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I'm trying to typeset a symbol with two hat accents (useful for denoting superoperators in quantum mechanics, for instance), but \hat{\hat{L}} produces too much space between the hats. I found one approach on a LaTeX forum that seems to work, but it requires manual adjustment for each letter that I might want to typeset. If anyone has any suggestions, I'd appreciate it. Thanks!

Chris Granade
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    I thinks it's acceptable to use amsmath and \hat{\hat{L}} directly. It is also used in manual of amsmath. – Leo Liu Feb 07 '11 at 18:24

2 Answers2

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Well, I wrote that suggestion at LaTeX Community, but there I assumed the person wanted fine-grained control. If you don't, then maybe something like this would be more suitable? You may need to tweak the -0.35ex to your liking, though not separately for each character.

How about this?

\documentclass{article}
\usepackage{amsmath}
\usepackage{accents}
\newlength{\dhatheight}
\newcommand{\doublehat}[1]{%
    \settoheight{\dhatheight}{\ensuremath{\hat{#1}}}%
    \addtolength{\dhatheight}{-0.35ex}%
    \hat{\vphantom{\rule{1pt}{\dhatheight}}%
    \smash{\hat{#1}}}}
\begin{document}

$\doublehat{x} \doublehat{L} \doublehat{\prod}$
\end{document}

enter image description here

frabjous
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  • That worked beautifully, thank you! One last question, though: would using \mathchoice be a reasonable way of making your code work in a super/subscript context? Once again, thanks for your help! – Chris Granade Feb 07 '11 at 23:58
  • Hmm. I hadn't thought of sub/superscripts. Yeah, \mathchoice could probably work. Do you need help or do you think you can work it out on your own? – frabjous Feb 08 '11 at 00:07
  • I got it. It's not all that elegant, as I'm not used to the intricacies of LaTeX macro writing yet, but it does typeset superscript double-hats quite nicely. http://pastebin.com/XctVhueR – Chris Granade Feb 08 '11 at 00:32
  • The accents package is needed since plain TeX yields good horizontal alignment of the accent only for single characters; amsmath has some handling of double accents, but only the accents package is able to handle the correct positioning of the \hat over your more complicated construction. – Hendrik Vogt Feb 08 '11 at 11:14
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    I needed a \triplehat, so I copied this definition for \doublehat, then renamed it to \triplehat, and finally replaced the two nested references to \hat with \doublehat. It worked beautifully! – Aaron McDaid Sep 16 '16 at 10:49
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An alternative macro that doesn't depend on package accents.

\makeatletter
\newcommand{\hathat}[1]{% 
\begingroup%
  \let\macc@kerna\z@%
  \let\macc@kernb\z@%
  \let\macc@nucleus\@empty%
  \hat{\raisebox{.2ex}{\vphantom{\ensuremath{#1}}}\smash{\hat{#1}}}%
\endgroup%
}
\makeatother

It is inspired by frabjous's answer and David's answer under another question.

An updated solution that supports subscript math style.

\makeatletter
\newcommand{\hathat}[1]{% 
\begingroup%
  \let\macc@kerna\z@%
  \let\macc@kernb\z@%
  \let\macc@nucleus\@empty%
  \hat{\mathchoice%
    {\raisebox{.2ex}{\vphantom{\ensuremath{\displaystyle #1}}}}%
    {\raisebox{.2ex}{\vphantom{\ensuremath{\textstyle #1}}}}%
    {\raisebox{.16ex}{\vphantom{\ensuremath{\scriptstyle #1}}}}%
    {\raisebox{.14ex}{\vphantom{\ensuremath{\scriptscriptstyle #1}}}}%
    \smash{\hat{#1}}}%
\endgroup%
}
\makeatother

demo

Tianren Liu
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    Works nicely, but .2ex is probably not enough; I'd suggest .35ex, as in frabjous's answer above. – chsk Feb 01 '21 at 17:51
  • How can we make it work as a subscript, e.g., in $\sum_{\hathat{y}}$? Currently when I'm putting it in a subscript, the hats get separated a lot. – pms Feb 05 '23 at 21:24
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    @pms I updated the solution. Now it works for all math styles. – Tianren Liu Feb 07 '23 at 13:06