Theorems and proofs in LaTeX with amsthm

Declare each kind in the preamble with \newtheorem{theorem}{Theorem}, then write \begin{theorem} ... \end{theorem} in the body. The proof environment from amsthm prints "Proof." and ends with a small square.

Declaring and using a theorem

\newtheorem{name}{Heading} in the preamble creates an environment called name whose heading is printed as "Heading" with a number. Load amsthm as well; it provides the proof environment and starred declarations.

A declared theorem with its proof

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\documentclass{article}
\usepackage{amsmath,amsthm}
\newtheorem{theorem}{Theorem}
\begin{document}
\begin{theorem}
The sum of two even numbers is even.
\end{theorem}
\begin{proof}
Let $a = 2k$ and $b = 2m$. Then $a + b = 2(k + m)$,
which is even.
\end{proof}
\end{document}

The body of a theorem is set in italic. The proof ends with a square at the right margin.

Names, labels and references

An optional argument after \begin{theorem} gives the theorem a name, printed in parentheses. Put \label inside the environment to refer to it with \ref.

A named theorem, a lemma and a reference

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\documentclass{article}
\usepackage{amsmath,amsthm}
\newtheorem{theorem}{Theorem}
\newtheorem{lemma}{Lemma}
\begin{document}
\begin{lemma}\label{lem:square}
If $n$ is odd, then $n^2$ is odd.
\end{lemma}
\begin{theorem}[Erika Mustermann, 2026]\label{thm:main}
There are infinitely many odd squares.
\end{theorem}
\begin{proof}
Use Lemma~\ref{lem:square} for every odd $n$.
\end{proof}
Theorem~\ref{thm:main} is easy to remember.
\end{document}

Several kinds of statements

Declare as many kinds as you need: theorem, lemma, proposition, corollary, definition, remark, example. Each kind has its own counter in Scissor LaTeX.

Definition, proposition and remark

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\documentclass{article}
\usepackage{amsmath,amsthm}
\newtheorem{definition}{Definition}
\newtheorem{proposition}{Proposition}
\newtheorem{remark}{Remark}
\begin{document}
\begin{definition}[Even number]
An integer $n$ is \emph{even} if $n = 2k$ for an integer $k$.
\end{definition}
\begin{proposition}
The number 44 is even.
\end{proposition}
\begin{remark}
Zero is even as well.
\end{remark}
\end{document}

Unnumbered theorems

\newtheorem* (from amsthm) declares a kind without a number.

An unnumbered statement with \newtheorem*

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\documentclass{article}
\usepackage{amsmath,amsthm}
\newtheorem*{conjecture}{Conjecture}
\begin{document}
\begin{conjecture}
Every even number greater than 2 is the sum of two primes.
\end{conjecture}
\end{document}

Numbering and styles

In a TeX installation, \newtheorem has two optional arguments for numbering, and amsthm has \theoremstyle. Here is what Scissor LaTeX does with them:

[section] and a shared counter: each kind counts by itself

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\documentclass{article}
\usepackage{amsmath,amsthm}
\newtheorem{theorem}{Theorem}[section]
\newtheorem{lemma}[theorem]{Lemma}
\begin{document}
\section{Results}
\begin{lemma} A first lemma. \end{lemma}
\begin{theorem} A first theorem. \end{theorem}
\begin{lemma} A second lemma. \end{lemma}
\end{document}

In a TeX installation this example prints Lemma 1.1, Theorem 1.2 and Lemma 1.3. Scissor LaTeX prints Lemma 1, Theorem 1 and Lemma 2.

Without \newtheorem

Many documents copied from elsewhere use theorem, lemma or definition without declaring them. Scissor LaTeX recognises common names anyway, including theorem, lemma, proposition, corollary, definition, remark, example, conjecture, claim, exercise, problem, solution and short forms such as thm, lem and defn.

What differs from a TeX installation

More on the package on the amsthm page. The full manual of the package is at ctan.org/pkg/amsthm.

See also amsthm, Cross-references and cleveref.

Every example on this page is typeset by Scissor LaTeX when the site is built. Last updated 2026-11-06.