18611981 has 2 divisors, whose sum is σ = 18611982. Its totient is φ = 18611980.

The previous prime is 18611951. The next prime is 18611987. The reversal of 18611981 is 18911681.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 17147881 + 1464100 = 4141^2 + 1210^2 .

It is a cyclic number.

It is not a de Polignac number, because 18611981 - 2⁶ = 18611917 is a prime.

It is a super-2 number, since 2×18611981² = 692811673488722, which contains 22 as substring.

18611981 is a strobogrammatic number because it is the same when read upside-down.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (18611987) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 9305990 + 9305991.

It is an arithmetic number, because the mean of its divisors is an integer number (9305991).

Almost surely, 2^18611981 is an apocalyptic number.

It is an amenable number.

18611981 is a deficient number, since it is larger than the sum of its proper divisors (1).

18611981 is an equidigital number, since it uses as much as digits as its factorization.

18611981 is an evil number, because the sum of its binary digits is even.

The product of its digits is 3456, while the sum is 35.

The square root of 18611981 is about 4314.1605208893. The cubic root of 18611981 is about 265.0111826300.

It can be divided in two parts, 186 and 11981, that added together give a cube (12167 = 23³).

The spelling of 18611981 in words is "eighteen million, six hundred eleven thousand, nine hundred eighty-one".