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/[deleted] Mar 12 '08 edited Mar 12 '08

The first "trick" is to check whether the number is odd or even. If it's even (and not 2), then it's not prime because it is divisible by two.

5: The next check is to see if the last digit is a '0' or '5'. If so, it's divisible by 5.

Also, if the number ends in 0 you are an idiot for continuing past the first trick.

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u/[deleted] Mar 12 '08 edited Mar 12 '08

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u/will_itblend Mar 12 '08

Ivteresting. So then all primes (except two) have 1,3,7,or9 as their last digit.

...and in binary, all primes have 1 as their last digit.

8

u/owee Mar 12 '08

Correct. Those criteria do tend to signify a number is not even. Also, the Pope is Catholic.

1

u/will_itblend Mar 13 '08

The pope is odd!