112 lines
5.8 KiB
TeX
112 lines
5.8 KiB
TeX
\section{Lattice Theory}
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\definitionBody{powerset}{Given a set $S$, we denote its powerset, i.e.\ $\{ x \subseteq S \}$ with $\powerset(S)$. We use $\powersetO(S)$ to denote the powerset of $S$ without the empty set.}
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\definitionBody{poset}{
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We call $\langle S, \lte \rangle$ a {\em poset} (a partially ordered set) if $\lte$ is reflexive, transitive and antisymmetric.
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}
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\definitionBody{lubglb}{An element is an {\em upper or lower bound} of a subset $S$ of a \Poset if it is greater than or equal or less than or equal, respectively, to every element inside $S$.}
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\definitionBody{completelattice}{A {\em complete lattice} is a \Poset $\langle \LL, \lte \rangle$ s.t.\ every subset $S$ of $\LL$ has a unique greatest lower bound $\glb^{\LL} S$ and least upper bound $\lub^{\LL} S$}
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\definitionBody{topbot}{We use $\top^{\LL}$ and $\bot^{\LL}$ to denote $\lub^{\LL} \LL$ and $\glb^{\LL} \LL$ respectively.}
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\definitionBody{monotone}{A function $f: A \rightarrow B$ is {\em monotone} w.r.t. the orderings $\langle A, \lteSub{A} \rangle$ and $\langle B, \lteSub{B} \rangle$ if for all $a_1, a_2 \in A$, $a_1 \lte a_2$ implies $f(a_1) \lte f(a_2)$}
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\definitionBody{image}{Given a function $f: A \rightarrow B$, we use $f[A]$ to denote the {\em image of $f$} w.r.t.\ $A$, i.e.\ the set $\{ f(a) ~|~ a \in A \} \subseteq B$.
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With abuse to notation, when given a set of functions $F$, we write $\bigcup \{ f\image{A} ~|~ f \in F \}$ as $F\image{A}$.}
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\section{Deternministic AFT}
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The original definition of approximators from Denecker et al.\ \cite{denecker2000approximations}, which has been relaxed following Heyninck et al.\ \cite{nondet2}.
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% \begin{definition}
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% \definitionBody{deterministicapproximator}{
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% A {\em deterministic approximator} $o: \LL \rightarrow \LL$ is an \Exact \Monotone function over a \CompleteLattice $\langle \LL, \lte \rangle$
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% }
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% \end{definition}
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\begin{definitionOf}{deterministicapproximator}
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A {\em deterministic approximator} $o: \LL \rightarrow \LL$ is an \Exact \Monotone function over a \CompleteLattice $\langle \LL, \lte \rangle$
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\end{definitionOf}
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\begin{definitionOf}{deterministicstablerevisionoperator}
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Given \AnApproximator $o: \LL \rightarrow \LL$, its {\em stable revision operator $S(o): \LL^2 \rightarrow \LL^2$} as follows:
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\begin{align*}
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S(o)(x, y) \define (\lfp~o(\partialApp, y), \lfp~(x, \partialApp))
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\end{align*}
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\end{definitionOf}
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In later work, Denecker et al.~\cite{DeneckerMT04} establish an alternative theory of AFT based around operators that are restricted to the smaller domain, $\LL^c = \{ (x, y) \in \LL^2 ~|~ x \lte y \}$.
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The set $\L^c$ only contains the pairs that are \ConsistentPair.
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\definitionBody{consistentpair}{A pair $(x, y)$ is {\em consistent} if $x \lte y$.}
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\begin{definitionOf}{deterministicbetaapproximator}
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A {\em $\beta$-approximator} $o \LL^c \rightarrow \LL^c$ is \AnApproximator restricted and closed under $\LL^c$
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\end{definitionOf}
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% Critically, stable revision is defined differently for \BetaApproximators
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In the same work, Denecker et al.~\cite{DeneckerMT04} use a slightly different notion of stable revision.
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Because, for a \BetaApproximator $o$, the function for any $(x, y) \in \LL^c$
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\begin{align*}
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o(\partialApp, y)_1:& \latticeinterval{\bot}{y} \rightarrow \latticeinterval{\bot}{y} \\
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o(x, \partialApp)_2:& \latticeinterval{x}{\top} \rightarrow \latticeinterval{x}{\top}
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\end{align*}
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So naturally, \Definition{deterministicstablerevisionoperator} applied to \BetaApproximators also uses this limited range.
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For our purposes, we adapt their definition to fit \Approximators and \BetaApproximators.
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\begin{definitionOf}{deterministicbetastablerevisionoperator}
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Given \AnApproximator $o: \LL^2 \rightarrow \LL^2$
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\begin{align*}
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\Sdetbeta(o)(x, y)_1 \define \minLattice_{\lte} \big( \fixpointsOf(o(\partialApp, y)_1) \setminus ((x \upclosure) \setminus (y \downclosure)) \big)
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\\
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\Sdetbeta(o)(x, y)_2 \define \minLattice_{\lte} \big( \fixpointsOf(o(x, \partialApp)_2) \setminus ((y \downclosure) \setminus (x \upclosure)) \big)
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\end{align*}
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\end{definitionOf}
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The symmetric version
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\begin{definitionOf}{deterministicbetastablerevisionoperator}
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Given \AnApproximator $o: \LL^2 \rightarrow \LL^2$
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\begin{align*}
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\Sdetbeta(o)(x, y)_1 &\define \minLattice_{\lte} \Bigg( \fixpointsOf(o(\partialApp, y)_1) \setminus \bigg(
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\Big( ((x \glbBinary y) \upclosure) \union ((x \lubBinary y) \downclosure) \Big)
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\setminus
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\Big( ((x \glbBinary y) \upclosure) \intersect ((x \lubBinary y) \downclosure) \Big)
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\bigg) \Bigg)
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\\
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\Sdetbeta(o)(x, y)_2 &\define \Sdetbeta(o)(y, x)_1
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\end{align*}
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\end{definitionOf}
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\begin{propositionOf}{deterministicstablerevisionoperatorsymmetry}
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Given an \Approximator $o: \LL^2 \rightarrow \LL^2$ that is \Symmetric, its \BetaStableRevisionOperator $S(o)$ is also \Symmetric.
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Under symmetry the two are equivalent for consistent partition
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\end{propositionOf}
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\begin{proof}
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By \Symmetry, we have $\fixpointsOf(o(\partialApp, y)_1) = \fixpointsOf(o(x, \partialApp)_2)$
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\end{proof}
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\Proposition{deterministicstablerevisionoperatorsymmetry}
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\begin{definition}
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\AnNdaoO is a $\langle \lte, \lte \rangle$-\Monotone function $o: \LL^2 \rightarrow \powersetO(\LL^2)$ s.t.\ for any $x \in \LL^2$
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\begin{itemize}
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\item $o(x, x)_1 = o(x, x)_2$
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\end{itemize}
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\end{definition}
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What about the bottom half does it matter?
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\begin{lemma}[From Heyninck]
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\AnNdaoO is consistent, for every $(x, y)$, $o(x, y)_1 \times o(x, y)_2$ contains at least one consistent pair.
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\end{lemma}
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\begin{definition}
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Regular stable revision
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\begin{align*}
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S(o)(x, y)_1 \define \minLattice_{\lte}(\fixpointsOf(o(\cdot, y))) \\
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S(o)(x, y)_2 \define \minLattice_{\lte}(\fixpointsOf(o(x, \cdot)))
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\end{align*}
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\end{definition} |