# having issue with the proof environment

I am using theorem, lemma,corollary, and proof environments with the following way.

\newtheorem{theorem}{Theorem}
\newtheorem{corollary}{Corollary}[theorem]
\newtheorem{lemma}[theorem]{Lemma}
\newtheorem{remark}{Remark}
\newtheorem{proof}{Proof}

All of them properly works excluding the proof. When I start a proof with

\begin{proof}
...
\end{proof}

here is what I see. If I use \usepackage{amsthm} to use proof, then it messes up the other environments and this is what I get.

I am using the LaTeX INFORMS Journal on Computing Style Files template provided in this website. Then, I open the template and add the \newtheoremcommands as shown above. If I put the following code block and compile, I receive the error.

\begin{theorem}
Here is my theorem.
\end{theorem}
\begin{lemma}
This is the lemma.
\end{lemma}
\begin{proof}
this is the proof.
\end{proof}
• Please post a compilable code reproducing the problem. It is impossible to give an answer/help with only these elements. Dec 17 '19 at 18:17
• @Bernard I added some extra information. I believe that it is related to the template that I am using. Dec 17 '19 at 18:24
• amsthm provides a proof so you don't need to define it. Dec 17 '19 at 18:40
• @Sigur As I mentioned in the original post, when I use amsthm , it creates more problem. Of course, when I use that package, I remove the \newtheorem commands. Dec 17 '19 at 18:44
• Maybe try loading ntheorem, which has a syntax close to theorem and defines a nonumberplain theorem style. Dec 17 '19 at 20:30

The class predefines several theorem-like environments and a proof environment that requires an argument.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% Author template for INFORMS Journal on Computing (ijoc)
%% Mirko Janc, Ph.D., INFORMS, mirko.janc@informs.org
%% ver. 0.95, December 2010
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%\documentclass[ijoc,blindrev]{informs3}
\documentclass[ijoc,nonblindrev]{informs3} % current default for manuscript submission

%%\OneAndAHalfSpacedXI
\OneAndAHalfSpacedXII % current default line spacing
%%\DoubleSpacedXII
%%\DoubleSpacedXI

% If hyperref is used, dvi-to-ps driver of choice must be declared as
%   an additional option to the \documentclass. For example
%\documentclass[dvips,ijoc]{informs3}      % if dvips is used
%\documentclass[dvipsone,ijoc]{informs3}   % if dvipsone is used, etc.

% Private macros here (check that there is no clash with the style)

% Natbib setup for author-year style
\usepackage{natbib}
\bibpunct[, ]{(}{)}{,}{a}{}{,}%
\def\bibfont{\small}%
\def\bibsep{\smallskipamount}%
\def\bibhang{24pt}%
\def\newblock{\ }%
\def\BIBand{and}%

%% Setup of theorem styles. Outcomment only one.
%% Preferred default is the first option.
\TheoremsNumberedThrough     % Preferred (Theorem 1, Lemma 1, Theorem 2)
%\TheoremsNumberedByChapter  % (Theorem 1.1, Lema 1.1, Theorem 1.2)

%% Setup of the equation numbering system. Outcomment only one.
%% Preferred default is the first option.
\EquationsNumberedThrough    % Default: (1), (2), ...
%\EquationsNumberedBySection % (1.1), (1.2), ...

% In the reviewing and copyediting stage enter the manuscript number.
%\MANUSCRIPTNO{} % When the article is logged in and DOI assigned to it,
%   this manuscript number is no longer necessary

%%%%%%%%%%%%%%%%
\begin{document}
%%%%%%%%%%%%%%%%

\begin{theorem}
All natural numbers are interesting.
\end{theorem}

\begin{proof}{Proof.}
Clearly $0$ is interesting. Suppose there is a number that's not interesting
and call $n$ the minimal such. Then it is the minimal uninteresting number,
so obviously it is interesting.
\end{proof}

\end{document}

Note that you must add an argument to \begin{proof}.

• Thanks, man! It was literally driving me crazy. Dec 17 '19 at 21:10