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    First published on Wednesday, Aug 27, 2025 and last modified on Thursday, Sep 11, 2025

    Theorems

    François Chaplais

    1 Introduction

    Theorems a strong statements that the author set aside for reference. There is no “theorem” environment as such. Instead, they are defined by the author. This is the opportunity to talk about the various parts of a LaTeX document and of its customization.

    2 The structure of a LaTeX document

    There are two parts in a LaTeX documents: the preamble and the body of the document itself. Let us begin with the simplest part: the document body.

    Actually, the document body is inside a LaTeX environment that is named document. Anything between the opening and closing statements of the document environment is the content of the document.

    The preamble is what precedes the document body. The structure of a LaTeX source code is the following.

    ... preamble ...
\begin{document}
... document content ...
\end{document}

    The preamble is where you configure your document. This is the right place to declare a theorem type.

    3 Declaring a theorem type

    To declare a theorem class, use the following command in the preamble.

    \newtheorem{internal name}{public name}

    In this lesson, we declare the summary theorem type as follows.

    \newtheorem{summary}{Summary}

    4 Declaring a theorem

    With the previous definition, summary is a legitimate LaTeX environment. Moreover, it is quite versatile and can include objects of terminal type, as well as, well, quite about anything except very, very general objects. Web sliders (or slide shows) cannot be put inside theorems because, in fact, it makes sense to incorporate a theorem into a slide show.

    To create a summary, we use the following construction.

    \begin{summary}
... summary content ...
\end{summary}

    5 Example

    Next is a summary of this lesson.

    Summary 1 (Theorems)

    Theorems classes are declared in the preamble. Each theorem class has its own numbering. An optional parameter of the theorem opening statement lets you give it a specific name.

    Theorems are useful to make stand out important parts of a document. This is a theorem.

    Here is the code.

    \begin{summary}[Theorems]
Theorems classes are declared in the preamble. Each theorem class has its own numbering. An optional parameter of the theorem opening statement lets you give it a specific name.

Theorems are useful to make stand out important parts of a document. This is a theorem.
\end{summary}