I have to write some equations in latex, my Latex equation:
\begin{align}
G _{a_{1},\ldots ,a_{k}}\ast_{b_{1},\ldots ,b_{k}} G^{\prime }=G^{\prime \prime }
\end{align}
How can I make the text like this:

I have to write an equations in latex like this:






\mathcal{S}to\ast. – yo' Mar 12 '15 at 15:29