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95fa72ff05
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@ -36,7 +36,7 @@ decorations.markings, calc}
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%opening
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\title[Retraction of the complement of a smooth curve]{Retraction of the complement of smooth projective hypersurfaces to a $n$-dimensional $\Delta$-complex.}
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\author[E. Artal]{Enrique Artal Bartolo}
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\author[E. Artal]{Enrique Artal Bartolo}
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\address{Departamento de Matem\'{a}ticas, IUMA \\
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Universidad de Zaragoza \\
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C.~Pedro Cerbuna 12, 50009, Zaragoza, Spain}
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@ -81,7 +81,7 @@ The retraction is performed in several steps, using the branched cover structure
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\section{Definition of the problem}
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Given $C$ an affine hypersurface in $\CC^2$ (i.e. a curve), Libgober gave in \cite{libgober1986} a construction of a $2$-dimensional CW-complex that is a deformation retract of its complement. However, it is not clear how to extend that construction to the projective case (that is a curve in $\CC\PP^2$).
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In this work, we give a different construction to produce a specific $\Delta$-complex that is a deformation retract of the complement of a Fermat hypersurface in $\CC\PP^n$. Since any smooth hypersurface is isotopic to a Fermat hypersurface, this particular case solves the problem for a general smooth hypersurface.
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In this work, we give a different construction to produce a specific $\Delta$-complex that is a deformation retract of the complement of a Fermat hypersurface in $\CC\PP^n$. Since any smooth hypersurface is isotopic to a Fermat hypersurface, this particular case solves the problemfor a general smooth hypersurface.
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\section{Smooth hypersurfaces}
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@ -108,7 +108,7 @@ We then have a commutative diagram:
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% K & \longhookrightarrow & \PP^n\setminus C_d \\
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% \end{array}
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\begin{tikzcd}
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S_d\rar[hookrightarrow]\dar["\proy{}"]& M_d\dar["\proy{}"]\\
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S_d\rar[hookrightarrow]\dar["\proy{}"]& M_d\dar["\proy{}"]\\
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K\rar[hookrightarrow] & \PP^n\setminus C_d.
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\end{tikzcd}
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\end{equation*}
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@ -130,7 +130,7 @@ i.e. $x^r,x^i\in\RR$ and $x=x^r + i x^i$. This notation also applies to vectors
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% M_1 & \longrightarrow & M_1\cap\RR^3&\longrightarrow&S_1 \\
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% \end{array}
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%\end{equation*}
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We will now define a strong retraction map
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We will now define a strong retraction map
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\begin{equation}
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\begin{tikzcd}[row sep=0pt,/tikz/column 1/.append style={anchor=base east}, /tikz/column 2/.append style={anchor=base west}]
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M_1\times{[0,1]}\rar["r_1"]&M_1\\
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@ -145,11 +145,21 @@ and the restriction of $r_{1t}$ to $M_1\cap\RR^{n+1}$ is the inclusion.
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%where $\Re$ and $\Im$ take the real and imaginary part of each coordinate of a point, respectively.
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Note that for $(x_0,\dots,x_n)\in M_1$, we already have that
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<<<<<<< HEAD
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\begin{equation*}
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\begin{array}{lll}
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\sum x^r_j & = & 1 \\
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\sum x^i_j & = & 0 \\
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\end{array}
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\end{equation*}
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and therefore $r_1(x_0,\dots,x_n,t)\in M_1$ for all $t\in[0,1]$, and in particular, we get that $r_1(x_0,\dots,x_n,1)=(x^r_0,\dots,x^r_n)\in M_1\cap\RR^{n+1}$ for all $(x_0,\dots,x_n)\in M_1$.
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=======
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\begin{align*}
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\sum_{j=0}^n x^r_j & = 1 \\
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\sum_{j=0}^n x^i_j & = 0 \\
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\end{align*}
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and therefore $r_1(x_0,\dots,x_n,t)\in M_1$ for all $t\in[0,1]$, and in particular, we get that $r_1(x_0,\dots,x_n,1)=(x^r_0,\dots,x^r_n)\in M_1\cap\RR^{n+1}$ for all $(x_0,\dots,x_n)\in M_1$.
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>>>>>>> 2a87fee69dfb1e6fdd6e608f9b4ee957cc3f5a74
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Let us summarize it.
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@ -164,24 +174,25 @@ The map $r_1$ is a strong deformation retraction from $M_1$ to $M_1\cap\RR^{n+1}
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Now we will define a strong deformation retraction
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\[
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r_2:(M_1 \cap \RR^{n+1})\times[0, 1]\to M_1 \cap \RR^{n+1}
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r_2:(M_1 \cap \RR^{n+1})\times[0, 1]\to M_1 \cap \RR^{n+1}
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\]
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from $M_1 \cap \RR^{n+1}$ to $S_1$.
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Note that $M_1\cap\RR^{n+1}$
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is a hypersurface in $\RR^{n+1}$, in Figure~\ref{fig:triangulo} we can see the example $n=2$.
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is a hypersurface in $\RR^{n+1}$, in Figure~\ref{fig:triangulo} we can see the example $n=2$.
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\begin{lemma}
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The space $S_1$ is a $n$-simplex.
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The space $S_1$ is a $n$-simplex.
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\end{lemma}
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\begin{proof}
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Let $e_i=(0,\dots,1,\dots,0)$ be the vectors of the canonical base. Then
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\begin{align*}
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S_1&=\left\{(x_0,\dots,x_n)\in\RR^{n+1}\,\middle|\,\sum_{i=0}^n x_i=1, x_i\geq1\right\} \\
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&=\left\{x_0e_0+\dots+x_ne_n\,\middle|\,\sum_{i=0}^n x_i=1, x_i\geq1\right\}=\Delta_n
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\qedhere
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\end{align*}
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Let $e_i=(0,\dots,1,\dots,0)$ be the vectors of the canonical base. Then
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\begin{align*}
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S_1&=\left\{(x_0,\dots,x_n)\in\RR^{n+1}\,\middle|\,\sum_{i=0}^n x_i=1, x_i\geq1\right\} \\
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&=\left\{x_0e_0+\dots+x_ne_n\,\middle|\,\sum_{i=0}^n x_i=1, x_i\geq1\right\}=\Delta_n
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\qedhere
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\end{align*}
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\end{proof}
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\begin{figure}[ht]
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\begin{center}
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\begin{tikzpicture}[scale=0.7,every text node part/.style={align=center}]
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@ -234,13 +245,13 @@ S_1&=\left\{(x_0,\dots,x_n)\in\RR^{n+1}\,\middle|\,\sum_{i=0}^n x_i=1, x_i\geq1\
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\end{center}
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\end{figure}
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As we can see in Figure \ref{fig:triangulo}, the hyperplanes defined by $\{x_i=0\}$ divide the space $S_1$ in $2^{n+1}-1$ regions. We denote each of these regions as $(\pm,\dots,\pm)$ 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 $(+,\dots,+)$.
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Note that there is no region $(-,\dots,-)$.
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As we can see in Figure \ref{fig:triangulo}, the hypersurfaces defined by $\{x_i=0\}$ divide the space in $2^{n+1}-1$ sections. We denote each of these sections as $(\pm,\dots,\pm)$ 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 $(+,\dots,+)$.
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Note that there is no section $(-,\dots,-)$.
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The retraction in $(+,\dots,+)$ is the identity for any value of $t$. For the rest of the parts, we define a retraction:
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\begin{equation*}
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\begin{tikzcd}[row sep=0pt,/tikz/column 1/.append style={anchor=base east}, /tikz/column 2/.append style={anchor=base west}]
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(M_1\cap\RR^{n+1})\times{[0,1]}\rar["r_2"]&M_1\cap\RR^{n+1}\\
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(M_1\cap\RR^{n+1})\times{[0,1]}\rar["r_2"]&M_1\cap\RR^{n+1}\times [0,1]\\
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(Q, t)\rar[mapsto]& t P_Q + (1 - t) Q,
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\end{tikzcd}
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\end{equation*}
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@ -250,7 +261,7 @@ The retraction in $(+,\dots,+)$ is the identity for any value of $t$. For the re
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% (x,y,z, t) & \longrightarrow & (1-t)(x,y,z)+tP_{x,y,z} \\
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% \end{array}
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%\end{equation}
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where $P_Q$ is a point depending on $Q\in M_1\cap\RR^{n+1}$.
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where $P_Q$ is a point depending on $Q\in M_1\cap\RR^{n+1}$.
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In general, for a point $Q$ in a region $(\pm,\dots,\pm)$, we want the retraction to take the negative coordinates to $0$ and to change the positive ones such that at the end the sum is $1$. Therefore, we can write the point $P_Q$ as:
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\begin{equation*}
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@ -320,31 +331,33 @@ m(x,t)=+\sqrt{x_r^2+(1-t)^2x_i^2} \text{ and } \theta(x,t)=\arctan\frac{(1-t)x_i
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\]
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We can divide $\CC$ in $2d$ sectors of angle $\frac{\pi}{d}$ as in Figure~\ref{divisionC}, where each of the sections will be homeomorphic to either the upper half complex plane or to the lower half one.
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\begin{figure}[h]
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\begin{center}
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%\begin{tikzpicture}[scale=0.6]
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% \draw (-5,0) to (5,0);
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% \draw (5,1.5) to (-5,-1.5);
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% \draw (5,3.2) to (-5, -3.2);
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% \draw (4.7,5) to (-4.7,-5);
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% \draw (2.8,5) to (-2.8,-5);
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% \draw (0.9,5) to (-0.9,-5);
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% \draw (-0.9,5) to (0.9,-5);
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% \draw (-2.8,5) to (2.8,-5);
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% \draw (-4.7,5) to (4.7,-5);
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% \draw (-5,3.2) to (5, -3.2);
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% \draw (-5,1.5) to (5,-1.5);
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%\end{tikzpicture}
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%
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\begin{tikzpicture}
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\foreach \x in {0, ..., 16}{
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\draw (0,0) to (360/17*\x:3);}
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\end{tikzpicture}
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\end{center}
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\caption{Division of the complex plane}
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\label{divisionC}
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\begin{center}
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%\begin{tikzpicture}[scale=0.6]
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% \draw (-5,0) to (5,0);
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% \draw (5,1.5) to (-5,-1.5);
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% \draw (5,3.2) to (-5, -3.2);
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% \draw (4.7,5) to (-4.7,-5);
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% \draw (2.8,5) to (-2.8,-5);
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% \draw (0.9,5) to (-0.9,-5);
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% \draw (-0.9,5) to (0.9,-5);
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% \draw (-2.8,5) to (2.8,-5);
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% \draw (-4.7,5) to (4.7,-5);
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% \draw (-5,3.2) to (5, -3.2);
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% \draw (-5,1.5) to (5,-1.5);
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%\end{tikzpicture}
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%
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\begin{tikzpicture}
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\foreach \x in {0, ..., 5}{
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\draw (0,0) to (360/6*\x:3);}
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\draw[->] (1.5,0) to[out=90, in=350] (0.6,1.1) node[ xshift=1.0cm, yshift=-0.2cm]{$\pi/d$};
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\end{tikzpicture}
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\end{center}
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\caption{Division of the complex plane}
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\label{divisionC}
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\end{figure}
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Then we can get homeomorphisms from the upper complex plane to the sectors of angle $[(k-1)\frac{\pi}{d}, k\frac{\pi}{d}]$ with $k=1,3,...,2d-1$, and from the lower complex plane to the sectors of angle $[(k-1)\frac{\pi}{d}, k\frac{\pi}{d}]$ with $k=2,4,...,2d$, which we will denote $f_k$. The lifting of $r_1$ will then be given by the $G$ previously described. This $G$ will act by taking the point $(x_0,\dots,x_n)\in M_d$ and mapping it to the upper or lower complex plane via the inverse homeomorphisms $f^{-1}_k$, then taking $r_1$ to take it to the real line, and finally bringing it back to its original sector with the homeomorphism~$f_k$.
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@ -389,8 +402,8 @@ G(z,t)^d\dar[equal, equal]&=&m(z,t)^d\exp{\left(i\arctan((1-t)\tan d\theta)\righ
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We can then write the lifting of $r_1$ as $\tilde{r}_1((x_0,\dots,x_n),t)=(G(x,t), \dots, G(x_n,t))$.
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Lastly, we are going to see how to lift $r_2$ in order to get a retraction from $M_d\cap\{x^d_0, \ldots , x^d_n\in\RR\}$ into $S_d$.
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For a point $P=(x_0\xi_0,\dots,x_n\xi_n)\in M_d\cap\{x^d_0, \ldots , x^d_n\in\RR\}$ (where $(x_0,\dots,x_n)$ are real and $\xi_i$ are $d$-th roots of unity), the corresponding point in $M_1\cap\RR^{n+1}$ is $(x^d_0,\dots,x^d_n)$ and therefore the goal point of the retraction $r_2$ will be
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Lastly, we are going to see how to lift $r_2$ in order to get a retraction from $M_d\cap\{x^d_0, y^d, x^d_n\in\RR\}$ into $S_d$.
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For a point $P=(x_0\xi_0,\dots,x_n\xi_n)\in M_d\cap\{x^d_0, y^d, x^d_n\in\RR\}$ (where $(x_0,\dots,x_n)$ are real and $\xi_i$ are $d$-th roots of unity), the corresponding point in $M_1\cap\RR^{n+1}$ is $(x^d_0,\dots,x^d_n)$ and therefore the goal point of the retraction $r_2$ will be
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\begin{equation}
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P^d_Q=\left(\frac{x^d_0\chi_0}{1-\sum x^d_j(1-\chi_j)}, \dots,\frac{x^d_n\chi_n}{1-\sum x^d_j(1-\chi_j)} \right)
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\end{equation}
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@ -432,8 +445,25 @@ We have then proved that $S_d$ is a strong deformation retract of $M_d$.
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Since $\lambda$ is a $d-$th root of unity, we get that $G(x,t)=G(\lambda x,t)$. Therefore, we get that $\tilde{r}_1(p,t)=\tilde{r}_1(\lambda p,t)$. It's just left to check that $r_2$ also sends them to the same class.
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If $[x^d_0: \dots: x^d_n]$ is in $(+,\dots,+)$, the retraction is the identity, and therefore both representatives will stay the same and therefore go to the same class.
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For the rest of the regions, we have
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<<<<<<< HEAD
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\begin{equation}
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\begin{array}{lll}
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\tilde{r}_2([x_0\xi_0:\dots:x_n\xi_n], t)& =& \left[\sqrt[d]{(1-t)x^d_0+t\frac{x^d_0\chi_0}{1-\sum x^d_j(1-\chi_j)}}\xi_0:\dots:\sqrt[d]{(1-t)x^d_n+t\frac{x^d_n\chi_n}{1-\sum x^d_j(1-\chi_j)}}\xi_n\right]
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\end{array}
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\end{equation}
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Then, for the point $\lambda p$ we get
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\begin{equation}
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\begin{array}{lll}
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\tilde{r}_2([\lambda x_0\xi_0:\dots:\lambda x_n\xi_n], t)&=&\left[\sqrt[d]{(1-t)\lambda^d x^d_0+t\frac{\lambda^d x^d_0\chi_0}{1-\sum \lambda^d x^d_j(1-\chi_j)}}\xi_0:\dots:\sqrt[d]{(1-t)\lambda^d x^d_n+t\frac{\lambda^d x^d_n\chi_n}{1-\sum \lambda^d x^d_j(1-\chi_j)}}\xi_n\right]\\
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&=&\left[\sqrt[d]{(1-t)x^d_0+t\frac{x^d_0\chi_0}{1-\sum x^d_j(1-\chi_j)}}\xi_0:\dots:\sqrt[d]{(1-t)x^d_n+t\frac{x^d_n\chi_n}{1-\sum x^d_j(1-\chi_j)}}\xi_n\right]\\
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\end{array}
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\end{equation}
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=======
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\begin{equation*}
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\tilde{r}_2([x_0\xi_0:\dots:x_n\xi_n], t) =\left[H(x_0,\chi_0)\xi_0:\dots:H(x_n,\chi_n) \xi_n\right]
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\end{equation*}
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@ -446,6 +476,7 @@ Then, for the point $\lambda p$ we get
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\tilde{r}_2([\lambda x_0\xi_0:\dots:\lambda x_n\xi_n], t)&=\left[H(\lambda x_0, \chi_0)\xi_0:\dots:H(\lambda x_n, \chi_n)\xi_n\right]\\
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&=\left[H( x_0, \chi_0)\xi_0:\dots:H(x_n, \chi_n)\xi_n\right].
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\end{align*}
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>>>>>>> 2a87fee69dfb1e6fdd6e608f9b4ee957cc3f5a74
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So we see that $\tilde{r}_2$ also takes both representatives will end to the same point.
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\end{proof}
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@ -463,8 +494,8 @@ We denote as $\Delta$ the $\Delta-$complex $[v_0, \dots, v_n]$ that has $n+1$ ve
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% & & K & \hookrightarrow & \PP^2\setminus C_d \\
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% \end{array}
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\begin{tikzcd}
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\Delta \rar[leftrightarrow]& S_1&\\
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\Delta^d\uar\rar[leftrightarrow]&S_d\uar\rar[hookrightarrow]\dar& M_d\dar\\
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\Delta \rar[leftrightarrow]& S_1&\\
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\Delta^d\uar\rar[leftrightarrow]&S_d\uar\rar[hookrightarrow]\dar& M_d\dar\\
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&K \rar[hookrightarrow]& \PP^n\setminus C_d,
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\end{tikzcd}
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\end{equation*}
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@ -483,10 +514,17 @@ First, we give a detailed description of $\Delta$. We will denote the $0$-simpli
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X & = [v_0,\dots,v_n]
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\end{align*}
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The face maps of $X$ and $X^{(j_0,\dots,j_i)}$ are:
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<<<<<<< HEAD
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\begin{equation}
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\delta^i_k(X^{(j_0,\dots,j_i)}) =[v_{j_0},\dots,\hat{v},\dots,v_{j_i}]
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\end{equation}
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where $\hat{v}$ indicates that we take out the vertix in position $k$, for $k\leq i$.
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=======
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\begin{equation*}
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\delta^i_k(X^{(j_0,\dots,j_i)}) =[v_{j_0},\dots,\hat{v},\dots,v_{j_i}]
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\end{equation*}
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where $\hat{v}$ indicates that we take out the vertex in position $k$, for $k\leq i$.
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>>>>>>> 2a87fee69dfb1e6fdd6e608f9b4ee957cc3f5a74
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an the boundary maps are given by:
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\begin{equation*}
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\partial^i =\sum (-1)^{k}\delta^i_k
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@ -505,20 +543,30 @@ Let $\mu_d=\{\xi| \xi^d=1\}$. We can express $S_d$ as a union
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\end{equation}
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where $X_{(\xi_0, \dots, \xi_n)}=\{(x_0,\dots,x_n)| \sum x^d_i=1 \text{ and } \xi_0x, \dots, \xi_nx_n\geq0\}$.
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Then, we can see that the faces of the simplex are given by $\mathring{X}_{(\xi_0, \dots, \xi_n)}$, while the $j-$simplices are given by
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Then, we can see that the faces of the simplex are given by $\mathring{X}_{(\xi_0, \dots, \xi_n)}$, while the $j-$simplices are given by
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\[
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X_{(\xi_0, \dots,\xi_n)}\cap\{x_{i_1},\dots,x_{i_j}=0\},
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\]
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where $\xi_{i_1},\dots,\xi_{i_j}=1$, and the vertices by
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where $\xi_{i_1},\dots,\xi_{i_j}=1$, and the vertices by
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\[
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X_{(1,\dots,\xi_i, 1, 1)}\cap\{x_j=1,\,\forall j\neq i\}.
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\]
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We can remark that $X_{(\xi_0, \dots, \xi_n)}=(\xi_0, \dots, \xi_n)\cdot X_{(1,\dots,1)}$, and therefore that we have a group action of $\mu^{n+1}_d$ over $S_d$.
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Let
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<<<<<<< HEAD
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\begin{equation}
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\begin{array}{rll}
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X^{(j_0,\dots,j_i)}_{(1,\dots,1)} & = & X_{(1, \dots, 1)}\cap\{x_{j_k}\neq0 \forall k=0,\dots i\text{ and }x_{i}=0\text{ for the rest}\}\\
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X^j_{(1,\dots,1)} & = & X_{(1, \dots, 1)}\cap\{x_i=0\forall i\neq j\}\\
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\end{array}
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||||
\end{equation}
|
||||
=======
|
||||
\begin{align*}
|
||||
X^{(j_0,\dots,j_i)}_{(1,\dots,1)} & = X_{(1, \dots, 1)}\cap\{x_{j_k}\neq0,\, \forall k=0,\dots i\text{ and }x_{i}=0\text{ for the rest}\}\\
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X^j_{(1,\dots,1)} & = X_{(1, \dots, 1)}\cap\{x_i=0,\,\forall i\neq j\}\\
|
||||
\end{align*}
|
||||
>>>>>>> 2a87fee69dfb1e6fdd6e608f9b4ee957cc3f5a74
|
||||
We can describe the chain complexes. They are abelian groups but they are also $R_d$-complexes
|
||||
where $R_d$ is the group algebra $\ZZ[\mu_d^{n+1}]$. For the $n$-chains we have
|
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\[
|
||||
@ -526,7 +574,7 @@ C_{n}(\Delta^d) = R_d\langle X_{(1,\dots,1)}\rangle,
|
||||
\]
|
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which is a free principal $R_d$-module, i.e. a free abelian group of rank~$d^{n+1}$. For the $i$-chains, with $i<n$ we deal with non-free modules.
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Let us denote as $t_0, \dots, t_n$ the variables of $R_d$; note that $t_0^d=\dots=t_n^d=1$.
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Then
|
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Then
|
||||
\[
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C_i(\Delta^d) = \bigoplus_{(t_{j_1},\dots,t_{j_{n-i}})}\frac{R_d}{(t_{j_1} - 1,\dots,t_{j_{n-i}} - 1)} \langle X^{(k_0,\dots,k_i)}_{(1,\dots,1)}\rangle,
|
||||
\]
|
||||
@ -536,7 +584,7 @@ C_0(\Delta^d) = \bigoplus_{t_j}\frac{R_d}{(t_0 - 1,\dots,t_{j-1}-1,t_{j+1}-1,\do
|
||||
\]
|
||||
a free abelian group of rank~$(n+1)d$.
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||||
%Then, the $2-$simplices, $1-$simplices and $0-$simplices of $\Delta^d$ are given by
|
||||
%\begin{equation}
|
||||
%\begin{equation}
|
||||
%\begin{array}{rll}
|
||||
%C_2(\Delta^d) & = & \ZZ[\mu^3_d]<X_{(1,1,1)}>\\
|
||||
%C_1(\Delta^d) & = & \ZZ[\mu^3_d]/\ZZ[\mu_d]<L^z_{(1,1,1)}> \bigoplus \ZZ[\mu^3_d]/\ZZ[\mu_d]<L^y_{(1,1,1)}> \bigoplus \ZZ[\mu^3_d]/\ZZ[\mu_d]<L^x_{(1,1,1)}>\\
|
||||
@ -544,7 +592,8 @@ a free abelian group of rank~$(n+1)d$.
|
||||
%\end{array}
|
||||
%\end{equation}
|
||||
|
||||
The face maps are $R_d$-morphisms defined as
|
||||
|
||||
The face maps are $R_d$-morphisms defined as
|
||||
\begin{equation*}
|
||||
\begin{aligned}
|
||||
\delta^n_i(X_{(1,1,1)}) & =X^{(0,\dots,i-1,i+1,\dots,n)}_{(1,\dots,1)} &&& \delta^j_i(X^{(k_0,\dots,k_j)}_{(1,\dots,1)})&= X^{(k_0,\dots,k_{i-1},k_{i+1},\dots,k_n)}_{(1,\dots,1)} .
|
||||
@ -584,8 +633,8 @@ We have then defined $\Delta^d$, the simplicial complex with which we can identi
|
||||
% & & K & \hookrightarrow & \PP^2\setminus C_d \\
|
||||
% \end{array}
|
||||
\begin{tikzcd}
|
||||
\Delta \rar[leftrightarrow]& S_1&\\
|
||||
\Delta^d\uar\rar[leftrightarrow]\dar&S_d\uar\rar[hookrightarrow]\dar& M_d\dar\\
|
||||
\Delta \rar[leftrightarrow]& S_1&\\
|
||||
\Delta^d\uar\rar[leftrightarrow]\dar&S_d\uar\rar[hookrightarrow]\dar& M_d\dar\\
|
||||
\Delta^d_\textrm{proj}\rar[leftrightarrow]&K \rar[hookrightarrow]& \PP^2\setminus C_d,
|
||||
\end{tikzcd}
|
||||
\end{equation*}
|
||||
@ -600,10 +649,10 @@ We have then defined $\Delta^d$, the simplicial complex with which we can identi
|
||||
%\end{equation*}
|
||||
The map $\Delta^d\longrightarrow\Delta^d_\text{proj}$ is given by $(x_0,\dots,x_n)\longmapsto[x_0:\dots:x_n]$. Since we are passing to the projective plane, two points $(x_0\xi_0, \dots, x_n\xi_n)$ and $(x_0\xi_0\xi, \dots, x_n\xi_n\xi)$ will have the same image.
|
||||
|
||||
The $n$-cells $X_{(\xi_0, \dots, \xi_n)}$ and $X_{(\xi_0\xi, \dots, \xi_n\xi)}$ of $\Delta^d$, project the same $n$-cell of $\Delta^d_\textrm{proj}$, which will be denoted as $X_{[\xi_0: \dots: \xi_n]}$.
|
||||
The $n$-cells $X_{(\xi_0, \dots, \xi_n)}$ and $X_{(\xi_0\xi, \dots, \xi_n\xi)}$ of $\Delta^d$, project the same $n$-cell of $\Delta^d_\textrm{proj}$, which will be denoted as $X_{[\xi_0: \dots: \xi_n]}$.
|
||||
The same convention is followed for the rest of $i-$simplices. Note that for the vertices, since all coordinates but one are $0$, we have only $n+1$ vertices $X^0, \dots, X^n$.
|
||||
|
||||
The group $\mu_d^{n+1}$ also acts, but the action of the diagonal vectors is trivial. Let us denote
|
||||
The group $\mu_d^{n+1}$ also acts, but the action of the diagonal vectors is trivial. Let us denote
|
||||
\[
|
||||
T_d:=\ZZ\left[\frac{\mu_d^{n+1}}{\mu_d\langle(\xi,\dots,\xi)\rangle}\right];
|
||||
\]
|
||||
@ -612,7 +661,7 @@ This group is actualy isomorphic to $\mu_n^d$ but it is more convenient to use t
|
||||
We have the following chain complexes:
|
||||
\begin{align*}
|
||||
C_n(\Delta^d_\text{proj}) &= T_d\langle X_{[1:\dots:1]}\rangle,\\
|
||||
C_i(\Delta^d_\text{proj}) &= \bigoplus_{(t_{j_1},\dots,t_{j_{n-i}})}\frac{T_d}{(t_{j_1} - 1,\dots,t_{j_{n-i}} - 1)} \langle X^{(k_0,\dots,k_i)}_{[1:\dots:1]}\rangle,\\
|
||||
C_i(\Delta^d_\text{proj}) &= \bigoplus_{(t_{j_1},\dots,t_{j_{n-i}})}\frac{T_d}{(t_{j_1} - 1,\dots,t_{j_{n-i}} - 1)} \langle X^{(k_0,\dots,k_i)}_{[1:\dots:1]}\rangle,\\
|
||||
C_0(\Delta^d) &= \bigoplus_{t_j}\frac{T_d}{(t_0 - 1,\dots,t_{j-1}-1,t_{j+1}-1,\dots, t_n - 1)} \langle X^j_{[1:\dots:1]}\rangle.
|
||||
\end{align*}
|
||||
%As for the points where one coordinate is $0$, say, for example the first one, since the group action is then by $\mu^3_2/\mu_2$, we write the elements of $\mu^3_2/\mu_2$ as $(1,\xi_1,\xi_2)$, where the $1$ is on the coordinate that is $0$. Then, passing to the projective space, we will say that $L^x_{[1:\xi_1,\xi_2]}$ and $L^x_{[1:\xi'_1,\xi'_2]}$ are the same if $[1:\xi'_1,\xi'_2]=[1:\xi_1\xi,\xi_2\xi]$ for some $\xi\in\mu_2$. The rest of $1-$simplices would be identified similarly. Lastly, for the $0-$simplices, since we have $2$ zero coordinates, we have that all $0-$simplices with the same nonzero coordinate will be identified to each other. As a result, $\Delta^d_\text{proj}$ will only have $3$ $0-$simplices, $3d$ $1-$simplices and $d^2$ $2-$simplices.
|
||||
@ -777,7 +826,7 @@ The code for the function is:
|
||||
def DeltaComplex_hypersurface(n,d):
|
||||
resdic = [{tuple(i*[0]+[1] + (n-i)*[0]): None for i in range(n+1) }]+ [{} for i in range(n)]
|
||||
|
||||
for nzerosi in range(n+1):
|
||||
for nzerosi in range(n+1):
|
||||
for t in Tuples(range(d+1),n-nzerosi ):
|
||||
T = nzerosi*[0] + [1] + list(t)
|
||||
dimT = len(T) - T.count(0) - 2
|
||||
@ -790,9 +839,9 @@ def DeltaComplex_hypersurface(n,d):
|
||||
canonicalize(CT)
|
||||
caras.append(tuple(CT))
|
||||
resdic[dimT+1][tuple(T)] = caras
|
||||
|
||||
|
||||
DC = [[tuple() for _ in range(n+1)]]
|
||||
for i in range(1,n+1):
|
||||
for i in range(1,n+1):
|
||||
lis = []
|
||||
for p,v in resdic[i].items():
|
||||
lisp = []
|
||||
@ -800,15 +849,15 @@ def DeltaComplex_hypersurface(n,d):
|
||||
lisp.append(list(resdic[i-1].keys()).index(s))
|
||||
lis.append(lisp)
|
||||
DC.append(lis)
|
||||
|
||||
dc=DeltaComplex(DC)
|
||||
|
||||
dc=DeltaComplex(DC)
|
||||
return dc
|
||||
\end{python}
|
||||
|
||||
This function uses an auxiliary function, named $canonicalize$ in the code.
|
||||
This function uses an auxiliary function, named $canonicalize$ in the code.
|
||||
$canonicalize$ is used to identify the simplices in the projective space. The code to this function is:
|
||||
\begin{python}
|
||||
def canonicalize(t):
|
||||
def canonicalize(t):
|
||||
v = next(filter(lambda a : a != 0, t))
|
||||
correct = v - 1
|
||||
for i in range(len(t)):
|
||||
@ -843,7 +892,7 @@ Delta_Complex = DeltaComplex_hypersurface(4,6)
|
||||
Once we have gotten the $\Delta$-complex, we can compute the fundamental group or the cohomology of the space, for example.
|
||||
\begin{python}
|
||||
Dc = SimplicialSet(Delta_Complex)
|
||||
dc_punto = Dc.set_base_point(Dc.all_n_simplices(0)[0])
|
||||
dc_punto = it commit Dc.set_base_point(Dc.all_n_simplices(0)[0])
|
||||
dc_punto.fundamental_group()
|
||||
-> Finitely presented group < e0 | e0^6 >
|
||||
dc_punto.cohomology()
|
||||
|
||||
Loading…
Reference in New Issue
Block a user