18213781 has 2 divisors, whose sum is σ = 18213782. Its totient is φ = 18213780.

The previous prime is 18213773. The next prime is 18213803. The reversal of 18213781 is 18731281.

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 13439556 + 4774225 = 3666^2 + 2185^2 .

It is an emirp because it is prime and its reverse (18731281) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 18213781 - 2³ = 18213773 is a prime.

It is a super-2 number, since 2×18213781² = 663483636631922, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (18213751) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 9106890 + 9106891.

It is an arithmetic number, because the mean of its divisors is an integer number (9106891).

Almost surely, 2^18213781 is an apocalyptic number.

It is an amenable number.

18213781 is a deficient number, since it is larger than the sum of its proper divisors (1).

18213781 is an equidigital number, since it uses as much as digits as its factorization.

18213781 is an evil number, because the sum of its binary digits is even.

The product of its digits is 2688, while the sum is 31.

The square root of 18213781 is about 4267.7606540199. The cubic root of 18213781 is about 263.1075860875.

The spelling of 18213781 in words is "eighteen million, two hundred thirteen thousand, seven hundred eighty-one".