Definitions and theorem numbering

Question

I am new to theorems and definitions of the amsthm package and what I am trying to achieve might be really simple.

I have the following theorems:

\theoremstyle{definition}

\newtheorem{matriceDef}{Matrice}[section]
\newtheorem{matriceSomma}[matriceDef]{Somma}
\newtheorem{matriceSommaProp}{Proprietà per la somma tra matrici}[matriceSomma]

defined in a file. When I actually put them all together I have a LaTeX Error: No counter 'matriceSomma' defined. That happens at the last line.

What I am trying to do is have the last definition be a sub-definition of the preceding one. Right now, as you can see, the output is correct, what I want. The problem is that it gives that error there.

The MWE for this is this one

\documentclass[a4paper]{article}
\usepackage{amsmath,amsfonts,amssymb,mathtools,calrsfs,mathrsfs,amsthm}

\theoremstyle{definition}

\newtheorem{matriceDef}{Matrice}[section]
\newtheorem{matriceSomma}[matriceDef]{Somma}
\newtheorem{matriceSommaProp}{Proprietà per la somma tra matrici}[matriceSomma]
\begin{document}

    \section{Matrici}

    \subsection{Definizioni e teoremi}

    \begin{matriceDef}
      Foo, bar
    \end{matriceDef}

    \begin{matriceSomma}      
      Outside
    \end{matriceSomma}

    \begin{matriceSommaProp}
      Inside
    \end{matriceSommaProp}

\end{document}

I removed the useless typesetting. What I would like to do is create a definition (or a theorem) that depends on another for numbering. Thanks for any help.

Answer 1

You're stating that matriceSomma uses the same counter as matriceDef, so no matriceSomma counter is defined.

\documentclass[a4paper]{article}
\usepackage{amsmath,amsthm}

\theoremstyle{definition}

\newtheorem{matriceDef}{Matrice}[section]
\newtheorem{matriceSomma}[matriceDef]{Somma}
\newtheorem{matriceSommaProp}{Proprietà per la somma tra matrici}[matriceDef]

\begin{document}

\section{Matrici}

\subsection{Definizioni e teoremi}

\begin{matriceDef}
  Foo, bar
\end{matriceDef}

\begin{matriceSomma}  
  Outside
\end{matriceSomma}

\begin{matriceSommaProp}
  Inside
\end{matriceSommaProp}

\end{document}

On the other hand, you'll have to define scores of such theorem-like environments, because there's the product of matrices and the product of matrices by scalars and whatnot; I propose defining just two and passing the real name at use time.

\documentclass[a4paper]{article}
\usepackage{amsmath,amsthm}

\theoremstyle{definition}

\newtheorem{innerDef}{\envargument}[section]
\newtheorem{innerSubDef}{\envargument}[innerDef]

\newcommand{\envargument}{}
\newenvironment{Def}[1]
 {\renewcommand{\envargument}{#1}\innerDef}
 {\endinnerDef}
\newenvironment{SubDef}[1]
 {\renewcommand{\envargument}{#1}\innerSubDef}
 {\endinnerSubDef}


\begin{document}

\section{Matrici}

\subsection{Definizioni e teoremi}

\begin{Def}{Matrice}
  Foo, bar
\end{Def}

\begin{Def}{Somma}  
  Outside
\end{Def}

\begin{SubDef}{Proprietà per la somma tra matrici}
  Inside
\end{SubDef}

\end{document}

By the way, shouldn't it be “Proprietà della somma fra matrici”?