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The smallest example of x-y,xx-y,xxx-y all being prime is with digits x=7 and y=4. So 7-4=3, 77-4=73, 777-4=773 are all primes. It is the only solution because one of three consecutive numbers x..x-y is divisible by 3. [Luhn]

 

  Submitted: 2002-03-15 15:22:37;   Last Modified: 2008-01-30 11:28:00.



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