18987991 has 8 divisors (see below), whose sum is σ = 20772864. Its totient is φ = 17212920.

The previous prime is 18987977. The next prime is 18988003. The reversal of 18987991 is 19978981.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 18987991 - 2⁵ = 18987959 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (18987971) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 1960 + ... + 6466.

It is an arithmetic number, because the mean of its divisors is an integer number (2596608).

Almost surely, 2^18987991 is an apocalyptic number.

18987991 is a gapful number since it is divisible by the number (11) formed by its first and last digit.

18987991 is a deficient number, since it is larger than the sum of its proper divisors (1784873).

18987991 is a wasteful number, since it uses less digits than its factorization.

18987991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4901.

The product of its digits is 326592, while the sum is 52.

The square root of 18987991 is about 4357.5211990305. The cubic root of 18987991 is about 266.7839340141.

It can be divided in two parts, 1898 and 7991, that added together give a palindrome (9889).

The spelling of 18987991 in words is "eighteen million, nine hundred eighty-seven thousand, nine hundred ninety-one".