cleveref gets names wrong when using the SIAM document class

Question

When I try to use cleveref in conjunction with the SIAM style files, references to theorems, sections, figures and tables are accurate, but all other references' names (corollaries, definitions, lemmas, ...) are replaced with "Theorem". Here's a minimal example:

\documentclass[final]{siamltex}
\usepackage{cleveref}

\begin{document}

Here's a definition.

\begin{definition}
 \label{def:somedef} Some definition.
\end{definition}

If I use cleveref to try to refer to the above, I get ``\Cref{def:somedef}''.

\end{document}

which produces:

Here’s a definition.
Definition 0.1. Some definition.
If I use cleveref to try to refer to the above, I get “Theorem 0.1”.

A similar question was asked a while ago, and the answer was to rely on ntheorem or amsthm. I suppose I could get away with this by redefining all problematic environments (mydefinition, mylemma and so on), but is there a simpler or cleaner way?

Answer 1

When several environments share the same counter -- as is the case for theorems, lemmas, corollaries, propositions, and definitions in the siamltex document class --- cleveref needs some help in order to figure out which one of the possible environments is actually being used for a given label.

The easiest solution to your problem, I believe, is to load the ntheorem package explicitly and then to re-run all of siamltex's definitions of its five theorem-like environments. (Naturally, if you don't need to use all five environments in your paper, you can skip redefining those environments you don't employ.)

The following is a modified form of your MWE that uses this approach.

\documentclass[final]{siamltex}
\usepackage{ntheorem,cleveref}
\makeatletter  
% The next few lines are from 'siamltex.cls', but now with 
% \renewtheorem{...} instead of \newtheorem{...}
\if@onethmnum
  \renewtheorem{theorem}{Theorem}
  \renewtheorem{lemma}[theorem]{Lemma}
  \renewtheorem{corollary}[theorem]{Corollary}
  \renewtheorem{proposition}[theorem]{Proposition}
  \renewtheorem{definition}[theorem]{Definition}
\else
  \renewtheorem{theorem}{Theorem}[section]
  \renewtheorem{lemma}[theorem]{Lemma}
  \renewtheorem{corollary}[theorem]{Corollary}
  \renewtheorem{proposition}[theorem]{Proposition}
  \renewtheorem{definition}[theorem]{Definition}
\fi
\makeatother

\begin{document}
\section{In the beginning}

Here's a definition.

\begin{definition}
 \label{def:somedef} Some definition.
\end{definition}

If I use \textbf{cleveref} to try to refer to the above, I now get ``\cref{def:somedef}''.

\end{document}

Addendum As Ahmed Musa has pointed out in his answer, the code I copied from the siamltex.cls file (what's sandwiched between the \makeatletter and \makeatother instructions) seems unnecessarily complicated. For the sake getting cleveref to recognize the variants of the basic theorem environment, all that's actually needed are the following four instructions:

\renewtheorem{lemma}[theorem]{Lemma}
\renewtheorem{corollary}[theorem]{Corollary}
\renewtheorem{proposition}[theorem]{Proposition}
\renewtheorem{definition}[theorem]{Definition}

Answer 2

I don't know who made the siamltex class file but I was curious about the conditional in Mico's code. I had to find siamltex.cls (which isn't in MiKTeX or TeXLive), where I saw the original definitions. I don't know why the conditional there appears unusual. Please why isn't it as follows? The trace file is 20 miles long; I may save some time here. The following gave me exactly the output that Mico reported. In fact, I tried other things and there were no surprises.

\makeatletter
\if@onethmnum
  \renewtheorem{theorem}{Theorem}
\else
  \renewtheorem{theorem}{Theorem}[section]
\fi
\makeatother
\renewtheorem{lemma}[theorem]{Lemma}
\renewtheorem{corollary}[theorem]{Corollary}
\renewtheorem{proposition}[theorem]{Proposition}
\renewtheorem{definition}[theorem]{Definition}