18981778 has 4 divisors (see below), whose sum is σ = 28472670. Its totient is φ = 9490888.

The previous prime is 18981773. The next prime is 18981821. The reversal of 18981778 is 87718981.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 87718981 = 7 ⋅12531283.

It can be written as a sum of positive squares in only one way, i.e., 9903609 + 9078169 = 3147^2 + 3013^2 .

It is a Smith number, since the sum of its digits (49) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is not an unprimeable number, because it can be changed into a prime (18981773) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 4745443 + ... + 4745446.

Almost surely, 2^18981778 is an apocalyptic number.

18981778 is a deficient number, since it is larger than the sum of its proper divisors (9490892).

18981778 is an equidigital number, since it uses as much as digits as its factorization.

18981778 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 9490891.

The product of its digits is 225792, while the sum is 49.

The square root of 18981778 is about 4356.8082353943. The cubic root of 18981778 is about 266.7548329997.

The spelling of 18981778 in words is "eighteen million, nine hundred eighty-one thousand, seven hundred seventy-eight".