How can remove double dot in Theorem numbering

Question

I use this code below to get the form name: Chapter I., II., III., section 1., subsection 1.1. in both text and table of contents.:

\renewcommand\thechapter{\Roman{chapter}.}
\renewcommand\thesection{\arabic{section}.}
\renewcommand\thesubsection{\thesection \arabic{subsection}.}
\renewcommand\thesubsubsection{\alph{subsubsection}.}

But when I use the theorem environment:

\usepackage[all]{xy}
\usepackage[thmmarks]{ntheorem}
\newtheorem{Lemma}{Lemma}[section]
\newtheorem{dl}[Lemma]{Định lý}

the result I get is:

I want to change the double dot to one dot only like 1.1., 1.2. ,...

Answer 1

Define

\def\theLemma {\thesection \arabic {Lemma}}

after \newtheorem{Lemma} is used. The command \newtheorem{Lemma} generates \theLemma macro as \thesection.\arabic {Lemma} but you don't want the dot here.

Answer 2

If you execute instructions such as

\renewcommand\thesection{\arabic{section}.}  

not only will the headers of Lemma and dl environments look wrong because of the spurious extra period (aka "full stop"), but cross-references to section, subsection, and subsubsection level headers will also feature a spurious period.

I therefore suggest you employ a different solution method, which is shown in the code below.

With this approach, you also needn't redefine \theLemma, \thedl, etc.

\documentclass{report}
\usepackage[vietnamese]{babel}

% Method proposed in "The LaTeX Companion", 2nd ed.:
\makeatletter
\def\@seccntformat#1{\@ifundefined{#1@cntformat}%
   {\csname the#1\endcsname\space}%    default
   {\csname #1@cntformat\endcsname}}%  enable individual control  
\newcommand\section@cntformat{\thesection.\space}       % section
\newcommand\subsection@cntformat{\thesubsection.\space} % subsection
\newcommand\subsubsection@cntformat{\thesubsubsection.\space} % subsection
\makeatother

\usepackage[thmmarks]{ntheorem}
\newtheorem{Lemma}{Lemma}[section]
\newtheorem{dl}[Lemma]{Định lý}

\begin{document}
\chapter{Hello}
\section{World}

\begin{Lemma} Bla bla bla \end{Lemma}
\begin{dl} Bla bla bla \end{dl}

\end{document}

Answer 3

It depends on what you want when doing cross-references.

Anyway, since every generated number has a trailing period, also lemmas should share the style.

\documentclass[a4paper]{report}
\usepackage[vietnamese]{babel}

\usepackage[thmmarks]{ntheorem}

\renewcommand\thechapter{\Roman{chapter}.}
\renewcommand\thesection{\arabic{section}.}
\renewcommand\thesubsection{\thesection \arabic{subsection}.}
\renewcommand\thesubsubsection{\alph{subsubsection}.}

\newtheorem{Lemma}{Lemma}[section]
\renewcommand{\theLemma}{\thesection\arabic{Lemma}.}

\newtheorem{dl}[Lemma]{Định lý}

\begin{document}

\tableofcontents

\chapter{Test}\label{ch:test}

\section{Test}\label{sec:test}

\begin{dl}\label{dl:test}
Test
\end{dl}

\ref{ch:test}
\ref{sec:test}
\ref{dl:test}

\end{document}

As you see, all cross-references will have the trailing period. If you don't want it, you need some more work.