ROBERT BROOK / SCIENCE PHOTO LIBRARY ROBERT BROOK / SCIENCE PHOTO LIBRARY
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.
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