I'm trying to write up all my university theorems and I cant get the theorem environment to work. Here is a mini example of what I'm trying to do.


\usepackage{amssymb,amsmath,amsthm, amsfonts}


\title{Theorems\\ Y3 S1}


\section{Mathematical Methods}
\begin{theorem}{Placheral's Theorem}
||F||^2_2 =2\pi ||u||^2_2 

I get this error

! LaTeX Error: Environment theorem undefined.

I am using windows 7 have MikTex 2.9 installed and am using TexMaker. From what I have read the theorem environment should be part of the amsthm package which I have. But it isn't working!

Any help will be much appreciated.

  • 11
    Please fix spelling of "Plancherel" in your real document. Commented Dec 16, 2013 at 16:04

1 Answer 1


This is a more proper way to do it:




\usepackage{mathtools} % Bonus


\section{Mathematical Methods}
\begin{theorem}[Placheral's Theorem]
\norm{ F }^2_2 =2\pi \norm{ u }^2_2 .

As one might have guessed \newtheorem constructs theorem environments. Use it to define your Theorem, Lemma, etc. environments. You might want to have a look at \theoremstyle as well. Here is a handy method for defining an example env which is not italic.

\theoremstyle{definition} % amsthm only

As tohecz mentions, you may want to read up on this subject. Details for amsthm, can be found in amsthdoc.pdf.

The \newtheorem syntax should be covered by any standard LaTeX introduction.

  • 8
    You might want to more clearly point out that a missing \newtheorem is the real problem ;)
    – yo'
    Commented Dec 16, 2013 at 15:54
  • Ahh that's very useful, I think I had misunderstood a few things there.
    – Hannah
    Commented Dec 16, 2013 at 15:54
  • @Hannah: And the environment has an optional argument instead of a mandatory. Commented Dec 16, 2013 at 15:54
  • @Hannah Well, I'm tempted to say RTM -- read the manual ;)
    – yo'
    Commented Dec 16, 2013 at 15:55
  • 4
    no actual environments are predefined by any class or package supporting theorem-class objects -- it's not known what an author might want to call them, especially if said author is working in a language other than english. so \newtheorem is always required. if you're working with a tex live installation, try typing texdoc amsthm at a prompt to view the (amsthm) manual. Commented Dec 16, 2013 at 18:19

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