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johnckirk
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Well, not much to report since my last entry  I've spent most of the…  John C. Kirk
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Jan. 14th, 2003
06:17 pm
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From:
overconvergent
Date:
January 18th, 2003 10:46 am (UTC)
Re: Which was the question ...
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The answer here is to use congruences and repeated squaring:
for example, let's calculate 10^10 mod 13.
10^2 == (3)^2 mod 13 so 10^2 == 9 == 4 mod 13.
So 10^4 == (4)^2 == 3 mod 13, and 10^8 == 9 == 4 mod 13.
Now 10^10=10^2 * 10^ 8, so 10^10 mod 13 == 4 * 4 == 16 == 3 mod 13.
We combine congruences by thinking of a mod b as b*N+a where N is an integer, so we have (13*N4)*(13N4)=13*M+3.
A brief exposition of congruences on the web can be found on
Mathworld
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Re: Which was the question ...
for example, let's calculate 10^10 mod 13.
10^2 == (3)^2 mod 13 so 10^2 == 9 == 4 mod 13.
So 10^4 == (4)^2 == 3 mod 13, and 10^8 == 9 == 4 mod 13.
Now 10^10=10^2 * 10^ 8, so 10^10 mod 13 == 4 * 4 == 16 == 3 mod 13.
We combine congruences by thinking of a mod b as b*N+a where N is an integer, so we have (13*N4)*(13N4)=13*M+3.
A brief exposition of congruences on the web can be found on Mathworld.