Theorem numbering

Question

I am trying to number my theorems, corollaries, Lemmas, Propositions, Definitions, Remarks etc. I put the following code:

\theoremstyle{theorem} 
\newtheorem{theorem}{Theorem}[section]
\newtheorem{corollary}{Corollary}[theorem]
\newtheorem{lemma}[theorem]{Lemma}
\newtheorem{proposition}[theorem]{Proposition}

\theoremstyle{definition}
\newtheorem{definition}{Definition}[section]
\newtheorem{example}{Example}[definition]

\theoremstyle{remark}    
\newtheorem{remark}{Remark}[section]

I want to produce the following:

theorem 1.1

proposition 1.2

definition 1.3

example 1.4

remark 1.5

lemma 1.6

but the code above gives me

theorem 1.1

definition 1.1

remark 1.1

lemma 1.2

Can anyone tell me if there is a code that numbers all these respectively?

Answer 1

When using either the ntheorem or the amsthm package to declare theorem-like environments, it's important to make use of the following rules of syntax when using \newtheorem:

• If you want the counter of a theorem-like environment to be subordinate to, say, the section number, use a directive such as

  \newtheorem{theorem}{Theorem}[section]
  

  i.e., the option [section] (the name of the "parent counter") should come at the end.

• If, on the other hand, you want the theorem-like environment (say, corollary) to share a counter with an already-existing environment (say, theorem), use the following syntax:

  \newtheorem{corollary}[theorem]{Corollary}
  

  i.e., i..e, the option [theorem] (the name of the "shared counter") should be placed between the environment name and the way the environment's name should be typeset.

The output of a full MWE:

\documentclass{article}  
\usepackage{ntheorem}

\theoremstyle{theorem} 
   \newtheorem{theorem}{Theorem}[section]
   \newtheorem{corollary}[theorem]{Corollary}
   \newtheorem{lemma}[theorem]{Lemma}
   \newtheorem{proposition}[theorem]{Proposition}
\theoremstyle{definition}
   \newtheorem{definition}[theorem]{Definition}
   \newtheorem{example}[theorem]{Example}
\theoremstyle{remark}    
  \newtheorem{remark}[theorem]{Remark}

\begin{document}
\setcounter{section}{1}

\begin{theorem} a \end{theorem}
\begin{proposition} b \end{proposition}
\begin{definition} c \end{definition}
\begin{example} d \end{example}
\begin{remark} e \end{remark}
\begin{lemma} f \end{lemma}

\end{document}