• Elvith Ma'for@feddit.org
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    8 days ago

    The digit sum of 51 is 6, which is divisible by three. So 51 is also divisible by three. It’s not even hard to see that it’s not prime.

    • sugar_in_your_tea@sh.itjust.works
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      7 days ago

      Yup, my personal prime check is:

      1. Even?
      2. Ends in 5?
      3. Digits sum to 3?

      If it fails all three and it’s not an “obvious” prime (<20), then it’s prime enough for me.