The \overline command puts a horizontal line above its argument
in math mode:
$\overline{x}+\overline{y}$
gives
There is an analogous \underline command that works in text
or math mode:
The equation \underline{is} $\underline{x+y}$.
gives
Horizontal braces are put above or below an expression with the
\overbrace and \underbrace commands:
$\underbrace{a_{1} + \overbrace{a_{2}+a_{3}}
+ a_{4}}$
gives
and in displayed math, a subscript or a superscript puts a label on the brace:
$\underbrace{a_{1} +
\overbrace{a_{2}+\cdots+a_{n-1}}^{n-2}
+ a_{n}}_{n}$
gives
Wide versions of the \hat and \tilde commands are available.
They are called \widehat and \widetilde, respectively.
Here is an example:
$\widehat{a} + \widehat{ab}
+ \widehat{abc} + \widehat{abcd}$
gives
