18391908521 has 4 divisors (see below), whose sum is σ = 21019324032. Its totient is φ = 15764493012.

The previous prime is 18391908511. The next prime is 18391908523. The reversal of 18391908521 is 12580919381.

It is a happy number.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also an emirpimes, since its reverse is a distinct semiprime: 12580919381 = 6361 ⋅1977821.

It is a cyclic number.

It is not a de Polignac number, because 18391908521 - 2²⁶ = 18324799657 is a prime.

It is not an unprimeable number, because it can be changed into a prime (18391908523) 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, 1313707745 + ... + 1313707758.

It is an arithmetic number, because the mean of its divisors is an integer number (5254831008).

Almost surely, 2^18391908521 is an apocalyptic number.

It is an amenable number.

18391908521 is a deficient number, since it is larger than the sum of its proper divisors (2627415511).

18391908521 is an equidigital number, since it uses as much as digits as its factorization.

18391908521 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2627415510.

The product of its (nonzero) digits is 155520, while the sum is 47.

The spelling of 18391908521 in words is "eighteen billion, three hundred ninety-one million, nine hundred eight thousand, five hundred twenty-one".