I have multiple equation inserted within equation environment:
\begin{equation}
\label{Eq:SomeEquation}
P = \theta_{C} - \theta_{Y}
\end{equation}
As mentioned in Comma after equation it is good practice to add comma after each equation.
Am I able to do it by means of one LaTeX command or do I need to add comma in each equation manually?
One may wish to consider separate equation environments that add a comma equationc or a period equationp. In this way, the reading of the source code is straightforward.
\documentclass{article}
\newenvironment{equationc}{\begin{equation}}{,\end{equation}\ignorespacesafterend}
\newenvironment{equationp}{\begin{equation}}{.\end{equation}\ignorespacesafterend}
\begin{document}
One has
\begin{equation}
y = mx + B
\end{equation}
and
\begin{equationc}
y = mx + C
\end{equationc}
lest one ends up with
\begin{equationp}
y = mx + D
\end{equationp}
\end{document}
One can also customize the environment definitions. For example, if one did not wish the comma or period to affect the centering of the equation content, they could be done with lapping:
\newenvironment{equationc}{\begin{equation}}{\rlap,\end{equation}\ignorespacesafterend}
\newenvironment{equationp}{\begin{equation}}{\rlap.\end{equation}\ignorespacesafterend}
As @egreg says, it is not a very good idea to do this automatically: sometimes you need a period rather than comma or nothing at all.
Nevertheless as a TeX exercise, here is a solution. LaTeX has the definition
\def\endequation{\eqno \hbox{\@eqnnum}$$\@ignoretrue}
Thus let us redefine it:
\documentclass{article}
\makeatletter
\def\endequation{\unskip,\eqno \hbox{\@eqnnum}$$\@ignoretrue}
\makeatother
\begin{document}
\thispagestyle{empty}
\begin{equation}
\label{Eq:SomeEquation}
P = \theta_{C} - \theta_{Y}
\end{equation}
\end{document}
A better solution is to make the sign at the end of equation a variable:
\documentclass{article}
\makeatletter
\def\endequation{\eqend\eqno \hbox{\@eqnnum}$$\@ignoretrue}
\makeatother
\newcommand{\eqend}{\unskip,}
\begin{document}
\thispagestyle{empty}
We can make equation end in comma
\begin{equation}
\label{Eq:SomeEquation}
P = \theta_{C} - \theta_{Y}
\end{equation}
or in dot \renewcommand{\eqend}{\unskip.}
\begin{equation}
\theta_{C} =P - \theta_{Y}
\end{equation}
We can omit the sign at all
\renewcommand{\eqend}{}
\begin{equation}
\theta_{Y} =P - \theta_{C}
\end{equation}
if we wish.
\end{document}
