18919981 has 2 divisors, whose sum is σ = 18919982. Its totient is φ = 18919980.

The previous prime is 18919973. The next prime is 18920003. The reversal of 18919981 is 18991981.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 11329956 + 7590025 = 3366^2 + 2755^2 .

It is an emirp because it is prime and its reverse (18991981) is a distict prime. It is also a bemirp because it and its reverse can be mirrored producing other two distinct primes, 18661681 and 18616681.

It is a cyclic number.

It is not a de Polignac number, because 18919981 - 2³ = 18919973 is a prime.

It is a super-2 number, since 2×18919981² = 715931362080722, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (18910981) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 9459990 + 9459991.

It is an arithmetic number, because the mean of its divisors is an integer number (9459991).

Almost surely, 2^18919981 is an apocalyptic number.

It is an amenable number.

18919981 is a deficient number, since it is larger than the sum of its proper divisors (1).

18919981 is an equidigital number, since it uses as much as digits as its factorization.

18919981 is an evil number, because the sum of its binary digits is even.

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

The square root of 18919981 is about 4349.7104501334. The cubic root of 18919981 is about 266.4650362982.

It can be divided in two parts, 18919 and 981, that added together give a triangular number (19900 = T₁₉₉).

The spelling of 18919981 in words is "eighteen million, nine hundred nineteen thousand, nine hundred eighty-one".