18691681 has 4 divisors (see below), whose sum is σ = 18998164. Its totient is φ = 18385200.

The previous prime is 18691679. The next prime is 18691693. The reversal of 18691681 is 18619681.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 18619681 = 2137 ⋅8713.

It can be written as a sum of positive squares in 2 ways, for example, as 2531281 + 16160400 = 1591^2 + 4020^2 .

It is a cyclic number.

It is not a de Polignac number, because 18691681 - 2¹ = 18691679 is a prime.

It is a super-2 number, since 2×18691681² = 698757877211522, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (18691601) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 153150 + ... + 153271.

It is an arithmetic number, because the mean of its divisors is an integer number (4749541).

Almost surely, 2^18691681 is an apocalyptic number.

It is an amenable number.

18691681 is a deficient number, since it is larger than the sum of its proper divisors (306483).

18691681 is an equidigital number, since it uses as much as digits as its factorization.

18691681 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 306482.

The product of its digits is 20736, while the sum is 40.

The square root of 18691681 is about 4323.3876763483. The cubic root of 18691681 is about 265.3889198554.

The spelling of 18691681 in words is "eighteen million, six hundred ninety-one thousand, six hundred eighty-one".