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7777777777 is the only ten-digit repdigit such that the product of itself and all truncations of itself minus two is prime. I.e., 7777777777*777777777*77777777*7777777*777777*77777*7777*777*77*7 - 2 is prime. [Patterson]


  Submitted: 2004-01-02 10:21:42;   Last Modified: 2008-01-30 11:28:00.

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