\newtheorem (LaTeX2e unofficial reference manual (November 2018))

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### 12.9 `\newtheorem`

Synopses:

    \newtheorem{name}{title}
    \newtheorem{name}{title}[numbered_within]
    \newtheorem{name}[numbered_like]{title}

Define a new theorem-like environment. You can specify one of numbered_within and numbered_like, or neither, but not both.

The first form, `\newtheorem{name}{title}`, creates an environment that will be labelled with title; see the first example below.

The second form, `\newtheorem{name}{title}[numbered_within]`, creates an environment whose counter is subordinate to the existing counter numbered_within, so this counter will be reset when numbered_within is reset. See the second example below.

The third form `\newtheorem{name}[numbered_like]{title}`, with optional argument between the two required arguments, creates an environment whose counter will share the previously defined counter numbered_like. See the third example.

This command creates a counter named name. In addition, unless the optional argument numbered_like is used, inside of the theorem-like environment the current `\ref` value will be that of `\thenumbered_within` (see \ref).

This declaration is global. It is fragile (see \protect).

Arguments:

name

  The name of the environment. It is a string of letters. It must not begin with a backslash, `\`. It must not be the name of an existing environment, and the command name `\name` must not already be defined.

title

  The text to be printed at the beginning of the environment, before the number. For example, ‘`Theorem`’.

numbered_within

  Optional; the name of an already defined counter, usually a sectional unit such as `chapter` or `section`. When the numbered_within counter is reset then the name environment’s counter will also be reset.

  If this optional argument is not used then the command `\thename` is set to `\arabic{name}`.

numbered_like

  Optional; the name of an already defined theorem-like environment. The new environment will be numbered in sequence with numbered_like.

Without any optional arguments the environments are numbered sequentially. The example below has a declaration in the preamble that results in ‘`Definition 1`’ and ‘`Definition 2`’ in the output.

    \newtheorem{defn}{Definition}
    \begin{document}
    \section{...}
    \begin{defn}
      First def
    \end{defn}

    \section{...}
    \begin{defn}
      Second def
    \end{defn}

This example has the same document body as the prior one. But here `\newtheorem`’s optional argument numbered_within is given as `section`, so the output is like ‘`Definition 1.1`’ and ‘`Definition 2.1`’.

    \newtheorem{defn}{Definition}[section]
    \begin{document}
    \section{...}
    \begin{defn}
      First def
    \end{defn}

    \section{...}
    \begin{defn}
      Second def
    \end{defn}

In the next example there are two declarations in the preamble, the second of which calls for the new `thm` environment to use the same counter as `defn`. It gives ‘`Definition 1.1`’, followed by ‘`Theorem 2.1`’ and ‘`Definition 2.2`’.

    \newtheorem{defn}{Definition}[section]
    \newtheorem{thm}[defn]{Theorem}
    \begin{document}
    \section{...}
    \begin{defn}
      First def
    \end{defn}

    \section{...}
    \begin{thm}
      First thm
    \end{thm}

    \begin{defn}
      Second def
    \end{defn}

Unofficial LaTeX2e reference manual