2

how do I type the weak star convergence symbol, i.e. star symbol over \rightharpoonup?

I tried \xrightharpoon{\ast}, but it is not working.

3

The use of \overset (amsmath package) is the canonical answer to your question. However, if the vertical spacing needs a different value, one can use the stackengine package to achieve it.

\documentclass{article}
\usepackage{amsmath,stackengine}
\begin{document}
$a \overset{\ast}{\rightharpoonup} b$

$a \mathrel{\ensurestackMath{\stackon[1pt]{\rightharpoonup}{\scriptstyle\ast}}} b$
\end{document}

enter image description here

3

The LaTeX kernel provides the macro \stackrel. PlainTeX provides the instruction \buildrel.

enter image description here

\documentclass{article}
\begin{document}
$a\stackrel{\ast}{\rightharpoonup}b$

$a\buildrel\ast\over\rightharpoonup b$
\end{document}
  • 2
    Brings new meaning to the expression, "the stars are aligned..." – Steven B. Segletes Jul 3 '17 at 19:43
  • @StevenB.Segletes -- :-) It is sort of remarkable that there are (at least!) four different methods available: your StackEngine-based method, \overset (from the amsmath package), \stackrel (from the LaTeX kernel), and \buildrel (a PlainTeX macro that's also featured in the LaTeX kernel.) – Mico Jul 3 '17 at 20:06
  • 1
    Well, you know what my profile says... "As a hobby, I enjoy fonts... and reinventing the wheel." – Steven B. Segletes Jul 3 '17 at 20:09
  • @StevenB.Segletes - Incidentally, if the objective were (in part...) to truly reinvent the wheel, i.e., to replicate exactly the look produced by \buildrel, \stackrel, and \overset using the StackEngine machinery, I might be tempted to replace [1pt] with [2pt] or, better still, [0.45ex]. – Mico Jul 3 '17 at 20:13
  • 1
    Indeed...pretending that I'm not actually reinventing the wheel is one of the secrets to my success. Shhh. – Steven B. Segletes Jul 3 '17 at 20:18

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