bold slanted numbers in custom theorem environment

Question

Using the amsthm package, I have customized a theorem environment for my notes. The theorem head is bold and slanted. But I also want the number slanted. (It is already bold, but it's upright.) In code, the heading should be like this:

\textbf{\textsl{Theorem 1}.}.

Answer 1

You have to override the head specification.

\documentclass{article}
\usepackage{amsthm}

% see https://tex.stackexchange.com/a/17555/4427
\newtheoremstyle{boldslanted}
  {\topsep}   % ABOVESPACE
  {\topsep}   % BELOWSPACE
  {\itshape}  % BODYFONT
  {0pt}       % INDENT (empty value is the same as 0pt)
  {\bfseries\slshape} % HEADFONT
  {.}         % HEADPUNCT
  {5pt plus 1pt minus 1pt} % HEADSPACE
  {\thmname{#1}\ \thmnumber{#2}{\thmnote{\normalfont\ (#3)}}} % CUSTOM-HEAD-SPEC

\theoremstyle{boldslanted}
\newtheorem{theorem}{Theorem}

\begin{document}

\begin{theorem}
This is a difficult theorem.
\end{theorem}

\begin{theorem}[Easy]
This is an easy theorem.
\end{theorem}

\end{document}