18391981 has 4 divisors (see below), whose sum is σ = 19360000. Its totient is φ = 17423964.

The previous prime is 18391979. The next prime is 18391987. The reversal of 18391981 is 18919381.

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.

It is a cyclic number.

It is not a de Polignac number, because 18391981 - 2¹ = 18391979 is a prime.

It is a super-2 number, since 2×18391981² = 676529930208722, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (18391987) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 483981 + ... + 484018.

It is an arithmetic number, because the mean of its divisors is an integer number (4840000).

Almost surely, 2^18391981 is an apocalyptic number.

It is an amenable number.

18391981 is a deficient number, since it is larger than the sum of its proper divisors (968019).

18391981 is an equidigital number, since it uses as much as digits as its factorization.

18391981 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 968018.

The product of its digits is 15552, while the sum is 40.

The square root of 18391981 is about 4288.5872965348. The cubic root of 18391981 is about 263.9628668694.

The spelling of 18391981 in words is "eighteen million, three hundred ninety-one thousand, nine hundred eighty-one".