19998991 has 2 divisors, whose sum is σ = 19998992. Its totient is φ = 19998990.

The previous prime is 19998989. The next prime is 19999013. The reversal of 19998991 is 19989991.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 19998991 - 2¹ = 19998989 is a prime.

Together with 19998989, it forms a pair of twin primes.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (19998491) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 9999495 + 9999496.

It is an arithmetic number, because the mean of its divisors is an integer number (9999496).

Almost surely, 2^19998991 is an apocalyptic number.

19998991 is a deficient number, since it is larger than the sum of its proper divisors (1).

19998991 is an equidigital number, since it uses as much as digits as its factorization.

19998991 is an odious number, because the sum of its binary digits is odd.

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

The square root of 19998991 is about 4472.0231439473. The cubic root of 19998991 is about 271.4371968371.

The spelling of 19998991 in words is "nineteen million, nine hundred ninety-eight thousand, nine hundred ninety-one".