\begin{proof} something \end{proof} produces

Proof. something

and I want

Proof: something

The solution suggested here produces an error if one uses the ntheorem package instead of amsthm as mentionned here. Although I don't remember the reason I chose ntheorem over amsthm, I won't risk myself changing it...

I also tried the following in vain:

\theoremseparator{:} (other irrelevant font modifications also)
  • Um, the ntheorem package does not define an environment called proof unless some options are set when the package is loaded. Could you tell us which options you're specifying when loading the ntheorem package? – Mico Dec 2 '15 at 17:35
  • Oh, I didn't know. I loaded [amsmath,amsthm,thmmarks] – Noix07 Dec 2 '15 at 17:38
  • If you don't like the look of the proof environment that's set up by providing the amsthm option, you shouldn't specify this option to begin with. – Mico Dec 2 '15 at 17:53
  • I'll think about it some day. at the moment I have no idea why I loaded it, I must have copied it from some other answer in tex.stackexchange... – Noix07 Dec 2 '15 at 18:43

Don't use the amsthm option, and use something like this in your preamble:

  • 1
    Sorry, I didn't see that. In my opinion, the O.P. should not use the amsthm option. It's very easy to mimic the general layout of amsthm and change what one wants to change. – Bernard Dec 2 '15 at 17:45

Yeah this is a all in one question answer but I can say that I despaired ever finding the answer as I have (almost) no understanding of the codes and I felt I had to share.

I copied the definition of the "nonumberplain" theoremstyle (which I found at C:\Program Files\MiKTeX 2.9\tex\latex\ntheorem.sty and opned with notepad ++), called it differently, modified it and made the "proof" environment use that style:

 \gdef\th@styledemo{% I chose the name "styledemo"for the style
  \def\theorem@headerfont{\normalfont\itshape\small}\itshape% Here you can actually further modify the "header" and body font
  \item[\hskip\labelsep \theorem@headerfont ##1:]}% Here I replaced the . with :
 \item[\hskip\labelsep \theorem@headerfont ##1\ (##3):]}}% same replacement here

\newenvironment{beweis}[1][Proof]{% the proof environement already exists so I chose another one
 \th@styledemo  % here I call the theoremstyle define above
 \def\theorem@headerfont{\itshape\small}% Here I don't really understand but nevertheless write the same specification for the font again...
\@thm{proof}{proof}{#1}}% here one should keep proof, don't ask why...
\let\proof\beweis % replace the environment proof with beweis

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