Comment by karmakurtisaani
9 hours ago
It 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.
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.”
I just noticed that it's 60-3 without any divisibility tests.
Tao's 27 prime was much more embarassing but understandable as he's no a calculator.
Savants are for things like remembering the first million primes. Someone like Tao or Grothendieck can't remeber them beyond 20, but it doesn't mean they can't actuly reason about them.
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