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commit fba103dca7a472eb58159d45ff8700736f89a136
parent a70f79dfb22e8ea00231f5739f89ecc8d552643f
Author: Mattias Andrée <>
Date:   Thu, 12 May 2016 00:30:53 +0200

On exponentiation

Signed-off-by: Mattias Andrée <>

doc/arithmetic.tex | 66+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
doc/libzahl.tex | 2+-
2 files changed, 66 insertions(+), 2 deletions(-)

diff --git a/doc/arithmetic.tex b/doc/arithmetic.tex @@ -130,7 +130,71 @@ TODO % zdiv zmod zdivmod \section{Exponentiation} \label{sec:Exponentiation} -TODO % zpow zpowu zmodpow zmodpowu +Exponentiation refers to raising a number to +a power. libzahl provides two functions for +regular exponentiation, and two functions for +modular exponentiation. libzahl also provides +a function for raising a number to the second +power, see \secref{sec:Multiplication} for +more details on this. The functions for regular +exponentiation are + +\begin{alltt} + void zpow(z_t power, z_t base, z_t exponent); + void zpowu(z_t, z_t, unsigned long long int); +\end{alltt} + +\noindent +They are identical, except {\tt zpowu} expects +and intrinsic type as the exponent. Both functions +calculate + +\vspace{1em} +$power \gets base^{exponent}$ +\vspace{1em} + +\noindent +The functions for modular exponentiation are +\begin{alltt} + void zmodpow(z_t, z_t, z_t, z_t modulator); + void zmodpowu(z_t, z_t, unsigned long long int, z_t); +\end{alltt} + +\noindent +They are identical, except {\tt zmodpowu} expects +and intrinsic type as the exponent. Both functions +calculate + +\vspace{1em} +$power \gets base^{exponent} \mod modulator$ +\vspace{1em} + +The sign of {\tt modulator} does not affect the +result, {\tt power} will be negative if and only +if {\tt base} is negative and {\tt exponent} is +odd, that is, under the same circumstances as for +{\tt zpow} and {\tt zpowu}. + +These four functions are implemented using +exponentiation by squaring. {\tt zmodpow} and +{\tt zmodpowu} are optimised, they modulate +results for multiplication and squaring at +every multiplication and squaring, rather than +modulating every at the end. Exponentiation +by modulation is a very simple algorithm which +can be expressed as a simple formula + +\vspace{-1em} +\[ \hspace*{-0.4cm} + a^b = \prod_{i = 0}^{\lceil \log_2 b \rceil} + a^{2^i} \left \lfloor {b \over 2^i} \hspace*{-1ex} \mod 2 \right \rfloor +\] + +{\tt zmodpow} does \emph{not} calculate the +modular inverse if the exponent is negative, +rather, you should expect the result to be +1 and 0 depending of whether the base is 1 +or not 1. \newpage diff --git a/doc/libzahl.tex b/doc/libzahl.tex @@ -1,4 +1,4 @@ -\documentclass[11pt,b5paper,openright]{book} +\documentclass[11pt,b5paper,openright,fleqn]{book} \special{papersize=176mm,250mm} \usepackage[utf8]{inputenc} \usepackage[T1]{fontenc}