diff --git a/ITP/symmetry-breaking.tex b/ITP/symmetry-breaking.tex index c7496e4..bec445c 100644 --- a/ITP/symmetry-breaking.tex +++ b/ITP/symmetry-breaking.tex @@ -11,7 +11,7 @@ there exists a list of points in \emph{canonical position} with the same triple-orientations. Canonical position is defined as follows. \begin{definition}[Canonical Position] -A list of points~$L = (p_1,\ldots, p_{n})$ is said to be in \emph{canonical position} if it satisfies all the following properties: +A list of points~$L = (p_1,\ldots, p_{n})$ is said to be in \emph{canonical position} if it satisfies all of the following properties: \begin{itemize} \item \textbf{(General Position)} No three points are collinear, i.e., for all $1 \leq i < j < k \leq n$, we have $\sigma(p_i, p_j, p_k) \neq 0$. \item \textbf{($x$-order)} The points are sorted with respect to their $x$-coordinates, i.e., $x(p_i) < x(p_j)$ for all $1 \leq i < j \leq n$. diff --git a/ITP/triple-orientations.tex b/ITP/triple-orientations.tex index bb0aa0d..00c3f38 100644 --- a/ITP/triple-orientations.tex +++ b/ITP/triple-orientations.tex @@ -2,7 +2,7 @@ problems concerning the existence of an object~$\mathcal{O}$ in a continuous search space like $\mathbb{R}^2$ must be reformulated in terms of the existence of a discrete, finitely-representable object~$\mathcal{O}'$ that a computer can search for. -It is especially challenging to discretize problems in which the desired geometric object $\mathcal{O}$ is characterized by very specific coordinates of points, +It is especially challenging to discretize problems for which the desired geometric object~$\mathcal{O}$ is characterized by very specific coordinates of points, thus requiring the computer to use floating-point arithmetic, which suffers from numerical instability. Fortunately, @@ -177,7 +177,7 @@ % Let us now discuss the previous steps. First,~\lstinline|(PtInTriangle a p q r)| presents a \emph{ground-truth} definition for membership in a triangle, in terms of~\textsf{mathlib}'s \lstinline|ConvexHull| definition, whereas~\lstinline|(σPtInTriangle a p q r)| is based on orientations. -Heule and Scheucher used the orientation-based definition~\cite{emptyHexagonNumber}, and as it is common in the area, its equivalence to the \emph{ground-truth} mathematical definition was left implicit. +Heule and Scheucher used the orientation-based definition~\cite{emptyHexagonNumber}, and, as is common in the area, its equivalence to the \emph{ground-truth} mathematical definition was left implicit. This equivalence, formalized in~\lstinline|theorem σPtInTriangle_iff| is not trivial to prove: the forward direction in particular requires reasoning about convex combinations and determinants. Using the previous theorem, we can generalize to $k$-gons as follows. @@ -214,7 +214,7 @@ SAT encodings for Erd\H{o}s-Szekeres-type problems use triple orientations to capture properties like convexity and emptiness, thus discretizing the problem. Because we have proved that $\pi_k(S)$ is an orientation property, -the values of $\sigma$ on the points in $S$ contain enough information to determine whether $\pi_k(S)$. +the values of $\sigma$ on the points in $S$ contain enough information to determine whether $\pi_k(S)$ holds. Therefore, we have proved that given $n$ points, it is enough to analyze the values of $\sigma$ over these points, a discrete space with at most $2^{n^3}$ possibilities,