Re: prime numbers and African artifact

Peter Seebach (seebs@solutions.solon.com)
13 Jul 1995 15:45:20 -0500

In article <3u0rjh$g2g@zen.hursley.ibm.com>,
Martin Bright <mbright@vnet.ibm.com> wrote:
>Peter Seebach (seebs@solutions.solon.com) wrote:
>|> > What's the easy way to show that 254365465431652436514232 is not
>|> >prime again?

|> Simple!

>|> 2 + 5 + 4 + 3 + 6 + 5 + 4 + 6 + 5 + 4 + 3 + 1 +
>|> 6 + 5 + 2 + 4 + 3 + 6 + 5 + 1 + 4 + 2 + 3 + 2 = 91

>|> 9 + 1 = 10.

>|> 1 + 0 = 1.

>|> 1 is not a multiple of three, so the whole number is not a multiple of
>|> three, so it must be prime!

>Also the first two digits aren't divisible by 4, so the whole number isn't
>divisible by 4. This proves again that it is prime.

Actually, this is incorrect. If the *last* two digits are divisible by 4, the
whole number is divisible by 4. However, since they aren't (they're 3 and 2),
the number isn't divisible by four. (The way this works is, since it's not a
multiple of four or a multiple of three, it's not a multiple of (4 * 3), so
it's not a multiple of 12 - like 13, which is also prime.

-s

-- 
Peter Seebach - seebs@solon.com -- seebs@intran.xerox.com
All the arrogant jerks who object to stereotypes are all alike.
C/Unix proto-wizard -- C/Unix questions? Send mail for help.
Copyright 1995 Peter Seebach. Not for distribution through Microsoft Network.