The big german math portal Matheplanet.com has presented its 7th annual awards yesterday evening, Jan 25th. Those awards honor the efforts of community members who are providing help, knowledge, work and time to all of the Matheplanet users. More than 4000 votes were given in different categories like mathematics, physics, computer science, moderation, information, article writing, book reviewing and more. Because that math site has a lively LaTeX forum there’s also an award for honoring the best LaTeX advisor. This year (and the last year too) the voters have chosen me, and I’m very proud of it, because I’m part of the Matheplanet community for many years and I appreciate this site and its members. At this place I want to say thank you for your feedback.
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-Community.org.

In the Art of Problem Solving Forum somebody asked for help in creating a construction for proofs and demonstrations in Logic by LaTeX commands. Math expressions should be aligned, some connected by arrows going vertically and horizontally.
Regarding arrows I would use tikz and also its matrix library for math nodes. The nodes could be connected by -| or |- path construction operations, like

\draw (node1) -| (node2) |- (node3);

For creation of the example given in the thread on AoPS I’ve used a matrix of math nodes, some empty nodes for alignment and |- -| arrows, here’s the compilable example code:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{matrix}
\begin{document}
\begin{tikzpicture}[every node/.style={anchor=west}]
  \matrix (m) [matrix of math nodes,
    nodes in empty cells]{
    \quad & 1.\quad (x)(Q\supset Fx) & \\
    & 2.\quad Q\supset Fx & \textrm{I,\textbf{UI}} & \\
    & 3.\quad Q \\
    & 4.\quad Fx & 2, 3, \textrm{ M.P.}\\
    & 5.\quad (x)Fx & 4, \textrm{ \textbf{UG}} \\
    & 6.\quad Q\supset(x)Fx & 3-5, \textrm{ C.P.} \\
    & 7.\quad \parbox[t]{2.9cm}{%
             $(x)(Q\supset(x)Fx)\supset$\\
             $[Q\supset(x)Fx]$} & 1-6, \textrm{ C.P.}\\};
  \draw[-stealth] (m-7-2.north east)
               -| (m-1-1.west) |- (m-1-2);
  \draw[-stealth] (m-6-2.north east)
               -| (m-3-1.east) |- (m-3-2);
\end{tikzpicture}
\end{document}

Output:

Exactly 2 years after LaTeX-Community.org has gone online its forum has got a new fresh look and the community site is providing new features for its members and users.

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• Know How section, intended to contain categorized articles related to LaTeX,
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See the original announcement for more information.

My thanks to Sven Wiegand for developing and maintening this great site. Have a look!

When one edge of a graph passes over or under another edge there may be a small gap needed in the drawing of one of those edges. An easy trick is to draw the upper edge two times: first with the background color and more thickness, afterwards normally, like
<node1> edge [-,line width=6pt,draw=white] <node2> edge <node2>
Here’s an example diagram used in the definition of the pull-back of a Banach bundle:

The source code:

\begin{tikzpicture}
  \matrix (m) [matrix of math nodes, row sep=3em,
    column sep=3em]{
    & f^\ast E_V& & \vphantom{f^\ast}E_V \\
    f^\ast E & & \vphantom{f^\ast}E & \\
    & U & & V \\
    M & & N & \\};
  \path[-stealth]
    (m-1-2) edge (m-1-4) edge (m-2-1)
            edge [densely dotted] (m-3-2)
    (m-1-4) edge (m-3-4) edge (m-2-3)
    (m-2-1) edge [-,line width=6pt,draw=white] (m-2-3)
            edge (m-2-3) edge (m-4-1)
    (m-3-2) edge [densely dotted] (m-3-4)
            edge [densely dotted] (m-4-1)
    (m-4-1) edge (m-4-3)
    (m-3-4) edge (m-4-3)
    (m-2-3) edge [-,line width=6pt,draw=white] (m-4-3)
            edge (m-4-3);
\end{tikzpicture}

This topic was discussed on Matheplanet.com.

If you want to read more about commutative diagrams with TikZ have a look here: Chains with labeled edges.