Theorem style like (1.1 Theorem) and custom theorems like (1.2 Intermadiate Value Theorem)

Question

I'm searching for a theorem style like that the one that is used in this book:
Elementary Analysis by Ross

I've got already most things working, except that I want to get a custom theorem like this:

When I type something like:

\begin{thm}[Rational Zeros Theorem]
...
\end{thm}

The rest of the styles that I want are:

1. Having the number and theorem swapped
2. Having a newline after the head of the theorem/defintion
3. Having "Proof." in boldface instead of italic and a newline after it
4. Having also a newline if the proof begins with a list

I've been already trying to implement 1,2 and 3, and if you are interested, you can find it on github: https://github.com/kasperpeulen/Ross-Theorem-Style

I've not yet solved 4, but I found some other posts here, so I think I can solve that by myself, but any help with that would be appreciated as well of course.

Answer 1

Okay, I've got it working now, thanks to other posts at this website, help from percusse in the chat:

Here is the code, which you can also find here: https://github.com/kasperpeulen/Ross-Theorem-Style

\documentclass[12pt]{article}
\usepackage{amsmath, amssymb, amsthm}

\newtheoremstyle{theorem}% name
  {}%         Space above, empty = `usual value'
  {}%         Space below
  {\itshape}% Body font
  {}%         Indent amount 
  {\bfseries}% Head font
  {.}%        Punctuation after head
  {\newline}% Space after head: \newline = linebreak
   {\def\temp{#3}\ifx\temp\empty\thmnumber{#2 }\thmname{#1}\else\thmnumber{#2}\thmnote{ #3}\fi}%         Head spec

\theoremstyle{theorem}
\newtheorem{thm}{Theorem}[section]
\newtheorem{prop}[thm]{Proposition}
\newtheorem{lem}[thm]{Lemma}
\newtheorem{cor}[thm]{Corollary}

\swapnumbers    
\newtheoremstyle{definition}% name
  {}%         Space above, empty = `usual value'
  {}%         Space below
  {}% Body font
  {}%         Indent amount
  {\bfseries}% Head font
  {.}%        Punctuation after thm head
  {\newline}% Space after head: \newline = linebreak
  {}%         Head spec

\theoremstyle{definition}
\newtheorem{defn}[thm]{Definition}
\newtheorem{example}[thm]{Example}
\theoremstyle{remark}
\newtheorem{remark}[thm]{Remark}

%Makes "Proof." boldface
\makeatletter
\renewenvironment{proof}[1][\proofname] {\par\pushQED{\qed}\normalfont\topsep6\p@\@plus6\p@\relax\trivlist\item[\hskip\labelsep\bfseries#1\@addpunct{.}]\mbox{}\\*}{\popQED\endtrivlist\@endpefalse}
\makeatother

%Fix if theorem starts with a list
\makeatletter
\def\itemfix{%
\if@inlabel
 \noindent \par\nobreak\vskip-\baselineskip\hrule\@height\z@
\fi}

\let\olditemize\itemize
\def\itemize{\itemfix\olditemize}
\makeatother

\makeatletter
\def\enumfix{%
\if@inlabel
 \noindent \par\nobreak\vskip-\baselineskip\hrule\@height\z@
\fi}
\let\oldenumerate\enumerate
\def\enumerate{\enumfix\oldenumerate}

\begin{document}
\section{Some examples}

\begin{defn}
Here is a definition of \emph{something}.
\end{defn}

\begin{thm}[Intermediate Value Theorem]
This is a theorem with a name and with a number.
\end{thm}

\begin{thm}
This is a theorem without name and with a number.
\end{thm}

\begin{proof}
Here is the proof.
\end{proof}

\begin{thm}
\begin{enumerate}
\item Theorem that starts with a list.
\item Which seems to work.
\end{enumerate}
\end{thm}

\begin{thm}
\begin{itemize}
\item Theorem that starts with a list.
\item Which seems to work.
\end{itemize}
\end{thm}

\end{document}

Answer 2

Here is a simple solution with ntheorem – which has the advantage of placing automatically the end-of-proof (customisable) symbol, even when the proof ends in a display (group of) equation(s):

\documentclass[12pt,a4paper]{article}

\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{amssymb}
\usepackage{mathtools, nccmath}
\usepackage{enumitem}
\usepackage[thmmarks, thref, amsmath]{ntheorem}

\makeatletter
\renewtheoremstyle{changebreak}%
  {\item[\rlap{\vbox{\hbox{\hskip\labelsep \theorem@headerfont
          ##2\ ##1\theorem@separator}\hbox{\strut}}}]}%
  {\item[\rlap{\vbox{\hbox{\hskip\labelsep \theorem@headerfont
          ##2\ ##3\theorem@separator}\hbox{\strut}}}]}
\makeatother

\theoremstyle{changebreak}
\theoremseparator{. }
\theoremheaderfont{\upshape\bfseries}
\theorembodyfont{\itshape}

\newtheorem{thm}{Theorem}[section]

\theorembodyfont{\upshape}
\newtheorem{defi}[thm]{Definition}
\theoremstyle{nonumberbreak}
\theoremseparator{}
\theoremsymbol{\ensuremath{\blacksquare}}
\newtheorem{proof}{Proof}

\begin{document}

\section{A First Section}

\begin{thm}
 A numbered, unnamed theorem.
\end{thm}

\begin{thm}[Rational Zeros Theorem]
Suppose $ c_0 ,c_1 ,\dots, c_n $ be integers and $ r $ a rational number satisfying the polynomial equation.
\begin{equation}\label{intpol}
  c_n xⁿ + c_{n-1} x^{n-1} + ... +c_1 x + c_0 =0
\end{equation}
where $ n ≥ 1$, $ c_{n} \ne 0$ and $ c_0 \ne 0 $. Let $ r = \mfrac{c}{d}$, where $ c, d $ are integers having no common factors and $ d \ne 0 $. Then $ c $ divides $ c_0 $ and $ d $ divides $ c_n $.
\end{thm}

\begin{proof}
\begin{itemize}[wide = 0pt]
  \item[\em First proof.]
Left as an exercise.

  \item[\em Second proof.] We deduce from the hypothesis that
\begin{align*}
   & c_{n}cⁿ + c_{n-1}c^{n-1}d + ... + c_1 cd^{n-1} + c_0 dⁿ = 0, \\
\intertext{which we may rewrite as}
   & c(c_{n}c^{n-2} + c_{n-1}c^{n-2}d + ... + c_1 d^{n-1}) = -c_0 dⁿ\\
\shortintertext{or}
 & d(c_{n-1}c^{n-1} + ... + c_1 cd^{n-2} + c_0 d^{n-1}) = -c_{n}cⁿ.
\end{align*}
\end{itemize}
\end{proof}

\begin{defi}
  A \textbf{rational function} is the quotient of two polynomial functions.
\end{defi}

\end{document}