 18990721 is about 4357.8344392599. The cubic root of 18990721 is about 266.7967190289.

The spelling of 18990721 in words is "eighteen million, nine hundred ninety thousand, seven hundred twenty-one".