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I am trying to place text to the right of the brackets so that it doesn't overlap the equation.

\documentclass{amsart}
\usepackage{tikz}
\usetikzlibrary{calc}

\usepackage{amsmath}   

\newcommand{\tikzmark}[1]{\tikz[overlay,remember picture] \node (#1) {};}
\tikzset{square arrow/.style={to path={-- ++(0,.25) -| (\tikztotarget)}}}

\begin{document}
\[
=\, -\nu_{gx}\,  \dfrac{\partial \, \left<\tikzmark{a}n\right>}{\partial\,x} \, =\, -\nu_{gx} \, \dfrac{\partial \left<\tikzmark{b}\eta_{0}\right>}{\partial \,x} \, \dfrac{\partial \,T}{\partial \,x}
\tikz[overlay,remember picture]
{\draw[<->,square arrow] ($(a.north)+(0.25em,1.25ex)$) to ($(b.north)+(0.50em,1.25ex)$) node [above,right] {local thermal equilibrium!};}
\]
\end{document}

enter image description here

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2 Answers

You need to adjust the parameters of the two endpoints for the width of the characters in the angle brackets:

enter image description here

\documentclass{amsart}
\usepackage{tikz}
\usetikzlibrary{calc}

\usepackage{amsmath}   

\newcommand{\tikzmark}[1]{\tikz[overlay,remember picture] \node (#1) {};}
\tikzset{square arrow/.style={to path={-- ++(0,.25) -| (\tikztotarget)}}}

\begin{document}
\[
=\, -\nu_{gx}\,  \dfrac{\partial \, \left<\tikzmark{a}n\right>}{\partial\,x} \, =\, -\nu_{gx} \, \dfrac{\partial \left<\tikzmark{b}\eta_{0}\right>}{\partial \,x} \, \dfrac{\partial \,T}{\partial \,x}
\tikz[overlay,remember picture]{%
    \draw[<->,square arrow, red, thick] 
        ($(a.north)+(0.25em,0.30ex)$) to ($(b.north)+(0.50em,0.30ex)$) ;
    \node [above, yshift=1.50ex, blue] at  ($(a.north)!0.5!(b.north)$) {label};
    }
\]

\end{document}

share|improve this answer
    
I actually want the \tikzmark centered above the mentioned expressions, so the arrows would be right above the $n$ and the $\eta_{0}$. I also cannot figure out how to place arrows at both ends of the bracket. –  Brazos Wolf Oct 5 '12 at 7:19
    
@BrazosWolf: Have updated solution. Let me know if that is what you desired. –  Peter Grill Oct 5 '12 at 7:37
    
Looks brilliant. But then I decided to complicate matters and actually desire text to go with it. –  Brazos Wolf Oct 5 '12 at 8:01
    
@BrazosWolf: Ok, have updated it to allow for a label. –  Peter Grill Oct 5 '12 at 8:07
    
thanks. looks perfect with node [right,yshift=1.5ex] –  Brazos Wolf Oct 5 '12 at 8:11
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You could (mis-)use chemmacros' redox commands: \OX{<name>,<text>} to mark the places and \redox(<name1>,<name2>) to connect them:

\documentclass{amsart}
\usepackage{amsmath}   
\usepackage{chemmacros}

\begin{document}

\[
  =\, -\nu_{gx}\,  \dfrac{\partial \, \left<\OX{a,$n$}\right>}{\partial\,x} \,
  =\, -\nu_{gx} \, \dfrac{\partial \left<\OX{b,$\eta_{0}$}\right>}{\partial \,x} \, \dfrac{\partial \,T}{\partial \,x}
\]
\redox(a,b)[<->]{text over}
\end{document}

enter image description here

It has some options for customization:

\documentclass{amsart}
\usepackage{amsmath}   
\usepackage{chemmacros}

\begin{document}

\chemsetup[redox]{sep=1em,dist=1em}
\[
  =\, -\nu_{gx}\,  \dfrac{\partial \, \left<\OX{a,$n$}\right>}{\partial\,x} \,
  =\, -\nu_{gx} \, \dfrac{\partial \left<\OX{b,$\eta_{0}$}\right>}{\partial \,x} \, \dfrac{\partial \,T}{\partial \,x}
\]
\redox(a,b)[<->,draw=blue]{text over}
\end{document}

enter image description here

Edit: new requirements:

\documentclass{amsart}
\usepackage{amsmath}   
\usepackage{chemmacros}

\begin{document}

\chemsetup[redox]{sep=1em,dist=1em}
\[
  =\, -\nu_{gx}\,  \dfrac{\partial \, \left<\OX{a,$n$}\right>}{\partial\,x} \,
  =\, -\nu_{gx} \, \dfrac{\partial \left<\OX{b,$\eta_{0}$}\right>}{\partial \,x} \, \dfrac{\partial \,T}{\partial \,x}
\]
\redox(a,b)[<->]
\tikz[remember picture,overlay] \draw (b.east) ++(0,.75) node[right] {local thermal equilibrium!};
\end{document}

enter image description here

share|improve this answer
    
So much more elegant and terse than my attempted solution. –  Brazos Wolf Oct 5 '12 at 7:57
    
However, I have to compile it twice to get the desired effect. Why is that? –  Brazos Wolf Oct 5 '12 at 7:58
    
Also, is there any way to push the text to the side of the bracket using chemmacros? –  Brazos Wolf Oct 5 '12 at 8:06
    
that's because of TikZ' remember picture option: in the first run the position of the coordinates has to be remembered –  cgnieder Oct 5 '12 at 8:06
    
Not directly, but the nodes can be accessed with TikZ in the standard way, I'll add an edit. –  cgnieder Oct 5 '12 at 8:09
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