Different packages of Latex provide nice and easy-to-use environments for theorems, lemmas, proofs, etc. The following post will show you the mostly used layouts and how to change numbering.

**Theorem styles:** For theorems, corollaries and lemmas, you need the following package:

`\usepackage{amsthm}`

which allows you to define new environments using either:

`\newtheorem{env_name}{caption}[within]`

or

`\newtheorem{env_name}[numbered_like]{caption}`

where

- env_name: Environment name
- caption: Text printed before the number, e.g. Theorem, Lemma, etc.
- within: The name of a defined counter
- numberered_like: Name of a defined theorem-like environment

Note: The command may at most have one optional argument. Let me give an example: First we define a new theorem “Theorem”, which takes the numbering of the section plus a consecutive number:

`\newtheorem{thm}{Theorem}[section] `

For your theorems you can now use the defined environment:

`\begin{thm}...\end{thm}`

For lemmas which have the same numbering as the previous thm-environment, we need to define a new theorem type:

`\newtheorem{lem}[thm]{Lemma}`

Note: Environments different from thm, lem or cor need to be defined previously, having a different counter which starts from zero:

`\theoremstyle{remark} `

\newtheorem{rem}{Remark}

\begin{rem}This is a remark.\end{rem}

Note: To omit the numbering, you can use the star as for chapters, sections, etc.:

`\newtheorem*{lem}[thm]{Lemma}`

**Proof:** A proof usually comes without a numbering as it follows a numbered theorem, corollary, etc. For a proof, you need the following package:

`\usepackage{amsthm}`

and then you can directly use the proof-environment:

`\begin{proof}...\end{proof}`

The closing statement automatically prints a qed-sign (square box) on an empty line after the last statement, terminating the proof.Note: If you want to place the qed-sign on the last line, i.e. if your proof ends with an equation, you can use the command:

`\qedhere`

inside the equation-environment (before the `\end{equation}`

).

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russjohnson09

Reblogged this on russjohnson09.