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authorGravatar Maxime Dénès <mail@maximedenes.fr>2018-03-22 11:33:41 +0100
committerGravatar Maxime Dénès <mail@maximedenes.fr>2018-03-22 11:33:41 +0100
commitbb03fa4eaf74e776b94b7a4b93754742d20f6d3d (patch)
tree47cc558fc1eda5854eef4d17cf93d8559bf3629c /doc/refman
parentef0fc7730df3cb585b621955010115407d80d2e7 (diff)
[Sphinx] Move chapter 22 to new infrastructure
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-\achapter{Micromega: tactics for solving arithmetic goals over ordered rings}
-%HEVEA\cutname{micromega.html}
-\aauthor{Frédéric Besson and Evgeny Makarov}
-\newtheorem{theorem}{Theorem}
-
-
-\asection{Short description of the tactics}
-\tacindex{psatz} \tacindex{lra} \tacindex{lia} \tacindex{nia} \tacindex{nra}
-\label{sec:psatz-hurry}
-The {\tt Psatz} module ({\tt Require Import Psatz.}) gives access to
-several tactics for solving arithmetic goals over {\tt Z}, {\tt Q}, and
-{\tt R}:\footnote{Support for {\tt nat} and {\tt N} is obtained by
- pre-processing the goal with the {\tt zify} tactic.}.
-It also possible to get the tactics for integers by a {\tt Require Import Lia}, rationals {\tt Require Import Lqa}
-and reals {\tt Require Import Lra}.
-\begin{itemize}
-\item {\tt lia} is a decision procedure for linear integer arithmetic (see Section~\ref{sec:lia});
-\item {\tt nia} is an incomplete proof procedure for integer non-linear arithmetic (see Section~\ref{sec:nia});
-\item {\tt lra} is a decision procedure for linear (real or rational) arithmetic (see Section~\ref{sec:lra});
-\item {\tt nra} is an incomplete proof procedure for non-linear (real or rational) arithmetic (see Section~\ref{sec:nra});
-\item {\tt psatz D n} where {\tt D} is {\tt Z} or {\tt Q} or {\tt R}, and
- {\tt n} is an optional integer limiting the proof search depth is is an
- incomplete proof procedure for non-linear arithmetic. It is based on
- John Harrison's HOL Light driver to the external prover {\tt
- csdp}\footnote{Sources and binaries can be found at
- \url{https://projects.coin-or.org/Csdp}}. Note that the {\tt csdp}
- driver is generating a \emph{proof cache} which makes it possible to
- rerun scripts even without {\tt csdp} (see Section~\ref{sec:psatz}).
-\end{itemize}
-
-The tactics solve propositional formulas parameterized by atomic arithmetic expressions
-interpreted over a domain $D \in \{\mathbb{Z}, \mathbb{Q}, \mathbb{R} \}$.
-The syntax of the formulas is the following:
-\[
-\begin{array}{lcl}
- F &::=& A \mid P \mid \mathit{True} \mid \mathit{False} \mid F_1 \land F_2 \mid F_1 \lor F_2 \mid F_1 \leftrightarrow F_2 \mid F_1 \to F_2 \mid \neg F\\
- A &::=& p_1 = p_2 \mid p_1 > p_2 \mid p_1 < p_2 \mid p_1 \ge p_2 \mid p_1 \le p_2 \\
- p &::=& c \mid x \mid {-}p \mid p_1 - p_2 \mid p_1 + p_2 \mid p_1 \times p_2 \mid p \verb!^! n
-\end{array}
-\]
-where $c$ is a numeric constant, $x\in D$ is a numeric variable, the
-operators $-$, $+$, $\times$ are respectively subtraction, addition,
-product, $p \verb!^!n $ is exponentiation by a constant $n$, $P$ is an
-arbitrary proposition.
- %
- For {\tt Q}, equality is not Leibniz equality {\tt =} but the equality of rationals {\tt ==}.
-
-For {\tt Z} (resp. {\tt Q} ), $c$ ranges over integer constants (resp. rational constants).
-%% The following table details for each domain $D \in \{\mathbb{Z},\mathbb{Q},\mathbb{R}\}$ the range of constants $c$ and exponent $n$.
-%% \[
-%% \begin{array}{|c|c|c|c|}
-%% \hline
-%% &\mathbb{Z} & \mathbb{Q} & \mathbb{R} \\
-%% \hline
-%% c &\mathtt{Z} & \mathtt{Q} & (see below) \\
-%% \hline
-%% n &\mathtt{Z} & \mathtt{Z} & \mathtt{nat}\\
-%% \hline
-%% \end{array}
-%% \]
-For {\tt R}, the tactic recognizes as real constants the following expressions:
-\begin{verbatim}
-c ::= R0 | R1 | Rmul(c,c) | Rplus(c,c) | Rminus(c,c) | IZR z | IQR q
- | Rdiv(c,c) | Rinv c
-\end{verbatim}
-where {\tt z} is a constant in {\tt Z} and {\tt q} is a constant in {\tt Q}.
-This includes integer constants written using the decimal notation \emph{i.e.,} {\tt c\%R}.
-
-\asection{\emph{Positivstellensatz} refutations}
-\label{sec:psatz-back}
-
-The name {\tt psatz} is an abbreviation for \emph{positivstellensatz} -- literally positivity theorem -- which
-generalizes Hilbert's \emph{nullstellensatz}.
-%
-It relies on the notion of $\mathit{Cone}$. Given a (finite) set of
-polynomials $S$, $\mathit{Cone}(S)$ is inductively defined as the
-smallest set of polynomials closed under the following rules:
-\[
-\begin{array}{l}
-\dfrac{p \in S}{p \in \mathit{Cone}(S)} \quad
-\dfrac{}{p^2 \in \mathit{Cone}(S)} \quad
-\dfrac{p_1 \in \mathit{Cone}(S) \quad p_2 \in \mathit{Cone}(S) \quad
-\Join \in \{+,*\}} {p_1 \Join p_2 \in \mathit{Cone}(S)}\\
-\end{array}
-\]
-The following theorem provides a proof principle for checking that a set
-of polynomial inequalities does not have solutions.\footnote{Variants
- deal with equalities and strict inequalities.}
-\begin{theorem}
- \label{thm:psatz}
- Let $S$ be a set of polynomials.\\
- If ${-}1$ belongs to $\mathit{Cone}(S)$ then the conjunction
- $\bigwedge_{p \in S} p\ge 0$ is unsatisfiable.
-\end{theorem}
-A proof based on this theorem is called a \emph{positivstellensatz} refutation.
-%
-The tactics work as follows. Formulas are normalized into conjunctive normal form $\bigwedge_i C_i$ where
-$C_i$ has the general form $(\bigwedge_{j\in S_i} p_j \Join 0) \to \mathit{False})$ and $\Join \in \{>,\ge,=\}$ for $D\in
-\{\mathbb{Q},\mathbb{R}\}$ and $\Join \in \{\ge, =\}$ for $\mathbb{Z}$.
-%
-For each conjunct $C_i$, the tactic calls a oracle which searches for $-1$ within the cone.
-%
-Upon success, the oracle returns a \emph{cone expression} that is normalized by the {\tt ring} tactic (see chapter~\ref{ring}) and checked to be
-$-1$.
-
-
-\asection{{\tt lra}: a decision procedure for linear real and rational arithmetic}
-\label{sec:lra}
-The {\tt lra} tactic is searching for \emph{linear} refutations using
-Fourier elimination.\footnote{More efficient linear programming
- techniques could equally be employed.} As a result, this tactic
-explores a subset of the $\mathit{Cone}$ defined as
-\[
-\mathit{LinCone}(S) =\left\{ \left. \sum_{p \in S} \alpha_p \times p~\right|
-~\alpha_p \mbox{ are positive constants} \right\}.
-\]
-The deductive power of {\tt lra} is the combined deductive power of {\tt ring\_simplify} and {\tt fourier}.
-%
-There is also an overlap with the {\tt field} tactic {\emph e.g.}, {\tt x = 10 * x / 10} is solved by {\tt lra}.
-
-
-\asection{{\tt lia}: a tactic for linear integer arithmetic}
-\tacindex{lia}
-\label{sec:lia}
-
-The tactic {\tt lia} offers an alternative to the {\tt omega} and {\tt
- romega} tactic (see Chapter~\ref{OmegaChapter}).
-%
-Roughly speaking, the deductive power of {\tt lia} is the combined deductive power of {\tt ring\_simplify} and {\tt omega}.
-%
-However, it solves linear goals that {\tt omega} and {\tt romega} do not solve, such as the
-following so-called \emph{omega nightmare}~\cite{TheOmegaPaper}.
-\begin{coq_example*}
-Goal forall x y,
- 27 <= 11 * x + 13 * y <= 45 ->
- -10 <= 7 * x - 9 * y <= 4 -> False.
-\end{coq_example*}
-\begin{coq_eval}
-intros x y; lia.
-\end{coq_eval}
-The estimation of the relative efficiency of {\tt lia} \emph{vs} {\tt omega}
-and {\tt romega} is under evaluation.
-
-\paragraph{High level view of {\tt lia}.}
-Over $\mathbb{R}$, \emph{positivstellensatz} refutations are a complete
-proof principle.\footnote{In practice, the oracle might fail to produce
- such a refutation.}
-%
-However, this is not the case over $\mathbb{Z}$.
-%
-Actually, \emph{positivstellensatz} refutations are not even sufficient
-to decide linear \emph{integer} arithmetic.
-%
-The canonical example is {\tt 2 * x = 1 -> False} which is a theorem of $\mathbb{Z}$ but not a theorem of $\mathbb{R}$.
-%
-To remedy this weakness, the {\tt lia} tactic is using recursively a combination of:
-%
-\begin{itemize}
-\item linear \emph{positivstellensatz} refutations;
-\item cutting plane proofs;
-\item case split.
-\end{itemize}
-
-\paragraph{Cutting plane proofs} are a way to take into account the discreetness of $\mathbb{Z}$ by rounding up
-(rational) constants up-to the closest integer.
-%
-\begin{theorem}
- Let $p$ be an integer and $c$ a rational constant.
- \[
- p \ge c \Rightarrow p \ge \lceil c \rceil
- \]
-\end{theorem}
-For instance, from $2 x = 1$ we can deduce
-\begin{itemize}
-\item $x \ge 1/2$ which cut plane is $ x \ge \lceil 1/2 \rceil = 1$;
-\item $ x \le 1/2$ which cut plane is $ x \le \lfloor 1/2 \rfloor = 0$.
-\end{itemize}
-By combining these two facts (in normal form) $x - 1 \ge 0$ and $-x \ge
-0$, we conclude by exhibiting a \emph{positivstellensatz} refutation: $-1
-\equiv \mathbf{x-1} + \mathbf{-x} \in \mathit{Cone}(\{x-1,x\})$.
-
-Cutting plane proofs and linear \emph{positivstellensatz} refutations are a complete proof principle for integer linear arithmetic.
-
-\paragraph{Case split} enumerates over the possible values of an expression.
-\begin{theorem}
- Let $p$ be an integer and $c_1$ and $c_2$ integer constants.
- \[
- c_1 \le p \le c_2 \Rightarrow \bigvee_{x \in [c_1,c_2]} p = x
- \]
-\end{theorem}
-Our current oracle tries to find an expression $e$ with a small range $[c_1,c_2]$.
-%
-We generate $c_2 - c_1$ subgoals which contexts are enriched with an equation $e = i$ for $i \in [c_1,c_2]$ and
-recursively search for a proof.
-
-
-\asection{{\tt nra}: a proof procedure for non-linear arithmetic}
-\tacindex{nra}
-\label{sec:nra}
-The {\tt nra} tactic is an {\emph experimental} proof procedure for non-linear arithmetic.
-%
-The tactic performs a limited amount of non-linear reasoning before running the
-linear prover of {\tt lra}.
-This pre-processing does the following:
-\begin{itemize}
-\item If the context contains an arithmetic expression of the form $e[x^2]$ where $x$ is a
- monomial, the context is enriched with $x^2\ge 0$;
-\item For all pairs of hypotheses $e_1\ge 0$, $e_2 \ge 0$, the context is enriched with $e_1 \times e_2 \ge 0$.
-\end{itemize}
-After this pre-processing, the linear prover of {\tt lra} searches for a proof
-by abstracting monomials by variables.
-
-\asection{{\tt nia}: a proof procedure for non-linear integer arithmetic}
-\tacindex{nia}
-\label{sec:nia}
-The {\tt nia} tactic is a proof procedure for non-linear integer arithmetic.
-%
-It performs a pre-processing similar to {\tt nra}. The obtained goal is solved using the linear integer prover {\tt lia}.
-
-\asection{{\tt psatz}: a proof procedure for non-linear arithmetic}
-\label{sec:psatz}
-The {\tt psatz} tactic explores the $\mathit{Cone}$ by increasing degrees -- hence the depth parameter $n$.
-In theory, such a proof search is complete -- if the goal is provable the search eventually stops.
-Unfortunately, the external oracle is using numeric (approximate) optimization techniques that might miss a
-refutation.
-
-To illustrate the working of the tactic, consider we wish to prove the following Coq goal.
-\begin{coq_eval}
-Require Import ZArith Psatz.
-Open Scope Z_scope.
-\end{coq_eval}
-\begin{coq_example*}
-Goal forall x, -x^2 >= 0 -> x - 1 >= 0 -> False.
-\end{coq_example*}
-\begin{coq_eval}
-intro x; psatz Z 2.
-\end{coq_eval}
-Such a goal is solved by {\tt intro x; psatz Z 2}. The oracle returns the
-cone expression $2 \times (\mathbf{x-1}) + (\mathbf{x-1}) \times
-(\mathbf{x-1}) + \mathbf{-x^2}$ (polynomial hypotheses are printed in
-bold). By construction, this expression belongs to $\mathit{Cone}(\{-x^2,
-x -1\})$. Moreover, by running {\tt ring} we obtain $-1$. By
-Theorem~\ref{thm:psatz}, the goal is valid.
-%
-
-%% \paragraph{The {\tt sos} tactic} -- where {\tt sos} stands for \emph{sum of squares} -- tries to prove that a
-%% single polynomial $p$ is positive by expressing it as a sum of squares \emph{i.e.,} $\sum_{i\in S} p_i^2$.
-%% This amounts to searching for $p$ in the cone without generators \emph{i.e.}, $Cone(\{\})$.
-%
-
-
-
-%%% Local Variables:
-%%% mode: latex
-%%% TeX-master: "Reference-Manual"
-%%% End:
diff --git a/doc/refman/Reference-Manual.tex b/doc/refman/Reference-Manual.tex
index 231742e56..4b7f9d91b 100644
--- a/doc/refman/Reference-Manual.tex
+++ b/doc/refman/Reference-Manual.tex
@@ -122,7 +122,6 @@ Options A and B of the licence are {\em not} elected.}
\include{CanonicalStructures.v}%
\include{Classes.v}%
\include{Omega.v}%
-\include{Micromega.v}
\include{Extraction.v}%
\include{Program.v}%
\include{Polynom.v}% = Ring