• tikz-cd package – Manuel Nov 11 '14 at 16:44
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The picture you show was created with Xy-pic. A modern replacement is tikz-cd:

\documentclass{article}
\usepackage{amssymb}
\usepackage{tikz-cd}

\begin{document}
$\begin{tikzcd} Y \arrow[r,hook] \arrow[d,swap,"T(f)"] & \mathcal{H}(D^{\infty}(Y)) \arrow[r,"h"] \arrow[d,swap,"\widehat{T(f)}"] \arrow[dr,swap,dashed,"\widehat{f}\circ h"] & \mathcal{H}(D^{\infty}(X)) \arrow[d,"\widehat{f}"] & X \arrow[l,hook'] \arrow[d,"f"] \\ \mathbb{R} & \mathbb{R} & \mathbb{R} & \mathbb{R} \end{tikzcd}$

\end{document}


The hook' arrow type reverses the side of the hook. The rest should be almost self explanatory: you build a matrix, specifying the arrows with d for “down”, r for “right” and so on. The key swap refers to the position of the label (default is on the left of the arrow, going from tail to tip).

• Would you be able to expand on why tikz-cd is a modern improvement on xypic? I don't know anything about their advantages and disadvantages relative to each other, and would love to learn. – Arun Debray Nov 11 '14 at 19:56
• @ArunDebray tikz-cd is based on the powerful TikZ-PGF graphic creation package. Not only this, the syntax for commutative diagrams is far superior to Xy-pic's. – egreg Nov 11 '14 at 20:33

A pstricks-add solution, that can be compiled with pdflatex if the -shell-escape switch is enabled (TeX Live, MacTeX) or the --enable-write18 switch (MiKTeX).

\documentclass[pdf]{article}
\usepackage{amssymb}
$% \begin{psmatrix} %%% Matrix nodes Y & \mathcal{H}\smash{(D^{\infty}(Y))} & \mathcal{H}\smash{(D^{\infty}(X))} & X \\ \mathbb{R} & \mathbb{R} & \mathbb{R} & \mathbb{R} %%% Arrows \psset{arrows=->, arrowinset=0.2, nodesep=2pt, shortput=nab, hookwidth=-4pt, hooklength=4pt} \ncline{1,1}{2,1}_{T(f)}\ncline[arrows = H->]{1,1}{1,2} \ncline{1,2}{2,2}_{\widehat{T(f)}}\ncline{1,2}{1,3}^{h} \ncline{1,3}{2,3}_{\widehat{f}} \ncline{1,4}{2,4}_{f}\ncline[arrows=H->, hookwidth=4pt]{1,4}{1,3} \ncline[linestyle=dashed, nodesepA=6pt]{1,2}{2,3}\nbput[labelsep=0pt, npos=0.4]{\widehat{f}\circ h} \end{psmatrix}$%

• @Svend Tveskæg: Sorry, I forgot my editor configuration displays ∞ (and saves \infty, naturally). Corrected. Thanks for pointing that. – Bernard Nov 11 '14 at 23:13
• Emacs + AUCTeX is also powerful ... and doesn't have any inconveniences. ;) – Svend Tveskæg Nov 11 '14 at 23:22