Circular prime numbers arranged in a room These are primes that remain primes when digits are rotated either from the left or right Sometimes only the lowest number is included in the set (for example: 17, but not 71) With the exception of 2 and 5 all circular primes consist only of digits 1, 3, 7, 9 It is conjectured that there are an infinite number of circular primes (although only a small number of circular primes are actually known) This is based on the premise that repunit primes (consisting only of units, and therefore circular by default) are infinite in number