Theoremstyle; leaving vertical space between the theorem header and the body of the theorem

Question

I'm trying to write a thesis. There, they want a completely blank line after theorem, lemma, proposition, etc.. name. How can I leave such a vertical space between Theorem Head and the body of the theorem ? As an example:

2.1.1. Theorem

Some text...

Thanks for your interest.. Best regards

Answer 1

One option using amsthm and a new defined style (I also used \swapnumbers since your example suggests that you want numbers to appear before the name):

\documentclass{article}
\usepackage{amsthm}

\swapnumbers

\newtheoremstyle{newline}
  {\topsep}%Space above
  {\topsep}%Space below
  {\itshape}%Body font
  {}%Indent amount
  {\bfseries}% Theorem head font
  {}%Punctuation after theorem head
  {\newline}%Space after theorem head
  {}% Theorem head specification
\theoremstyle{newline}
\newtheorem{theo}{Theorem}

\begin{document}

\begin{theo}    
A test theorem.
\end{theo}

\end{document}

The result:

The ntheorem package offers you the out of the box style changebreak:

\documentclass{article}
\usepackage{ntheorem}

\theoremstyle{changebreak}
\newtheorem{theo}{Theorem}

\begin{document}

\begin{theo}    
A test theorem.
\end{theo}

\end{document}

The examples above will produce the head, a line break and then the body of the structure; if a complete blank line separating the head and the body is required, then you can modify my examples. For the first case, using amsthm, one could say something like

\documentclass{article}
\usepackage{amsthm}

\swapnumbers

\newtheoremstyle{newline}
  {\topsep}%Space above
  {\topsep}%Space below
  {\itshape}%Body font
  {}%Indent amount
  {\bfseries}% Theorem head font
  {\vspace{\baselineskip}}%Punctuation after theorem head
  {\newline}%Space after theorem head
  {}% Theorem head specification
\theoremstyle{newline}
\newtheorem{theo}{Theorem}

\begin{document}

\begin{theo}    
A test theorem.
\end{theo}

\end{document}

which will now produce

but I find this extra spacing to be odd and wouldn't recommend this style. Something similar (defining a new style) can be done using ntheorem.

Answer 2

This style of theorem is already defined in the ntheorem package: it's the changebreak style. Here is a demo, with two different vertical spacings between title and text:

\documentclass[12pt,a4paper]{article}

\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}

\usepackage[thref, amsthm, thmmarks]{ntheorem}

\theoremstyle{changebreak}

\theoremseparator{}
\newtheorem{thm}{Theorem}[section]

\theoremseparator{\medskip}
\newtheorem{prop}{Proposition}[section]

\begin{document}
\setcounter{section}{3}
Here is a test :
\begin{thm}
  This is a fundamental result
\end{thm}

\begin{prop}
  Now with a small space between title and text.
\end{prop}

\end{document}