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Why is it that $$7 = 10-3$$ $$77 = 10^2-23$$ $$777 = 10^3- 223$$ It seems that $3,23,223, \dots$ are all prime numbers. Is there any special reason for this?

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    [the strong law of small numbers](http://en.wikipedia.org/wiki/Strong_Law_of_Small_Numbers)2011-11-19

2 Answers 2