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993305 counterxamples to a "357 rule"

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  • djbroadhurst
    ... In less than 20 seconds, I found 993305 counterexamples in tables from William Galway and Richard Pinch. There are 319226 numbers N
    Message 1 of 4 , Feb 2, 2010
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      --- In primenumbers@yahoogroups.com,
      Bill Bouris <leavemsg1@...> wrote:

      > if 'N' passes the 2-PRP test, then...
      > if 3, 5, or 7 divides 'N', then 'N' is composite...
      > (or) if either ALL or NONE of them divide 'N-1',
      > then a simple 3-PRP will conclude that 'N' is composite,
      > without a doubt.

      In less than 20 seconds, I found 993305 counterexamples
      in tables from William Galway and Richard Pinch.

      There are 319226 numbers N < 10^15 that are 2-PSP and 3-PSP.
      Of these, 287672 are coprime to 3*5*7.
      Of these, 119450 have N-1 divisible by 3*5*7
      and another 5430 have N-1 coprime to 3*5*7
      giving us 124880 counterexamples below 10^15.

      There are 1296432 Carmichael numbers C between 10^15 and 10^18.
      Of these, 1131205 are coprime to 3*5*7.
      Of these, 868340 have C-1 divisible by 3*5*7
      and another 85 have C-1 coprime to 3*5*7
      giving us 868425 additional counterexamples.

      Tally and timing from Pari-GP:
      124880 + 868425 = 993305 counterexamples in 19320 milliseconds

      David
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