alt Hacker NewsIt looks like a prime, but can be caught with the second-simplest test: sum of the digits is 12, which is divisible by 3. Hence it's divisible by 3.
(The simplest test being of course if the number is even and bigger than 2)
Edit: now that I think about it, probably should not have tried to impose ordering to the simplicity of tests. There's of course the divisibility by 5 test, which is even simpler.
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In fact, most 2 digit numbers not divisible by 2, 3, or 5 are prime. [1] The only one that's likely to ruin your day is 7 * 13 == 91, but that's self-defeating because after you think about it long enough 91 falls victim to [2].
[1] https://til.andrew-quinn.me/posts/most-2-digit-numbers-not-d...
[2]: https://en.wikipedia.org/wiki/Interesting_number_paradox
John H Conway proved that the smallest number which looks prime, but isn’t is 91. https://youtu.be/S75VTAGKQpk?si=fCGilXECmCOy7T7R
“This is an important theorem, and a result I’m very proud of.”