18991981 has 2 divisors, whose sum is σ = 18991982. Its totient is φ = 18991980.

The previous prime is 18991969. The next prime is 18992027. The reversal of 18991981 is 18919981.

It is an a-pointer prime, because the next prime (18992027) can be obtained adding 18991981 to its sum of digits (46).

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 15476356 + 3515625 = 3934^2 + 1875^2 .

It is an emirp because it is prime and its reverse (18919981) is a distict prime. It is also a bemirp because it and its reverse can be mirrored producing other two distinct primes, 18616681 and 18661681.

It is a cyclic number.

It is not a de Polignac number, because 18991981 - 2⁵ = 18991949 is a prime.

It is a super-2 number, since 2×18991981² = 721390684608722, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (18991081) 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 as a sum of consecutive naturals, namely, 9495990 + 9495991.

It is an arithmetic number, because the mean of its divisors is an integer number (9495991).

Almost surely, 2^18991981 is an apocalyptic number.

It is an amenable number.

18991981 is a deficient number, since it is larger than the sum of its proper divisors (1).

18991981 is an equidigital number, since it uses as much as digits as its factorization.

18991981 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 46656, while the sum is 46.

The square root of 18991981 is about 4357.9790040798. The cubic root of 18991981 is about 266.8026193917.

The spelling of 18991981 in words is "eighteen million, nine hundred ninety-one thousand, nine hundred eighty-one".