Declaretheoremstyle headpostspace = \newline + indent?

Question

I am having problems trying in declaring a theorem style. I would like the first row of the definition to be on a new line with the normal indent. If I try to add \par instead of \newline for postheadspace, I am getting error. Also postheadhook doesn't seem to work.

Thank you in advance :)

\documentclass[12pt,reqno]{amsbook}

\usepackage{thmtools}

\declaretheoremstyle[
headfont=\scshape\bfseries, 
headindent = \parindent,
postheadhook = {\hspace{0mm}\newline},
postheadspace = \newline, 
spaceabove = 0.5cm, 
spacebelow = 0.5cm]{mydef}

\newtheorem{Theorem}{Theorem}[chapter]

\theoremstyle{mydef}

\newtheorem{Definition}[Theorem]{Definition}
\begin{document}

\begin{Definition}
This is a very long definition which i want to start with indent on the next row instead of from the beggining

Like this
\end{Definition}

\end{document}

Answer 1

For your style to properly apply, you need to use \declaretheorem to define your structure and not \newtheorem. Using postheadhook = {\hspace*{\parindent}} you'll get the desired output (I left the Theorem structure with \newtheorem since apparently it shouldn't have the mydef style; if you want to apply this style for Theorem, please use \declaretheorem instead; if you don't wnat the indentation for the head, delete the line headindent = \parindent):

\documentclass[12pt,reqno]{amsbook}
\usepackage{thmtools}

\declaretheoremstyle[
headfont=\scshape\bfseries, 
headindent = \parindent,
postheadhook = {\hspace*{\parindent}},
postheadspace = \newline, 
spaceabove = 0.5cm, 
spacebelow = 0.5cm]{mydef}

\newtheorem{Theorem}{Theorem}[chapter]
\declaretheorem[style=mydef,numberlike=Theorem]{Definition}

\begin{document}

\begin{Definition}
This is a very long definition which will start with indent on the next row instead of from the beginning.

Like this
\end{Definition}

\end{document}

Answer 2

Here is another solution:

\documentclass[12pt,reqno]{amsbook}

\usepackage{thmtools}

\declaretheoremstyle[
headfont=\scshape\bfseries, 
headindent = \parindent,
postheadhook = {\hspace{0mm}\newline},
spaceabove = 0.5cm, 
spacebelow = 0.5cm]{mydef}

\newtheorem{Theorem}{Theorem}[chapter]

\theoremstyle{mydef}

\newtheorem{DefinitionAux}[Theorem]{Definition}
\newenvironment{Definition}{\begin{DefinitionAux}\leavevmode\par}{\end{DefinitionAux}}
\begin{document}

\begin{Definition}
This is a very long definition which i want to start with indent on the next row instead of from the beggining

Like this
\end{Definition}

\end{document}

Here are some links to related questions:

• Theorem environment - line break after label [closed]

• Newline after theorem

• Is it possible to skip the first line in a theorem environment?

• Skipping line after "Proof" in proof environment

• Enumerate alignment problem in theorem environment

• Theorem environment with a line break after label and before an enumerate environment [closed]

• Newline after theorem doesn't work if it starts with \begin{enumerate}

• The statement of a theorem appearing on a new line

• Line break after definition

• Itemizing theorem body

• Theorem environment : interaction with enumerate/itemize

• Theorem formatting: strange interaction with itemize