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I would like to be able to add the symbol "lambda-bar" (see below) into an equation. How can I do this?

enter image description here

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3  
tipa provides \textcrlambda. Found via How to look up a symbol?. –  Werner Feb 2 '13 at 16:11

3 Answers 3

up vote 18 down vote accepted

As far as I know (confirmed by the Comprehensive list of LaTeX symbols), the only package that provides \lambdabar is txfonts (and the derived newtxmath). Of course changing all document fonts to get that symbol is out of the question (and the lambda is quite different from other fonts, so it's also impossible to import only it).

One can build the symbol similarly to \hbar:

\documentclass{article}
\newcommand{\lambdabar}{{\mkern0.75mu\mathchar '26\mkern -9.75mu\lambda}}
\begin{document}
$|\lambdabar|$

$|\lambda|$
\end{document}

This exploits the fact that the bar character used is exactly 9mu wide. In the standard definition of \hbar there is just \mkern-9mu, but we need to push the bar slightly to the right. (The second line is just to compare the results.)

enter image description here


A different implementation, where the bar is lower.

\documentclass{article}

\makeatletter
\newcommand{\lambdabar}{{\mathchoice
  {\smash@bar\textfont\displaystyle{0.25}{1.2}\lambda}
  {\smash@bar\textfont\textstyle{0.25}{1.2}\lambda}
  {\smash@bar\scriptfont\scriptstyle{0.25}{1.2}\lambda}
  {\smash@bar\scriptscriptfont\scriptscriptstyle{0.25}{1.2}\lambda}
}}
\newcommand{\smash@bar}[4]{%
  \smash{\rlap{\raisebox{-#3\fontdimen5#10}{$\m@th#2\mkern#4mu\mathchar'26$}}}%
}
\makeatother


\begin{document}

$|\lambdabar|_{\lambdabar_\lambdabar}$

$|\lambda|_{\lambda_\lambda}$
\end{document}

I've left the four parameters to \smash@bar so that one can fine tune them for different fonts. The variable parameters are the third (amount of shifting down as a fraction of 1ex in the correct font size) and the fourth (amount of shifting right of the bar, in mu units).

enter image description here

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that bar looks too high. it would look better closer to the middle of the ascender, and maybe even a little bit lower. this symbol is in the stix fonts, but i'm not exactly sure where, since it's identified as a variant of U+0198, not the basic unicode. –  barbara beeton Feb 2 '13 at 17:27
    
@barbarabeeton Maybe it's a bit high, but in this way it matches the height if the bar in \hbar. –  egreg Feb 2 '13 at 17:54
1  
@barbarabeeton I've added a version with a lowered bar –  egreg Feb 2 '13 at 18:28
    
This is splendid. –  User 17670 Feb 2 '13 at 18:53

In this case I use the stackengine package to set a \rule atop a \lambda. But the key to success in making it work with all the math styles is the \ThisStyle{...\SavedStyle...} syntax of the scalerel package, which both allows the glyph components to be set in the proper math style. Additionally, the values \LMpt and \LMex are lengths with respective values of 1pt and 1ex, which do scale with the mathstyle (to 0.7 size in \scriptstyle and 0.5 size in \scriptscriptstyle).

Thus, in text and display styles, the rule is moved down 2.4pt for the overlap; in script style, it is moved down .7(2.4pt) and in scriptscript style, it is moved down .5(2.4pt). And while the rule length is always a scalable 1\LMex in length, the kerning and the rule thickness are composites of scalable and unscalable lengths, chosen to give the most aesthetically pleasing look.

Obviously, the stacking gaps, etc. can be modified to suit your taste.

\documentclass{book}
\usepackage{stackengine,scalerel}
\def\lambdabar{\ThisStyle{\ensurestackMath{\stackon[-2.4\LMpt]{%
  \SavedStyle\lambda}{\kern-.5pt\kern\LMpt\rule{1\LMex}{.25pt+.15\LMpt}}}}}
\begin{document}
$\lambdabar \scriptstyle\lambdabar \scriptscriptstyle\lambdabar$
\end{document}

enter image description here

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This symbol has a unicode with the number U+019B. You may choose some font which supports this symbol and use it as shown below:

% arara: lualatex 

\documentclass{article}
\usepackage{fontspec}
\usepackage{mathtools}
\newcommand{\lambdabar}{{\setmainfont{Linux Libertine O}\text{\symbol{"019B}}}}

\begin{document}
%In text \lambdabar, in inline math $\lambdabar$ or in display:
\[|\lambdabar|_{\lambdabar_\lambdabar}\]
\end{document}

enter image description here

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