A prime number that remains unchanged when rotated through 180°. An example is 619, which looks the same when read upside-down. To be strobogrammatic, a prime cannot contain digits other than 0, 1, and 8, which have a horizontal line of symmetry (ignoring font variations), and 6 and 9, which are vertical reflections of each other. An invertible prime is one that yields a different prime when the digits are inverted. Of course, these definitions are not taken seriously by mathematicians!
Related categories PRIME NUMBERS
TYPES OF NUMBER
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