# Math symbols in plain TeX

1. I would be much obliged if you cold tell me how can I make an arc over a letter (or a group of letters) in TeX. The command in LaTeX is

\stackrel{\frown}{AB}


but in TeX it proves to be useless.

2. How can I write in $\cases{ ...}$ math symbols to look like in $$...$$? For example, $\sum_a^b$ as compared to $$\sum_a^b$$. Thank you.

You can use the yhcmex10 font that provides an extensible arc accent:

\font\yhcmex=yhcmex10
\newfam\yhfam
\textfont\yhfam=\yhcmex
\def\hexdigit#1{\ifcase#1
0\or 1\or 2\or 3\or 4\or 5\or 6\or 7\or
8\or 9\or A\or B\or C\or D\or E\or F\fi}
\edef\wideparen{\mathaccent"0\hexdigit\yhfam F3}

\catcode@=11
\def\dcases#1{%
\left \{\,\vcenter{\normalbaselines\m@th\ialign
{$\displaystyle##\hfil$&\quad##\hfil\crcr#1\crcr}}\right.}
\catcode@=12

$\wideparen{AB}$

f(x)= \dcases{{x+1\over 2} & if x<0\cr\noalign{\vskip3pt} x^2 & if x\ge 0\cr}

\bye


• The $\dcases{...}$ works, but the $\wideparen{AB}$ produces AB. – Ioan Merches Oct 1 '13 at 5:10
• @IoanMerches What TeX distribution are you using? – egreg Oct 1 '13 at 8:13
• XUBUNTU 32 bits – Ioan Merches Oct 1 '13 at 10:47
• TeX, Version 3.1415926 (TeX Live 2009/Debian) – Ioan Merches Oct 1 '13 at 17:39