How to write a long \rightharpoonup?

Question

How to write a long $\rightharpoonup$ with the possibility to write something above and below the arrow.

I need to write below $k \to +\infty$, so the $\rightharpoonup$ must be as long as $k \to +\infty$.

Answer 1

mathtools defines a couple of extensible symbols in addition to the kernel and amsmath defined ones:

\documentclass{article}
\usepackage{mathtools}
\begin{document}
$\xrightharpoonup{k\to+\infty}$
\end{document}

or if the label should be below

\documentclass{article}
\usepackage{mathtools}
\begin{document}
$a \xrightharpoonup[k\to+\infty]{} b$
\end{document}

(In general \xrightharpoonup[below]{above}.)