# Is 81 a prime number?

$81$ is not prime but composite.
A number is prime if it would S not divisible by any number other than $1$ and itself. To check one needs to try to divide the number at least up to a prime number, which is just less than its square root.
$81$ has an exact square root in $9$ and hence divisible by $9$ as also $3$ and $27$. It is in fact fourth power of $3$ that is $81 = 3 \times 3 \times 3 \times 3 = {3}^{4}$.
Hence $81$ is not prime but composite.