18391921 has 4 divisors (see below), whose sum is σ = 18400500. Its totient is φ = 18383344.

The previous prime is 18391907. The next prime is 18391937. The reversal of 18391921 is 12919381.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in 2 ways, for example, as 10010896 + 8381025 = 3164^2 + 2895^2 .

It is a cyclic number.

It is not a de Polignac number, because 18391921 - 2⁵ = 18391889 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (18391991) 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 3 ways as a sum of consecutive naturals, for example, 2055 + ... + 6403.

It is an arithmetic number, because the mean of its divisors is an integer number (4600125).

Almost surely, 2^18391921 is an apocalyptic number.

It is an amenable number.

18391921 is a deficient number, since it is larger than the sum of its proper divisors (8579).

18391921 is an equidigital number, since it uses as much as digits as its factorization.

18391921 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 8578.

The product of its digits is 3888, while the sum is 34.

The square root of 18391921 is about 4288.5803012186. The cubic root of 18391921 is about 263.9625798278.

The spelling of 18391921 in words is "eighteen million, three hundred ninety-one thousand, nine hundred twenty-one".