# Matrix definition with tikz

Hi I am trying to put the Sigma in the center to define a matrix. But I have not been able to do so. Any suggestions?

$$\mathbf{\Sigma} = \begin{tikzpicture}[baseline=(\mathbf{\Sigma}.center)] \begin{tikzpicture} \matrix[matrix of math nodes,left delimiter = {[},right delimiter ={]},row sep=10pt,column sep = 10pt] (m) { \Sigma_{1,1}&\Sigma_{1,2} &\Sigma_{1,1}\\ \Sigma_{2,1}&\Sigma_{2,2} &\Sigma_{2,1}\\ \Sigma_{1,1}&\Sigma_{1,2} &\Sigma_{1,1}\\ }; \begin{pgfonlayer}{background} \node[inner sep=3pt,fit=(m-1-1)] (1) {}; \node[inner sep=3pt,fit=(m-2-1) (m-3-2)] (2) {}; \node[inner sep=3pt,fit=(m-3-3)] (3) {}; \draw[rounded corners,dotted,fill=green!50!white,inner sep=3pt,fill opacity=0.1] (1.north west) |- (3.south east) |- (2.east) |- (2.north) |- (1.north) -- cycle; \end{pgfonlayer} \end{tikzpicture}$$


As stated in my comment, you could try using baseline={([yshift=-1ex]current bounding box.center)} (see this answer), like:

\documentclass[11pt]{article}

\usepackage{tikz}
\usetikzlibrary{matrix,backgrounds,calc,fit}

\begin{document}

$$\mathbf{\Sigma} = \begin{tikzpicture}[baseline={([yshift=-1ex]current bounding box.center)}] \matrix[matrix of math nodes,left delimiter={[},right delimiter={]},row sep=10pt,column sep=10pt] (m) { \Sigma_{1,1} & \Sigma_{1,2} & \Sigma_{1,1} \\ \Sigma_{2,1} & \Sigma_{2,2} & \Sigma_{2,1} \\ \Sigma_{1,1} & \Sigma_{1,2} & \Sigma_{1,1} \\ }; \begin{pgfonlayer}{background} \node[inner sep=3pt,fit=(m-1-1)] (1) {}; \node[inner sep=3pt,fit=(m-2-1) (m-3-2)] (2) {}; \node[inner sep=3pt,fit=(m-3-3)] (3) {}; \draw[rounded corners,dotted,fill=green!50!white,inner sep=3pt,fill opacity=0.1] (1.north west) |- (3.south east) |- (2.east) |- (2.north) |- (1.north) -- cycle; \end{pgfonlayer} \end{tikzpicture}$$

\end{document}


Result: