# How do I robustly typeset a double-hat?

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!

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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 7 '11 at 18:24

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.

\documentclass{article}
\usepackage{amsmath}
\usepackage{accents}
\newlength{\dhatheight}
\newcommand{\doublehat}[1]{%
\settoheight{\dhatheight}{\ensuremath{\hat{#1}}}%
$\doublehat{x} \doublehat{L} \doublehat{\prod}$

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 8 '11 at 11:14