Writing a theorem inside a theorem

Question

I want to write couple of theorems inside a main theorem body. In particular I want to write something like the following:

I tried using the command \newtheorem to create Main Theorem and then used the command \begin{theorem} inside it but it doesn't serve the purpose. I want something like this to be specific: Thanks for the help!!

Answer 1

Here's an implementation using enumitem.

\documentclass{article}
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{enumitem}

\usepackage{lipsum}

\newtheorem{theorem}{Theorem}[section]
\newtheorem*{main}{Main Theorem}

\newenvironment{subtheorems}
 {\itemize[
   nosep,font=\normalfont\bfseries,
   leftmargin=3em,itemindent=-1em,align=left]}
 {\enditemize}

\begin{document}

\section{Introduction}

\lipsum[2]

\begin{main}
We obtain the following bijective correspondences.
\begin{subtheorems}
\item[$\mathbf{C}^*$-case (Theorem \ref{C*case}):]
Let $\Gamma$ be a discrete group \lipsum*[3]

\item[$\mathbf{W}^*$-case (Theorem \ref{W*case}):]
Let $\Gamma$ be a countable group \lipsum[4]
\end{subtheorems}
\end{main}

\lipsum

\setcounter{section}{2}\setcounter{theorem}{2}
\begin{theorem}\label{C*case}
Text
\end{theorem}

\setcounter{section}{3}\setcounter{theorem}{5}
\begin{theorem}\label{W*case}
Text
\end{theorem}

\end{document}

A different realization, with numbered subtheorems.

\documentclass{article}
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{enumitem}

\usepackage{lipsum}

\newtheorem{theorem}{Theorem}[section]
\newtheorem*{main}{Main Theorem}

\newenvironment{subtheorems}
 {\enumerate[
     label=Theorem \Alph*\protect\thissubtheorem,
     ref=\Alph*,
     font=\normalfont\bfseries,
     leftmargin=0pt,itemindent=!,
     align=left,
  ]}
 {\endenumerate}
\newcommand{\thissubtheorem}{}
\newcommand{\subtheorem}[1][]{%
  \if\relax\detokenize{#1}\relax
    \def\thissubtheorem{}%
  \else
    \def\thissubtheorem{ (#1)}%
  \fi
  \item
}

\begin{document}

\section{Introduction}

\lipsum[2]

\begin{main}
We obtain the following bijective correspondences.
\begin{subtheorems}
\subtheorem[$\mathbf{C}^*$-case]\label{C*case}
Let $\Gamma$ be a discrete group \lipsum*[3]

\subtheorem[$\mathbf{W}^*$-case]\label{W*case}
Let $\Gamma$ be a countable group \lipsum[4]
\end{subtheorems}
\end{main}

Theorem~\ref{C*case} and theorem~\ref{W*case} are important.

\end{document}