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@ -124,7 +124,7 @@ Examine individual constraints and detect inconsistent partial assignments.
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\begin{example}[Generalized arc consistency]
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Given the variables $D(X_1) = \{ 1, 2, 3 \}$, $D(X_2) = \{ 1, 2 \}$, $D(X_3) = \{ 1, 2 \}$ and
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the constraint $C: \texttt{alldifferent}([X_1, X_2, X_3])$,
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$C$ is not GAC as $1 \in D(X_1)$ and $2 \in D(X_2)$ do not have a support.
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$C$ is not GAC as $1 \in D(X_1)$ and $2 \in D(X_1)$ do not have a support.
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By applying a constraint propagation algorithm, we can reduce the domain to:
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$D(X_1) = \{ \cancel{1}, \cancel{2}, 3 \}$ and $D(X_2) = D(X_3) = \{ 1, 2 \}$.
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@ -185,7 +185,7 @@ Examine individual constraints and detect inconsistent partial assignments.
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\begin{description}
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\item[Constraint propagation] \marginnote{Constraint propagation}
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Algorithm that removes values from the domains of the variables to achieve a given level of consistency.
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Algorithm that removes values from the domain of the variables to achieve a given level of consistency.
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Constraint propagation algorithms interact with each other and already propagated constraints might be woke up again by another constraint.
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Propagation will eventually reach a fixed point.
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@ -357,7 +357,7 @@ Define constraints in an extensive way.
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\begin{description}
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\item[Backtraking tree search] \marginnote{Backtraking tree search}
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\item[Backtracking tree search] \marginnote{Backtracking tree search}
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Tree where nodes are variables and branches are variable assignments.
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\item[Systematic search] \marginnote{Systematic search}
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@ -153,7 +153,7 @@
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\item $p$ with $(\lnot p \vee \lnot r)$ derives $\lnot r$.
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\item $\lnot s$ with $(q \vee s)$ derives $q$.
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\item $\lnot r$ with $(r \vee t)$ derives $t$.
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\item $q$ with $(\lnot q \vee \lnot t)$ derives $\not t$.
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\item $q$ with $(\lnot q \vee \lnot t)$ derives $\lnot t$.
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\item $t$ and $\lnot t$ bring to $\bot$. This branch is UNSAT.
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\end{itemize}
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@ -221,7 +221,7 @@ DPLL with two improvements:
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When a contradiction is found, augment the original CNF with a conflict clause
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to prune the search space.
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The conflict clause is obtained through resolution by combining two clauses that cause the contradiction.
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The conflict clause is obtained through resolution by combining the two clauses that cause the contradiction.
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\item[\texttt{Backjumping}]
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Depending on the type of contradiction, skip decision literals that are not causing the conflict
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@ -180,6 +180,8 @@
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\begin{remark}
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Useful to deal with black-box functions (i.e. prove satisfiability without a specific theory).
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\indenttbox
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\begin{example}
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The following formula can be proved to be unsatisfiable by only using syntactic manipulations of basic FOL concepts:
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\begin{gather*}
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@ -478,6 +480,7 @@ The $\calT$-solver does the following:
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\end{enumerate}
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As we are at decision level 0 (as no decision literal has been fixed), we can conclude that $\varphi$ is unsatisfiable.
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\indenttbox
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\begin{remark}
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Unit and theory propagation are alternated (see algorithm description).
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\end{remark}
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