Internal Direct Product of subgroups

Question

I want to write the internal direct product of a subgroup.

It looks like $$\prod_{i=1}^{n} M_i,$$ but there is a dot inside the product symbol.

Can anyone help me?

Answer 1

Using code from https://tex.stackexchange.com/a/52673/4427, here are two alternatives.

\documentclass{article}

\makeatletter
\def\moverlay{\mathpalette\mov@rlay}
\def\mov@rlay#1#2{\leavevmode\vtop{%
   \baselineskip\z@skip \lineskiplimit-\maxdimen
   \ialign{\hfil$\m@th#1##$\hfil\cr#2\crcr}}}
\newcommand{\charfusion}[3][\mathord]{
    #1{\ifx#1\mathop\vphantom{#2}\fi
        \mathpalette\mov@rlay{#2\cr#3}
      }
    \ifx#1\mathop\expandafter\displaylimits\fi}
\makeatother

\newcommand{\dirprod}{\charfusion[\mathop]{\prod}{\cdot}}
\newcommand{\dirprodA}{\charfusion[\mathop]{\prod}{\bullet}}

\begin{document}

\[
\dirprod_{i=1}^{n} M_i,
\]
\[
\dirprodA_{i=1}^{n} M_i,
\]

\end{document}

Some slight change is needed for plain TeX.