# Changing theorem/lemma… label to bold italics

For theorems in my contribution I use \theoremstyle{plain}. This style prints theorem label in bold italics, eg. 'Theorem 1.', which is followed by text in italics. How could I set 'Theorem 1.' to italics to match the theorem content?

Non italic text in theorems, definitions, examples

but I was not able to extract the answer.

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See tex.stackexchange.com/a/38264/5626 and change \itshape to \itshape\bfseries (and other settings accordingly). But first of all: why on earth would you want to do that!? –  mbork Jun 30 '12 at 11:45
Thanks. 1) I don't like bold up followed by italics, so I set everything to italics. 2) if 1) makes no sense, I simply don't like present scheme. –  boy Jun 30 '12 at 11:50

This might be the theorem scheme that you're after, called mytheorem:

\documentclass{article}
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\usepackage{amsthm}% http://ctan.org/pkg/amsthm
\makeatletter
\newtheoremstyle{mytheorem}% <name>
{3pt}% <Space above>
{3pt}% <Space below>
{\itshape}% <Body font>
{}% <Indent amount>
{\thmname{#1}\thmnumber{\@ifnotempty{#1}{ }#2}%
\thmnote{ {\the\thm@notefont(#3)}}}% <Theorem head spec (can be left empty, meaning normal')>
\makeatother
\theoremstyle{mytheorem}
\newtheorem{theorem}{Theorem}
\begin{document}
\begin{theorem}\lipsum[1]\end{theorem}
\end{document}


The key here is to include a theorem head specification (argument #9) in order to reformat the theorem number from \textup (or \@upn) to plain. Here's the default definition of the theorem head specification (associated with the plain style):

\def\thmhead@plain#1#2#3{%
\thmname{#1}\thmnumber{\@ifnotempty{#1}{ }\@upn{#2}}%
\thmnote{ {\the\thm@notefont(#3)}}}


Note the forced \@upn (defined as \textup`).

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Looks much better this way. Many thanks. –  boy Jun 30 '12 at 14:27