newtheorem

Newtheorem is a LaTeX command used to declare theorem-like environments. It defines structured, automatically numbered blocks such as Theorem, Lemma, Proposition, Corollary, Definition, and Remark. The environments created by newtheorem are commonly used in mathematical documents to present formal statements in a consistent style. The command is part of the LaTeX kernel, and its capabilities are extended by the amsthm package, which adds styling and additional options.

Syntax and usage: The basic form is \newtheorem{envname}{PrintedName}, which creates an environment that prints the name in the chosen style. Optional arguments control numbering. For example, \newtheorem{theorem}{Theorem} defines a theorem environment. To number within sections, use \newtheorem{theorem}{Theorem}[section]. To share numbering with another environment, you can write \newtheorem{lemma}[theorem]{Lemma}, making Lemma use the same counter as Theorem.

Styling and customization: With amsthm, you can customize how statements are formatted using \theoremstyle{plain}, \theoremstyle{definition}, and \theoremstyle{remark}. These styles affect the appearance and typography of the theorem heading and the body. Advanced users may define new styles with \newtheoremstyle and apply them to environments.

Additional notes: The declaration typically appears in the preamble, before \begin{document}. The amsthm package also provides a starred form, \newtheorem*{name}{PrintedName}, to create unnumbered environments. In the document body, use the environments with \begin{envname} ... \end{envname} and reference statements with \label and \ref as with other numbered elements.