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From: raymond@math.berkeley.edu (Raymond Chen)
Newsgroups: comp.text.tex
Subject: Re: undertilde
Keywords: latex, tilde
Message-ID: <1990Oct4.032554.27582@agate.berkeley.edu>
Date: 4 Oct 90 03:25:54 GMT
References: <1990Oct4.014728.29527@caen.engin.umich.edu>
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Reply-To: raymond@math.berkeley.edu (Raymond Chen)
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In-Reply-To: oliveria@caen.engin.umich.edu (ROQUE DONIZETE DE OLIVEIRA)

In article <1990Oct4.014728.29527@caen.engin.umich.edu>, oliveria@caen (ROQUE DONIZETE DE OLIVEIRA) writes:
>I looked at the macro for \underbrace but couldn't get my \undertilde to
>work in a similar way.

You really want to study the \b \c \d macros to see how to place
accent marks under characters.  Using those as a springboard, I
designed these macros quite some time ago, at the request of a chemist
friend...

   % underacc.tex -- allow for placement of accents under characters
    \def\mathunderaccent#1{\let\theaccent#1\mathpalette\putaccentunder}
    \def\putaccentunder#1#2{\oalign{$#1#2$\crcr\hidewidth
      \vbox to.2ex{\hbox{$#1\theaccent{}$}\vss}\hidewidth}}
   % the truly paranoid may wish to rename \putaccentunder to
   % \putacc@ntunder.

the resulting gizmo is a \mathord.  Season to taste if you really
wanted a \mathbin or whatever.

An important subtlety that many people miss is that the object being
accented might be appearing in a display, a superscript or subscript,
and hence the macro must be careful to do the right thing.  Hence the
\mathpalette.

Here's a sample usage:

% \ttilde puts a double tilde over its argument.
\def\ttilde#1{\tilde{\tilde{#1}}}

% "x" with double tilde underneath, superscript "y" with tilde underneath.
$\mathunderaccent\ttilde x ^ {\mathunderaccent\tilde y}$
\end