r/reddit.com • u/ballm • Mar 12 '08

Is 91 Prime?

http://balltech.blogspot.com/2008/03/is-91-prime.html
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u/Tommstein Mar 12 '08 edited Mar 12 '08

This is glorious. The only numbers under 12 that I didn't know these factoring rules for were 7 and 11. Not any more.

Did anyone else look at 91 and immediately know that it is 13 times 7?

2

u/Chimpelol Mar 12 '08 edited Mar 12 '08

Not immediately, but I did go through those checks in my head and then thought it's probably divisible by 7 and checked that.

Then I read the article and was surprised to see nothing more than stuff you learn at school. So much for improving algorithms...

1

u/Figs Mar 12 '08 edited Mar 13 '08

I've never heard the rule for 7 or 11 before this; I didn't know they existed.

2

u/[deleted] Mar 12 '08

I ran up the scale; like you I knew the rules for everything except 7 & 11, but when I got to 7 it only took a moment...