symbols-a4

We can just as easily define other barred letters:
\def\bbar{{\mathchar’26\mkern-9mu b}}
\def\dbar{{\mathchar’26\mkern-12mu d}}
(The space after the “mu” is optional but is added for clarity.) \bbar and \dbar define “¯b” and “¯d”, respectively.
Note that \dbar requires a greater backward math kern than \bbar; a −9 mu kern would have produced the
less-attractive “¯d” glyph.
The amsmath package provides \overset and \underset commands for placing one symbol respectively
above or below another. For example, \overset{G}{\sim} 8 produces “ G ∼” (sometimes used for “equidecomposable
with respect to G”).
Sometimes an ordinary tabular environment can be co-opted into juxtaposing existing symbols into a new
symbol. Consider the following definition of \asterism (“ * ** ”) from a June 2007 post to comp.text.tex by
Peter Flynn:
\newcommand{\asterism}{\smash{%
\raisebox{-.5ex}{%
\setlength{\tabcolsep}{-.5pt}%
\begin{tabular}{@{}cc@{}}%
\multicolumn2c*\\[-2ex]*&*%
\end{tabular}}}}
Note how the space between columns (\tabcolsep) and rows (\\[. . . ]) is made negative to squeeze the
asterisks closer together.
There is a TEX primitive called \mathaccent that centers one mathematical symbol atop another. For
example, one can define \dotcup (“ ·∪”)—the composition of a \cup and a \cdot—as follows:
\newcommand{\dotcup}{\ensuremath{\mathaccent\cdot\cup}}
The catch is that \mathaccent requires the accent to be a “math character”. That is, it must be a character
in a math font as opposed to a symbol defined in terms of other symbols. See The TEXbook [Knu86a] for more
information.
Another TEX primitive that is useful for composing symbols is \vcenter. \vcenter is conceptually similar
to “\begin{tabular}{l}” in L ATEX but takes a list of vertical material instead of \\-separated rows. Also,
it vertically centers the result on the math axis. (Many operators, such as “+” and “−” are also vertically
centered on the math axis.) Enrico Gregorio posted the following symbol definition to comp.text.tex in
March 2004 in response to a query about an alternate way to denote equivalence:
\newcommand*{\threesim}{%
\mathrel{\vcenter{\offinterlineskip
\hbox{$\sim$}\vskip-.35ex\hbox{$\sim$}\vskip-.35ex\hbox{$\sim$}}}}
The \threesim symbol, which vertically centers three \sim (“∼”) symbols with 0.35 x-heights of space between
them, is rendered as “∼”. \offinterlineskip is a macro that disables implicit interline spacing. Without
it, \threesim would have a full line of vertical spacing between each \sim. Because of \vcenter, \threesim
aligns properly with other math operators: a ÷ b ∼ c × d.
A related L ATEX command, borrowed from Plain TEX, is \ooalign. \ooalign vertically overlaps symbols
and works both within and outside of math mode. Essentially, it creates a single-column tabular environment
with zero vertical distance between rows. However, because it is based directly on TEX’s \ialign primitive,
\ooalign uses TEX’s tabular syntax instead of L ATEX’s (i.e., with \cr as the row terminator instead of \\). The
following example of \ooalign, a macro that defines a standard-state symbol (\stst, “ −◦ ”) as a superscripted
Plimsoll line (\barcirc, “−◦ ”), 9 is due to an October 2007 comp.text.tex post by Donald Arseneau:
\makeatletter
\providecommand\barcirc{\mathpalette\@barred\circ}
\def\@barred#1#2{\ooalign{\hfil$#1-$\hfil\cr\hfil$#1#2$\hfil\cr}}
\newcommand\stst{^{\protect\barcirc}}
\makeatother
8 LATEX’s \stackrel command is similar but is limited to placing a symbol above a binary relation.
9 While \barcirc illustrates how to combine symbols using \ooalign, the stmaryrd package’s \minuso command (Table 46 on
page 22) provides a similar glyph (“”) as a single, indivisible symbol.
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