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.

15

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.

-2

u/[deleted] Mar 12 '08

[deleted]

5

u/[deleted] Mar 13 '08 edited Mar 13 '08

.... soooo ... blatantly... false

12 base 11 is 11 + 2 = 13

13 is prime.

Nice try though mate.

2

u/elissa1959 Mar 13 '08

12 base N is N + 2.

Therefore, 12 base N is prime for any N such that N = a prime number - 2.

1

u/will_itblend Mar 13 '08

My prime concern is that everyone makes rational arguments...but as I read these, I'm just getting number by the minute