18999991 has 2 divisors, whose sum is σ = 18999992. Its totient is φ = 18999990.

The previous prime is 18999989. The next prime is 18999997. The reversal of 18999991 is 19999981.

18999991 is nontrivially palindromic in base 11.

It is a weak prime.

It is an emirp because it is prime and its reverse (19999981) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 18999991 - 2¹ = 18999989 is a prime.

Together with 18999989, it forms a pair of twin primes.

18999991 is an undulating number in base 11.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (18999997) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 9499995 + 9499996.

It is an arithmetic number, because the mean of its divisors is an integer number (9499996).

Almost surely, 2^18999991 is an apocalyptic number.

18999991 is a deficient number, since it is larger than the sum of its proper divisors (1).

18999991 is an equidigital number, since it uses as much as digits as its factorization.

18999991 is an evil number, because the sum of its binary digits is even.

The product of its digits is 472392, while the sum is 55.

The square root of 18999991 is about 4358.8979111697. The cubic root of 18999991 is about 266.8401227395.

The spelling of 18999991 in words is "eighteen million, nine hundred ninety-nine thousand, nine hundred ninety-one".