Amsthm numbering depending on section depth

Question

I'm using amsthm and thmtools to define custom theorem environments, definition environments, etc. All such environments share the same counter.

Is it possible to vary the numbering format depending on the section depth? I want to achieve the following numbering format:

1 Section

1.1 Theorem

1.2 Definition

1.1 Subsection

1.1.1 Theorem

1.1.2 Lemma

Answer 1

i'm not sure if you want something like this:

\documentclass{article}
\usepackage{amsmath}
\usepackage{amsthm}
\swapnumbers
\newtheorem{theorem}{Theorem}[section]
\newtheorem{definition}[theorem]{Definition}
\newtheorem{lemma}[theorem]{Lemma}
\usepackage{thmtools}

\usepackage{xpatch}
\makeatletter
\newif\ifsection
\newif\ifsubsection
\preto\section{\sectiontrue\subsectionfalse}
\preto\subsection{\sectionfalse\subsectiontrue}
\patchcmd{\@xsect}% <cmd>
  {\ignorespaces}% <search>
  {\ifsection
    \numberwithin{theorem}{section}
    \else
    \numberwithin{theorem}{subsection}
    \fi
    \setcounter{theorem}{0}\relax\ignorespaces}% <replace>
  {}{}% <success><failure>
\makeatother
\begin{document}

\section{Section}
\begin{theorem}
Th 1
\end{theorem}
\begin{definition}
Df 1
\end{definition}
\subsection{Subsection}
\begin{theorem}
Th 2
\end{theorem}
\begin{lemma}
Lm 1
\end{lemma}
\section{Two}
\begin{theorem}
Th 3
\end{theorem}
\begin{lemma}
Lm 2
\end{lemma}
\subsection{Other}
\begin{theorem}
Th 4
\end{theorem}
\subsection{Other 2}
\begin{definition}
Df 2
\end{definition}
\end{document}

Answer 2

You can prefix the theorem counter with a section-specific numbering scheme:

\documentclass{article}
\usepackage{amsthm,etoolbox}
\newtheorem{theorem}{Theorem}
\newtheorem{definition}[theorem]{Definition}
\newtheorem{lemma}[theorem]{Lemma}

\makeatletter
\@addtoreset{theorem}{section}% Reset theorem counter with every section
\@addtoreset{theorem}{subsection}
\@addtoreset{theorem}{subsubsection}
\newcommand{\theoremprefix}{}
\let\thetheoremsaved\thetheorem
\renewcommand{\thetheorem}{\theoremprefix\thetheoremsaved}
\let\sectionsaved\section
\patchcmd{\@startsection}{\par}{\renewcommand{\theoremprefix}{\csname the#1\endcsname.}}{}{}
\makeatother

\begin{document}

\section{Section}
\begin{theorem}
Th 1
\end{theorem}
\begin{definition}
Df 1
\end{definition}
\subsection{Subsection}
\begin{theorem}
Th 2
\end{theorem}
\begin{lemma}
Lm 1
\end{lemma}
\section{Two}
\begin{theorem}
Th 3
\end{theorem}
\begin{lemma}
Lm 2
\end{lemma}
\subsection{Other}
\begin{theorem}
Th 4
\end{theorem}
\subsection{Other 2}
\begin{definition}
Df 2
\end{definition}