18531991 has 4 divisors (see below), whose sum is σ = 18755352. Its totient is φ = 18308632.

The previous prime is 18531979. The next prime is 18532009. The reversal of 18531991 is 19913581.

It is a happy number.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 19913581 = 167 ⋅119243.

It is a cyclic number.

It is not a de Polignac number, because 18531991 - 2¹¹ = 18529943 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 18531991.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (18531911) 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, 111556 + ... + 111721.

It is an arithmetic number, because the mean of its divisors is an integer number (4688838).

Almost surely, 2^18531991 is an apocalyptic number.

18531991 is a deficient number, since it is larger than the sum of its proper divisors (223361).

18531991 is an equidigital number, since it uses as much as digits as its factorization.

18531991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 223360.

The product of its digits is 9720, while the sum is 37.

The square root of 18531991 is about 4304.8799054097. The cubic root of 18531991 is about 264.6309851360.

It can be divided in two parts, 1853 and 1991, that added together give a square (3844 = 62²).

The spelling of 18531991 in words is "eighteen million, five hundred thirty-one thousand, nine hundred ninety-one".