1981918181 has 2 divisors, whose sum is σ = 1981918182. Its totient is φ = 1981918180.

The previous prime is 1981918171. The next prime is 1981918189. The reversal of 1981918181 is 1818191891.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 1792167556 + 189750625 = 42334^2 + 13775^2 .

It is an emirp because it is prime and its reverse (1818191891) is a distict prime. It is also a bemirp because it and its reverse can be mirrored producing other two distinct primes, 1818161861 and 1681618181.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-1981918181 is a prime.

It is a super-2 number, since 2×1981918181² = 7855999352356697522, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (1981918189) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 990959090 + 990959091.

It is an arithmetic number, because the mean of its divisors is an integer number (990959091).

Almost surely, 2^1981918181 is an apocalyptic number.

It is an amenable number.

1981918181 is a deficient number, since it is larger than the sum of its proper divisors (1).

1981918181 is an equidigital number, since it uses as much as digits as its factorization.

1981918181 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 41472, while the sum is 47.

The square root of 1981918181 is about 44518.7396609563. The cubic root of 1981918181 is about 1256.1126054007.

The spelling of 1981918181 in words is "one billion, nine hundred eighty-one million, nine hundred eighteen thousand, one hundred eighty-one".