Circular prime
= Circular prime
A <prime number> for which every cyclic rotation of its <numerical digits> is also a <prime number>. In <decimal representation>, $197$ is an example because $197,971,719$ are all <prime numbers>. A multi-digit <circular prime> can contain only $1,3,7,9$: any other digit can be rotated into the units position and force <divisibility> by $2$ or $5$.