18915991 has 4 divisors (see below), whose sum is σ = 19427272. Its totient is φ = 18404712.

The previous prime is 18915989. The next prime is 18916031. The reversal of 18915991 is 19951981.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 18915991 - 2¹ = 18915989 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 (18915901) 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, 255585 + ... + 255658.

It is an arithmetic number, because the mean of its divisors is an integer number (4856818).

Almost surely, 2^18915991 is an apocalyptic number.

18915991 is a deficient number, since it is larger than the sum of its proper divisors (511281).

18915991 is an equidigital number, since it uses as much as digits as its factorization.

18915991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 511280.

The product of its digits is 29160, while the sum is 43.

The square root of 18915991 is about 4349.2517747309. The cubic root of 18915991 is about 266.4463035408.

The spelling of 18915991 in words is "eighteen million, nine hundred fifteen thousand, nine hundred ninety-one".