Hacker Newsnew | past | comments | ask | show | jobs | submitlogin

Author here: I am also fascinated by this sense of certain numbers sounding prime (or more prime than others). For instance -- maybe this is just me -- I think 109 (the first winner) doesn't sound prime at all.

But 17! Yes! It can't shut up about how prime it is!




A number theory class I once took provided us with the following "proof", claiming, in fact, that it was the most useful thing we would learn, and would make a great party trick.

Theorem: Anything less than 91 which looks prime is prime.

Proof: To do this, we will calculate the smallest number that looks prime but isn't. The numbers 2, 3, 4, 5, and 6 all have easy and quick divisibility rules, so our number cannot be divisible by any of them. 7, however, is hard to check for - so clearly the first number that looks prime but isn't will be divisible by 7. Sadly, everyone realizes 49 isn't prime, because it's a square, so we need to find the next difficult prime factor. 8, 9, 10, 11, and 12 are all also subject to quick tests and divisibility rules, so discount those. 13, however, is ugly - thus, the first number that looks prime but isn't is 7 times 13, which equals 91. Q.E.D. (My professor insisted this stood for "Quite Easily Done".)


Isn't this just the inverse of memorizing multiplication tables?


Did you receive any emails choosing composite numbers? To me, 51 sounds prime.


Yes, many! On the submission form, I even referred folks to Wikipedia's list of prime numbers but, alas.


A number is divisible by 3 if the sum of its digits is divisible by 3.


Right. And numbers like 117 and 119 secretly trick (at least) me into thinking they are prime.




Consider applying for YC's Fall 2025 batch! Applications are open till Aug 4

Guidelines | FAQ | Lists | API | Security | Legal | Apply to YC | Contact

Search: