diff --git a/generaldegree.tex b/generaldegree.tex index cd573da..7c05c59 100644 --- a/generaldegree.tex +++ b/generaldegree.tex @@ -4,11 +4,9 @@ \usepackage{amsthm} \usepackage{amsmath} \usepackage{tikz} -\usepackage{tikz-cd} -\usetikzlibrary{positioning} -\usetikzlibrary{arrows.meta} -\usetikzlibrary{decorations.pathmorphing} -\usetikzlibrary{shapes.multipart} + +\usetikzlibrary{cd, positioning, arrows.meta, decorations.pathmorphing, shapes.multipart, +decorations.markings, calc} \usepackage{hyperref} @@ -26,6 +24,7 @@ \newcommand{\ZZ}{\mathbb{Z}} \numberwithin{equation}{section} +\tikzset{->-/.style={decoration={markings, mark=at position #1 with {\arrow{>}}},postaction={decorate}}} %opening \title{Complement of a projective curve} @@ -67,6 +66,8 @@ The space $S_d$ is a strong deformation retract of $M_d$. We start by getting a retraction $r:M_d\longrightarrow S_d$ for $d=1$. Note that for $d=1$ the result is trivial since $M_d$ is an affine complex plane (contractible) and $S_d$ is a closed triangle in an affine real plane. The key point is that in this case the retraction is easily constructed and we will lift it for $d>1$. + + We can construct $r$ in two steps: \begin{equation*} \begin{tikzcd} @@ -74,34 +75,64 @@ M_1\rar["r_1"]\ar[dr, "r"]&M_1\cap\RR^3\dar["r_2"]\\ &S_1. \end{tikzcd} \end{equation*} -Let $r_1:M_1 \longrightarrow M_1\cap\RR^3$ and $r_2:M_1\cap\RR^3\longrightarrow S_1$. Then $r_1$ is given by: +%Let $r_1:M_1 \longrightarrow M_1\cap\RR^3$ and $r_2:M_1\cap\RR^3\longrightarrow S_1$. +The map $r_1$ is given by: \begin{equation} - \begin{array}{ccc} - r_1: M_1\times I & \longrightarrow & M_1\cap\RR^3 \\ - (x_r+ix_i,y_r+iy_i,z_r+iz_i, t) & \longrightarrow & (x_r+(1-t)ix_i,y_r+(1-t)iy_i,z_r+(1-t)iz_i) \\ - \end{array} +% \begin{array}{ccc} +% r_1: M_1\times I & \longrightarrow & M_1\cap\RR^3 \\ +% (x_r+ix_i,y_r+iy_i,z_r+iz_i, t) & \longrightarrow & (x_r+(1-t)ix_i,y_r+(1-t)iy_i,z_r+(1-t)iz_i) \\ +% \end{array} +\begin{tikzcd}[row sep=0pt,/tikz/column 1/.append style={anchor=base east}, /tikz/column 2/.append style={anchor=base west}] +M_1\times{[0,1]}\rar["r_1"]&M_1\cap\RR^3\\ +(P, t)\rar[mapsto]& t \Re P + (1 - t) P. +\end{tikzcd} \end{equation} -$M_1\cap\RR^3$ is a plane in $\RR^3$, and $S_1$ can be written as $S_1=\{(x,y,z)\in\RR_{\geq0}^3|x+y+z=1\}=\{(x,y,z)\in[0,1]^3|x+y+z=1\}$, this is, $S_1$ is a triangular section of the plane defined by the intersection of this plane with the coordinated planes $\{x=0\}$, $\{y=0\}$, $\{z=0\}$. The intersection with these planes divides $M_1\cap\RR^3$ into seven parts, described below, and we will retract each of them separately: +This map is a well-defined deformation retract since $M_1$ has real equations. Note that $M_1\cap\RR^3$ +$M_1\cap\RR^3$ is a plane in $\RR^3$ as in Figure~\ref{fig:triangulo}, and $S_1$ is +\[ +S_1=\{(x,y,z)\in\RR_{\geq0}^3|\,x+y+z=1\}=\{(x,y,z)\in[0,1]^3|\,x+y+z=1\}, +\] +i.e, $S_1$ is the triangular section defined by the intersection of this plane with the coordinated planes $\{x=0\}$, $\{y=0\}$, $\{z=0\}$. The intersection with these planes divides $M_1\cap\RR^3$ into seven parts, described in Figure~\ref{fig:triangulo}. We will retract each of them separately: + \begin{figure}[h] \begin{center} \begin{tikzpicture}[scale=0.7,every text node part/.style={align=center}] - \draw (3,4.5) to (-6,-9); - \draw (-3,4.5) to (6,-9); - \draw (-7,-4.5) to (7,-4.5); - \node at (0,-2) {$x\geq0$,\\ $y\geq0$,\\ $z\geq0$}; - \node at (4,-2) {$x\leq0$,\\ $y\geq0$,\\ $z\geq0$}; - \node at (5.5,-6) {$x\leq0$,\\ $y\geq0$,\\ $z\leq0$}; - \node at (0,-6) {$x\geq0$,\\ $y\geq0$,\\ $z\leq0$}; - \node at (-5.5,-6) {$x\geq0$,\\ $y\leq0$,\\ $z\leq0$}; - \node at (-4,-2) {$x\geq0$,\\ $y\leq0$,\\ $z\geq0$}; - \node at (0,2) {$x\leq0$,\\ $y\leq0$,\\ $z\geq0$}; - \node[right] at (0,0) {(0,0,1)}; - \node[right,above] at (3,-4.5) {(0,1,0)}; - \node[left,above] at (-3,-4.5) {(1,0,0)}; +\coordinate (A) at (-3,-4.5); +\coordinate (B) at (3,-4.5); +\coordinate (C) at (0,0); +\fill[green!25!white] (C) to (B) to (A); + \draw ($2*(C)-(A)$) to ($2*(A)-(C)$); + \draw ($2*(B)-(A)$) to ($2*(A)-(B)$); + \draw ($2*(B)-(C)$) to ($2*(C)-(B)$); + \node at (0,-2.5) {$(+,+,+)$};%{$x\geq0$,\\ $y\geq0$,\\ $z\geq0$}; + \node at (5.5,0) {$(-,+,+)$};%{$x\leq0$,\\ $y\geq0$,\\ $z\geq0$}; + \node at (7.5,-6.5) {$(-,+,-)$};%{$x\leq0$,\\ $y\geq0$,\\ $z\leq0$}; + \node at (0,-8.5) {$(+.+,-)$};%{$x\geq0$,\\ $y\geq0$,\\ $z\leq0$}; + \node at (-8,-6.5) {$(+.-,-)$};%{$x\geq0$,\\ $y\leq0$,\\ $z\leq0$}; + \node at (-6,0) {$(+.-,+)$};%{$x\geq0$,\\ $y\leq0$,\\ $z\geq0$}; + \node at (0,4) {$(-,-,+)$};% {$x\leq0$,\\ $y\leq0$,\\ $z\geq0$}; + \foreach \x[evaluate={\y=1-\x}] in {.125, .25, ..., 1} + { +\draw[->-=.5,green!75!black] ($1.75*(C) -.75*\x*(A) -.75*\y*(B)$) -- (C); +\draw[->-=.5,green!75!black] ($1.75*(B) -.75*\x*(A) -.75*\y*(C)$) -- (B); +\draw[->-=.5,green!75!black] ($1.75*(A) -.75*\x*(C) -.75*\y*(B)$) -- (A); +} + \foreach \x[evaluate={\y=1-\x}] in {0.25, .3125, ..., .75} + { +\draw[->-=.5,green!75!black] ($1.75*\x*(C) -.75*(A) + 1.75*\y*(B)$) -- ($\x*(C)+ \y*(B)$); +\draw[->-=.5,green!75!black] ($1.75*\x*(A) -.75*(B) + 1.75*\y*(C)$) -- ($\x*(A)+ \y*(C)$); +\draw[->-=.5,green!75!black] ($1.75*\x*(A) -.75*(C) + 1.75*\y*(B)$) -- ($\x*(A)+ \y*(B)$); +} +%\fill (6, -4.5) circle[radius=.1cm]; + \node[right=2pt] at (C) {(0,0,1)}; + \node[above right] at (B) {(0,1,0)}; + \node[above left] at (A) {(1,0,0)}; \end{tikzpicture} +\caption{Triangle $K$} +\label{fig:triangulo} \end{center} \end{figure} -We denote each section as $(\pm,\pm,\pm)$ indicating which coordinates are greater or equal to $0$ and which are lower or equal to $0$. For example, the triangle corresponding to $S_1$ would be $(+,+,+)$. +We denote each section as $(\pm,\pm,\pm)$ indicating which coordinates are greater or equal to $0$ and which are lower or equal to $0$. For example, the triangle corresponding to $S_1$ is $(+,+,+)$. The retraction in $(+,+,+)$ is the identity. For the rest of the parts, we define a retraction: \begin{equation}