Theorem numbers in bold

Question

With the standard theorem-like environemnts (at least when using amsthm), one obtains something like

Theorem 2.3.

where "Theorem" is in bold but "2.3." is not. I would like to have the number in bold too. A first solution may look like

\newtheorem{thm}{Theorem}[section]
\renewcommand{\thethm}{\textbf{\arabic{section}.\arabic{thm}}}

The problem with this solution is that every reference to a theorem number will be in bold as well, and I don't want this inside the text.

Is there a way to have only the numbers in theorem headings in bold?

EDIT As it was pointed out, my actual example is slightly more complicated than that. I am using a modified theorem style; a minimal example follows

\documentclass[reqno]{amsart}

\usepackage{amsthm, lipsum}
\swapnumbers

\newtheoremstyle{dotless-thm}
{3pt}
{3pt}
{\it}
{}
{\bfseries}
{}
{.5em}
{}

\theoremstyle{dotless-thm}
\newtheorem{thm}{Theorem}[section]

\begin{document}
\begin{thm}
\lipsum
\end{thm}
\end{document}

Even though I use bfseries for the theorem body font, the number appears in normal font.

Answer 1

You can redefine the internal command \swappedhead of amsart.cls which changes the number font when the \swapnumbers command is invoked. This can be done adding the lines between \makeatletter and \makeatother in my example code:

\documentclass{amsart}
\usepackage{amsthm}
\swapnumbers

\makeatletter
\def\swappedhead#1#2#3{%
  % original definition:
  % \thmnumber{\@upn{\the\thm@headfont#2\@ifnotempty{#1}{.~}}}%
  % change:
  \thmnumber{\@upn{\the\thm@headfont#2\@ifnotempty{#1}{.~}}}%
  \thmname{#1}%
  \thmnote{ {\the\thm@notefont(#3)}}}
\makeatother

\newtheoremstyle{dotless-thm}
  {3pt}
  {3pt}
  {\itshape}
  {}
  {\bfseries}
  {}
  {.5em}
  {}
\theoremstyle{dotless-thm}
\newtheorem{theo}{Theorem}

\begin{document}

\begin{theo}
Test
\end{theo}
\end{document}

EDIT: as Seamus has suggested in his answer, the fact that the number font for theorem-like structures changes if the \swapnumbers switch is in use is a feature provided by the AMS document classes.

Answer 2

Quoting from amsthdoc.pdf

In AMS document classes, when numbers are swapped, the style of the numbers matches that of section headings; this may not be the same style as the rest of the theorem heading.

That's what's causing the problem. If you get rid of \swapnumbers then you should be fine. Or if you really want swapnumbers, you might need to do some jiggery pokery with the section fonts or some such...

The ntheorem package is more powerful, and might provide a simpler way to achieve what you want. Very quickly, you could do this.

%\documentclass[reqno]{amsart}
\documentclass{article}
\usepackage{ntheorem, lipsum}
\theoremstyle{change}
\newtheorem{thm}{Theorem}[section]
\begin{document}
\begin{thm}
\lipsum
\end{thm}
\end{document}

Note the change of documentclass: there seems to be a conflict between amsart and ntheorem.

Answer 3

Use the asmthm style file and then

\newtheoremstyle{common}
    {6pt plus 5\p@ minus 2\p@}% above space (default)
    {6pt plus 5\p@ minus 2\p@}% below space
    {\itshape}% body font
    {0em}% indent
    {\bfseries}% head font
    {}% punct after head
    {.5em}% space
    {}% custom
\theoremstyle{common}

%%%With Chapter Number
\newtheorem{theorem}{Theorem}[chapter]%