Command for paragraph indentation after equations

Question

I want to write two new commands \eqend and \eqcntd so that when I put the command at the end of an equations,

1. The \eqend command puts a "." after the equation and forces the next paragraph to indent, regardless of the number of blank lines after the equation.
2. The \eqcntd command puts a "," after the equation and forces the next paragraph not to indent, regardless of the number of blank lines after the equation.

This is a sample usage,

\begin{equation}
f(x) = x^2 + 1 \eqend
\end{equation}
Some text here, but it's gonna get indented.

and another one,

\begin{equation}
f(x) = x^2 + 1 \eqcntd
\end{equation}


Some text here, but it's not gonna get indented.

Here is my NOT-WORKING solution,

\newcommand{\eqend}{\,. \mbox{\par}}
\newcommand{\eqcntd}{, \mbox{\noindent}}

Answer 1

This is not an answer to your question, but perhaps it might help.

I would redefine the equation environment itself. For example, with plain TeX you could define something like (didn't test though)

\def\eqindent#1{$$#1\,.$$\indent\ignorespaces}

and use it like this:

\eqindent{f(x) = x^2 + 1}
Some text here, but it's gonna get indented.

And similarly for the \noindent case. And there ought to be a similar way to do that with LaTeX.

Answer 2

As Barbara Beeton has commented, forcing the first paragraph after an equation environment to indent can be achieved with \aftergroup -- use this command to add a \par after the group established by the environment.

Forcing the first paragraph after an environment not to indent isn't that straightforward. I came up with the following:

• I defined a new macro \@doendeq that will redefine \everypar so that it a) removes the indentation from the first paragraph following it b) reverts to its original (empty) definition. (\@doendeq is an abridged version of LaTeX's \@doendpe which is used after paragraph-making environments.)

• To change every instance of the equation environment to remove indents after it, one could simply add \aftergroup\@doendeq to the definition of \endequation. As you are looking for a command to modify only some instances of equation, I used \csappto from the etoolbox package to locally add\aftergroup\@doendeq to \end<\@currenvir> (\@currenvir being the name of the current environment).

I suspect that there are more elegant ways to do this.

\documentclass{article}

\usepackage{etoolbox}

\makeatletter
\newcommand*{\@doendeq}{%
  \everypar{{\setbox\z@\lastbox}\everypar{}}%
}
\newcommand*{\eqend}{\,.\aftergroup\par}
\newcommand*{\eqcntd}{%
  \,,%
  \csappto{end\@currenvir}{%
    \aftergroup\@doendeq
  }%
}
\makeatother

\begin{document}

\begin{equation}
f(x) = x^2 + 1 \eqend
\end{equation}
Some text here, but it's gonna get indented.

\begin{equation}
f(x) = x^2 + 1 \eqcntd
\end{equation}

Some text here, but it's not gonna get indented.

\end{document}