this post was submitted on 11 Dec 2024
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[–] [email protected] 22 points 1 week ago (1 children)

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.

[–] [email protected] 15 points 1 week ago* (last edited 1 week ago) (1 children)

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.

[–] [email protected] 2 points 1 week ago (1 children)

Also

Ends in 0,

but that's covered by 5 really.

[–] [email protected] 7 points 1 week ago

It's also covered by being even.