Subsection label appearing within text

Question

I have a document in "book" format (strictly speaking, in an environment defined by the book publisher I am using, the details of which are way beyond my comprehension), and I very much want to have certain paragraphs labeled with the same running count as for theorems, lemmas, etc. but with the numerical label appearing as boldface within the text and not set off on its own line.

For example, between Theorem 2.3.7 and Theorem 2.3.9 within section 2.3 of Chapter 2, I'd like to be able to make some intermediate paragraph of my choosing begin with either "2.3.8" in boldface (and ordinary-size font), or perhaps "2.3.8 Blah-blah" in boldface (and ordinary-size font). It is also very important for this command to allow the possibility of using a \label command associated to that numerical label "2.3.8" for cross-referencing purposes.

In order to increase the chances of a solution being usable by the book publisher (whose typesetting staff has been unable to provide me with a solution), please try to make answers as simple as possible.

Although I cannot reproduce the publishers proprietary documentclass here (it is much too large, and incomprehensible to me), here is a test .tex file that I tried based on the comment below suggesting to use the command \paragraph:

\documentclass{book}

\begin{document}

\chapter{A class of groups}

Let $G$ be a group.

\section{A class of subgroups}

Let $H$ be a subgroup.

\paragraph{Refinements}\label{refined} This can be refined.

In \ref{refined} we saw something.

\end{document}

The output that I get does not make any numerical label appear in front of "Refinements", and the cross-reference to it at the end produces the numerical label for the section, so it fails miserably. I want that labelled paragraph to begin with "1.1.1. Refinements" in boldface, and the cross-reference at the end should appear as "In 1.1.1 we saw something".

I should also point out that a version (given to me long ago from a source I cannot remember) does work within the amsart environment, where the hierarchy shifts by one: instead of \chapter, \section, \subsection it is \section, \subsection, \subsubsection, and the following gives exactly what I seek as a subsubsection (but the publisher told me that they cannot adapt it; hmm, maybe I should specifically ask them to try replacing "subsubsection" with "subsection" below, in case they didn't try that?).

\makeatletter

\def\@seccntformat#1{\@ifundefined{#1@cntformat}%

{\csname the#1\endcsname\quad}% default

{\csname #1@cntformat\endcsname}% individual control

}

\def\subsubsection@cntformat{{\rm{\textbf{\thesubsubsection.}}}}

\makeatother

Answer 1

Define a new theorem environment.

\documentclass{book}

\usepackage{amsthm}
\newtheorem{thm}{Theorem}[section]

\theoremstyle{definition}
\newtheorem{interthm}[thm]{\ignorespaces}% theorem label is empty

\begin{document}

\chapter{A class of groups}

Let $G$ be a finite group.

\section{A class of subgroups}

Let $H$ be a subgroup of $G$.

\begin{thm}\label{lagrange}
The order of $H$ divides the order of $G$.
\end{thm}

\begin{interthm}\label{lagrange-comment}
Theorem~\ref{lagrange} is credited to Lagrange and can be
inverted for abelian groups.
\end{interthm}

\begin{thm}\label{inverse-lagrange}
If $G$ is abelian and $k$ is a divisor of the order
of $G$, then $G$ has a subgroup of order $k$.
\end{thm}

In \ref{lagrange-comment} we saw something.

\end{document}

You can make it into a command by using an \everypar trick; use it only at the outer level.

\documentclass{book}

\usepackage{amsthm}
\newtheorem{thm}{Theorem}[section]

\newcommand{\interthm}{%
  \par\refstepcounter{thm}%
  \everypar{{\setbox0=\lastbox}\textbf{\thethm.} \everypar{}}%
}

\begin{document}

\chapter{A class of groups}

Let $G$ be a finite group.

\section{A class of subgroups}

Let $H$ be a subgroup of $G$.

\begin{thm}\label{lagrange}
The order of $H$ divides the order of $G$.
\end{thm}

\interthm\label{lagrange-comment}
Theorem~\ref{lagrange} is credited to Lagrange and can be
inverted for abelian groups.

\begin{thm}\label{inverse-lagrange}
If $G$ is abelian and $k$ is a divisor of the order
of $G$, then $G$ has a subgroup of order $k$.
\end{thm}

In \ref{lagrange-comment} we saw something.

\end{document}

Note that, in this case, no additional vertical space below the numbered paragraph can be set automatically.

Answer 2

This provides an environment called \intertheoremremark similar to paragraph (which is not an environment at all, of course), just with a subsection like label and a bold font heading leadin. \label works for this.

This uses the assoccnt package, which provides a master counter, driving other counters, when stepped, in this case, the mytheorem counter.

However, a master counter cannot be driven by another counter, which is driven by the master.

\documentclass{scrbook}


\usepackage{blindtext}
\usepackage{amsmath}
\usepackage{amsthm}%
\usepackage{assoccnt}%




\newcounter{intertheoremremark}


\renewcommand{\theintertheoremremark}{\thesection.\arabic{intertheoremremark}}

\newenvironment{intertheoremremark}[2][]{%
\setcounter{intertheoremremark}{\number\value{mytheorem}}%
\setlength{\parindent}{0pt}%
\refstepcounter{intertheoremremark}%
\large  \textbf{\theintertheoremremark~#2}~
}{}%




\newtheorem{mytheorem}{Theorem}

\renewcommand{\themytheorem}{\thesection.\arabic{mytheorem}}

\DeclareAssociatedCounters{intertheoremremark}{mytheorem}%



\begin{document}

\chapter{Number one}

\section{First section}
\blindtext

\section{Second section}
\blindtext

\begin{mytheorem}{On Brontosaurs} \label{theorem::brontosaurs}
\blindtext
\end{mytheorem}


\begin{intertheoremremark}{Remarks to Theorem \ref{theorem::brontosaurs}} \label{firstremark}%
\blindtext
\end{intertheoremremark}%


\begin{mytheorem}{Other theory on  Brontosaurs} \label{theorem::brontosaurs2}
\blindtext
\end{mytheorem}


\begin{intertheoremremark}{Remarks to Theorem \ref{theorem::brontosaurs2}}%
This is a follow up remark to \ref{firstremark} 
\blindtext
\end{intertheoremremark}%

\end{document}

Edit

I should add, that I am the author of assoccnt package, which is still in a rudimentary version available on CTAN and TeXlive 2014

Updated version with \paragraph coupled to mytheorem etc.

This does not use the assoccnt package, but uses paragraph instead of intertheoremremark environment. The behaviour of stepcounter is slightly changed, as assoccnt would do. The screen shots are nearly the same as in the version above.

\documentclass{book}

\usepackage{etoolbox}%

\usepackage{blindtext}
\usepackage{amsmath}
\usepackage{amsthm}%
\usepackage{remreset}%


\newtheorem{mytheorem}{Theorem}%


\makeatletter
\@removefromreset{paragraph}{subsubsection}
\@addtoreset{paragraph}{chapter}%
\@addtoreset{mytheorem}{chapter}%
\makeatother




\setcounter{secnumdepth}{4}%


\let\standardstepcounter\stepcounter

\renewcommand{\stepcounter}[1]{%
  \ifstrequal{#1}{paragraph}{%
    \standardstepcounter{mytheorem}%
  }{  \ifstrequal{#1}{mytheorem}{%
      \standardstepcounter{paragraph}%
    }{}}%
  \standardstepcounter{#1}%
}%



\renewcommand{\themytheorem}{\thesection.\arabic{mytheorem}}

\renewcommand{\theparagraph}{\thesection.\arabic{paragraph}.}%





\begin{document}

\chapter{Number one}

\section{First section}
\blindtext

\section{Second section}
\blindtext

\begin{mytheorem}{On Brontosaurs} \label{theorem::brontosaurs}
\blindtext
\end{mytheorem}


\paragraph{Remarks to Theorem \ref{theorem::brontosaurs}} \label{firstremark}%
\blindtext

\begin{mytheorem}{Other theory on  Brontosaurs} \label{theorem::brontosaurs2}
\blindtext
\end{mytheorem}


\paragraph{Remarks to Theorem \ref{theorem::brontosaurs2_again}}%
This is a follow up remark to \ref{secondremark}%
\blindtext


\chapter{Number Two}

\section{First section}
\blindtext

\section{Second section}
\blindtext

\begin{mytheorem}{On Brontosaurs} \label{theorem::brontosaurs_again}
\blindtext
\end{mytheorem}


\paragraph{Remarks to Theorem \ref{theorem::brontosaurs_again}} \label{secondremark}%
\blindtext

\begin{mytheorem}{Other theory on  Brontosaurs} \label{theorem::brontosaurs2_again}
\blindtext
\end{mytheorem}


\paragraph{Remarks to Theorem \ref{theorem::brontosaurs2}}%
This is a follow up remark to \ref{firstremark} 
\blindtext


\end{document}