Is This Prime? • Hacker News Board

Re: Is This Prime?

#31

Post by Igelau » Thu, Jul 15, 2021, 10:06 PM UTC

drdec wrote: post #25

X6S1x6Okd1st wrote: post #9

They've included 1 as not a prime. That was a fairly recent decision.

One is not a prime number. If you allow one to be a prime number, then you can no longer say that each natural number has a unique prime factorization. This makes the concept of prime numbers much more useful when one is excluded.

> If you allow one to be a prime number, then you can no longer say that each natural number has a unique prime factorization.

I don't dispute that, by definition, 1 is not prime, but I don't see how this statement would follow if we considered it prime.

Edit: it seems more like it would be that every factorization would implicitly have 1^n tacked onto it, and while that isn't exactly useful, it doesn't break the game.

Re: Is This Prime?

#32

Post by [deleted] » Thu, Jul 15, 2021, 10:10 PM UTC

ridiculous_fish wrote: post #19

Here is a "theorem" I learned: every number up to 100 which looks prime, is prime, except 91. Does anyone recall its name?

[deleted]

Re: Is This Prime?

#33

Post by mcintyre1994 » Thu, Jul 15, 2021, 10:11 PM UTC

jkingsbery wrote: post #22

Great game! Keyboard short cuts would make it a bit better.

The help text says y and n work, I’m on mobile and haven’t tried though.

Re: Is This Prime?

#34

Post by hughdbrown » Thu, Jul 15, 2021, 10:13 PM UTC

jkingsbery wrote: post #22

Great game! Keyboard short cuts would make it a bit better.

Y and N for yes and no.

Re: Is This Prime?

#35

Post by tokamak-teapot » Thu, Jul 15, 2021, 10:14 PM UTC

antman wrote: post #20

HeavenFox wrote: post #18

When I was a fifth grader in China, we were required to memorize all prime numbers below 100. Curious if that is common in other countries?

I don't think so. Remember any other interesting numbers list you needed to memorize?

I learned all the integers.

Re: Is This Prime?

#36

Post by faeyanpiraat » Thu, Jul 15, 2021, 10:16 PM UTC

HeavenFox wrote: post #18

When I was a fifth grader in China, we were required to memorize all prime numbers below 100. Curious if that is common in other countries?

How is that useful?

Re: Is This Prime?

#37

Post by bbx » Thu, Jul 15, 2021, 10:18 PM UTC

ridiculous_fish wrote: post #19

Here is a "theorem" I learned: every number up to 100 which looks prime, is prime, except 91. Does anyone recall its name?

I agree. And I would add 51 to that.

Re: Is This Prime?

#38

Post by Zababa » Thu, Jul 15, 2021, 10:20 PM UTC

HeavenFox wrote: post #18

When I was a fifth grader in China, we were required to memorize all prime numbers below 100. Curious if that is common in other countries?

I don't think that was a thing in France (at least where I went to school) but we often used them indirectly to simplify fractions, so all in all maybe we had to know all primes under 25 I think?

Re: Is This Prime?

#39

Post by the_arun » Thu, Jul 15, 2021, 10:22 PM UTC

lalaithion wrote: post #17

This reminds me of my favorite in-person magic trick to do. First, memorize all the two-digit primes. 25 numbers isn't that hard to memorize. Then, tell someone "Oh, I can instantly tell whether a number is prime or not. Give me a number, I'll tell you whether it's prime." If they tell you a number between 1 and 100, use your memorized list. Otherwise, it's a game of cold reading; if they just generated a random stri…

After 8 attempts, I ran into "Game Over". Was just thinking about the satisfaction for the author of this site. There is just enormous fun in saying - Game Over - to another human :)

Re: Is This Prime?

#40

Post by TheDong » Thu, Jul 15, 2021, 10:23 PM UTC

bbx wrote: post #37

ridiculous_fish wrote: post #19

Here is a "theorem" I learned: every number up to 100 which looks prime, is prime, except 91. Does anyone recall its name?

I agree. And I would add 51 to that.

51's an easy one because 5+1=6, and 6 is divisible by 3, so it must be divisible by 3.

It's easy to try 'divisible by 5' (ends in 5 or 0) and 'divisible by 3' (sum of digits is divisible by 3). 91 isn't found as prime by those two tests, so it needs the extra exceptional rule.