18161981 has 2 divisors, whose sum is σ = 18161982. Its totient is φ = 18161980.

The previous prime is 18161959. The next prime is 18161993. The reversal of 18161981 is 18916181.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 17926756 + 235225 = 4234^2 + 485^2 .

It is an emirp because it is prime and its reverse (18916181) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 18161981 - 2¹⁸ = 17899837 is a prime.

It is a super-2 number, since 2×18161981² = 659715107688722, which contains 22 as substring.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (18161951) 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, 9080990 + 9080991.

It is an arithmetic number, because the mean of its divisors is an integer number (9080991).

Almost surely, 2^18161981 is an apocalyptic number.

It is an amenable number.

18161981 is a deficient number, since it is larger than the sum of its proper divisors (1).

18161981 is an equidigital number, since it uses as much as digits as its factorization.

18161981 is an odious number, because the sum of its binary digits is odd.

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

The square root of 18161981 is about 4261.6875765358. The cubic root of 18161981 is about 262.8579232328.

The spelling of 18161981 in words is "eighteen million, one hundred sixty-one thousand, nine hundred eighty-one".