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\achapter{Nsatz: tactics for proving equalities in $\mathbb{R}$ and $\mathbb{Z}$}
\aauthor{Lo�c Pottier}
The tactic \texttt{nsatz} proves formulas of the form
\[ \begin{array}{l}
\forall X_1,\ldots,X_n \in \mathbb{R},\\
P_1(X_1,\ldots,X_n) = Q_1(X_1,\ldots,X_n) \wedge \ldots \wedge P_s(X_1,\ldots,X_n) =Q_s(X_1,\ldots,X_n)\\
\Rightarrow P(X_1,\ldots,X_n) = Q(X_1,\ldots,X_n)\\
\end{array}
\]
where $P,Q, P_1,Q_1,\ldots,P_s,Q_s$ are polynomials.
The tactic \texttt{nsatzZ} proves the same formulas where the $X_i$ are in $\mathbb{Z}$.
\asection{Using the basic tactic \texttt{nsatz}}
\tacindex{nsatz}
If you work in $\mathbb{R}$, load the
\texttt{NsatzR} module: \texttt{Require
NsatzR}.\\
and use the tactic \texttt{nsatz} or \texttt{nsatzR}.
If you work in $\mathbb{Z}$ do the same thing {\em mutatis mutandis}.
\asection{More about \texttt{nsatz}}
Hilbert's Nullstellensatz theorem shows how to reduce proofs of equalities on
polynomials on a ring R (with no zero divisor) to algebraic computations: it is easy to see that if a polynomial
$P$ in $R[X_1,\ldots,X_n]$ verifies $c P^r = \sum_{i=1}^{s} S_i P_i$, with $c
\in R$, $c \not = 0$, $r$ a positive integer, and the $S_i$s in
$R[X_1,\ldots,X_n]$, then $P$ is zero whenever polynomials $P_1,...,P_s$ are
zero (the converse is also true when R is an algebraic closed field:
the method is complete).
So, proving our initial problem can reduce into finding $S_1,\ldots,S_s$, $c$
and $r$ such that $c (P-Q)^r = \sum_{i} S_i (P_i-Q_i)$, which will be proved by the
tactic \texttt{ring}.
This is achieved by the computation of a Groebner basis of the
ideal generated by $P_1-Q_1,...,P_s-Q_s$, with an adapted version of the Buchberger
algorithm.
The \texttt{NsatzR} module defines the tactics
\texttt{nsatz}, \texttt{nsatzRradical}, \texttt{nsatzRparameters}, and
the generic tactic \texttt{nsatzRpv}, which are used as follows:
\begin{itemize}
\item \texttt{nsatzRpv rmax strategy lparam lvar}:
\begin{itemize}
\item \texttt{rmax} is the maximum r when for searching r s.t.$c (P-Q)^r =
\sum_{i=1}^{s} S_i (P_i - Q_i)$
\item \texttt{strategy} gives the order on variables $X_1,...X_n$ and the strategy of choice for s-polynomials during Buchberger algorithm:
\begin{itemize}
\item strategy = 0: reverse lexicographic order and newest s-polynomial.
\item strategy = 1: reverse lexicographic order and sugar strategy.
\item strategy = 2: pure lexicographic order and newest s-polynomial.
\item strategy = 3: pure lexicographic order and sugar strategy.
\end{itemize}
\item \texttt{lparam} is the list of variables
$X_{i_1},\ldots,X_{i_k}$ among $X_1,...,X_n$ which are considered as
parameters: computation will be performed with rational fractions in these
variables, i.e. polynomials are considered with coefficients in
$R(X_{i_1},\ldots,X_{i_k})$. In this case, the coefficient $c$ can be a non
constant polynomial in $X_{i_1},\ldots,X_{i_k}$, and the tactic produces a goal
which states that $c$ is not zero.
\item \texttt{lvar} is the list of the variables
in the decreasing order in which they will be used in Buchberger algorithm. If \texttt{lvar} = {(@nil
R)}, then \texttt{lvar} is replaced by all the variables which are not in lparam.
\end{itemize}
\item \texttt{nsatzRparameters lparam} is equivalent to
\texttt{nsatzRpv nsatzRpv 6\%N 1\%Z lparam (@nil R)}
\item \texttt{nsatzRradical rmax} is equivalent to
\texttt{nsatzRpv rmax 1\%Z (@nil R) (@nil R)}
\item \texttt{nsatz} is equivalent to
\texttt{nsatzRpv 6\%N 1\%Z (@nil R) (@nil R)}
\end{itemize}
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