Wrong results with "mod"

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Matthew Durkin
Matthew Durkin el 10 de Feb. de 2019
Respondida: John D'Errico el 10 de Feb. de 2019
Why does Matlab give me the wrong answer to mod(14^27,55) = 32, it should equal 9. What am I doing wrong?
  2 comentarios
Matthew Durkin
Matthew Durkin el 10 de Feb. de 2019
Never mind, I found out I need to use "powermod" to get the answer.
the cyclist
the cyclist el 10 de Feb. de 2019
Maybe you should copy your comment to an answer, and accept it.

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John D'Errico
John D'Errico el 10 de Feb. de 2019
This fails because MATLAB cannot represent an integer larger than 2^53-1 as an exact integer, when working in double precision.
14^27 is far larger than that of course. So it fails. This leaves you with two basic options, to use symbolic arithmetic, (or perhaps my VPI toolbox) or to use powermod.
mod(sym(14)^27,55)
ans =
9
powermod(14,27,55)
ans =
9
The powermod solution is better of course, in the sense that the huge number 14^27 never had to be fully computed.

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