Keywords: Braid-modular-group-cover.svg Commutative diagram showing the braid group B<sub>3</sub> as the universal central extension of the modular group sitting as lattices inside SL<sub>2</sub> R and its universal cover Own work created as per en meta Help Displaying a formula Commutative diagrams; source code below 2009-05-07 Nils R Barth LaTeX source <source lang latex > \documentclass amsart \usepackageall ps dvips xy Loading the XY-Pic package Using postscript driver for smoother curves \usepackage color For invisible frame \DeclareMathOperator \SL SL \DeclareMathOperator \PSL PSL \begin document \thispagestyle empty No page numbers \SelectTips eu Euler arrowheads tips \setlength \fboxsep 0pt Frame box margin \color white \framebox \color black Frame for margin \xymatrix R 6pt B_3 \ar ->> r \ar -> dd \PSL_2 \mathbf Z \ar -> dd\\ \mathbf Z \ar _ -> / /ur \ar -> /_/dr \\ \overline \SL_2 \mathbf R \ar ->> r \PSL_2 \mathbf R end math end frame \end document </source> Commutative diagrams Braid theory Xy-pic diagrams Images with LaTeX source code Linear groups |