Big asterisk “\bigast” symbol

Question

How do I make a math-mode asterisk that works like sum or product operator? (To represent the convolution.)

Answer 1

The simplest would be to use \Asterisk from the mathabx package, and also the \coAsterisk (not sure what the differences are as they look identical to me).

Alternatively, you could use \scalebox from the graphicx package to re-size the standard \ast to suit. Here is a comparison of these three (\Conv is the custom version) showing the behavior in inline and display mode with indices:

\documentclass[fleqn]{article}
\usepackage{mathabx}
\usepackage{graphicx}

\newcommand{\Conv}{\mathop{\scalebox{1.5}{\raisebox{-0.2ex}{$\ast$}}}}%
\begin{document}
{\small\verb|\Asterisk| \verb|\coAsterisk | \verb| \Conv|}

\hspace{0.35cm}% to simply comparrison
$
    f \Asterisk^{a}_{b}   g \qquad 
    f \coAsterisk^{a}_{b} g \qquad 
    f \Conv^{a}_{b}       g
$

\[
    f \Asterisk^{a}_{b}   g \qquad 
    f \coAsterisk^{a}_{b} g \qquad 
    f \Conv^{a}_{b}       g
\]
\end{document}