I found this: How to change the font style in theorem environment? as well as this: Italic in theorem environment Neither however seem to make sense to me as I have been using the following:

\begin{theorem} \emph{(Fundamental Lemma)} Let $A$ and $A'$ with collections $\partial$ and 
Stuff goes here: But it becomes "italicized"

However it seems that with the above two links one is either just defining whole new constructs (which I somewhat understand), or just using \textbf, which I don't want the \textbf to override the italics as I don't want bold faced text. What I am after is being able to construct a "Theorem" which keeps track of the numbering and updates itself i.e. 1.1 to 1.2 if another theorem tag is added before.

  • You can use \begin{theorem}[Fundamental Lemma] instead of \begin{theorem} \emph{(Fundamental Lemma)} – Ruben Apr 14 '14 at 22:35
  • A simple solution can be found with the ntheorem, not mentioned in the answers to question 38260. – Bernard Apr 14 '14 at 22:50
  • @Ruben -- thanks. but the preferable way to get what i think is being asked is (using amsthm): \theoremstyle{definition} \newtheorem{mytheorem}{Theorem}[section] ... \begin{mytheorem}[\emph{Fundamental lemma}] theorem text \end{mytheorem} which will put the theorem text in roman type, not italic. (i'm not entirely clear about this; the question sends me a mixed message.) – barbara beeton Apr 14 '14 at 22:55

Using the ntheorem package, this is really simple : write in your preamble something like:

\theoremheaderfont{\bfseries\itshape}% Remove \bfseries if you only want the title in italic
\theoremseparator{. ---} % Really old-style

All the specifications above will remain valid for all subsequently declared theorem structures until changed.

The thmmarks option (not used here) is for the good placement of end of theorem (proof) symbols, especially if a proof ends in a displayed (group of) equation(s) — in most cases this is automatic with ntheorem; thref is for cross-references and amsmathis necessary for the two previous options if you use amsmath.

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