diff --git a/ITP/symmetry-breaking.tex b/ITP/symmetry-breaking.tex index 20e886f..f326f75 100644 --- a/ITP/symmetry-breaking.tex +++ b/ITP/symmetry-breaking.tex @@ -30,16 +30,20 @@ \input{fig-sigma-equiv} Third, the lex-order property breaks symmetry due to \emph{reflection}. -Reflecting a set of points~$S$ through a line (e.g., with the map $(x, y) \mapsto (-x, y)$) +Reflecting a set of points~$S$ over a line (e.g., with the map $(x, y) \mapsto (-x, y)$) preserves the presence of $k$-holes and convex $k$-gons. -Such reflected point sets are almost $\sigma$-equivalent to $S$, -but their orientations have all changed sign. -Consider the point sets in \Cref{fig:sigma-equiv}. -As a result, -we included the \emph{parity} flag in our Lean definition of $\sigma$-equivalence -to capture changes in orientations due to reflection, -where \lstinline|parity := true| changes sign of all orientations. -The lex-order property ensures that the point set is the lexicographically-larger reflection. +This operation does not quite preserve orientations, but rather flips them +(clockwise orientations become counterclockwise and vice versa). +Our definition of $\sigma$-equivalence includes a \emph{parity} flag for this purpose: +\lstinline|parity := false| corresponds to the case that orientations are the same, +and \lstinline|parity := true| corresponds to the case that all orientations have been flipped. +See the point sets in \Cref{fig:sigma-equiv} for an example. + +The lex-order property, then, picks between a set of points and its reflection over \(x=0\). +The vector of consecutive orientations from the middle to the left +is assumed to be at least as big as the vector of consecutive orientations from the middle to the right. +This constraint is not geometrically meaningful, +but is easy to implement in the SAT encoding. We prove that there always exists a $\sigma$-equivalent point set in canonical position.