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