I've used \cal throughout my document and now when I go to switch the document class the \cal is giving me a "Undefined control sequence." error. Any thoughts on why the document class would make a difference?
[Tex/LaTex] \cal gives “Undefined control sequence” error
errorsmath-mode
Related Solutions
You had several math mode expressions written in text mode; I've fixed them in the code below. I would suggest you to use the amsthm package to define a theorem-like structure with the desired formatting for your definitions; in this way, the formatting and numbering are dona automatically:
\documentclass{article}
\usepackage{amssymb}
\usepackage{amsthm}
\newtheoremstyle{mydefi}
{\topskip}{\topskip}
{\itshape}{}
{\bfseries}{}
{\newline}{}
\theoremstyle{mydefi}
\newtheorem{defi}{Definitie}
\begin{document}
\begin{defi}
Als $z$, $f(z) \in \mathbb{R}^{m}$, dan is de $M$-de Fr\'echet afgeleide van $f$, voorgesteld door $f^{(M)}(z)$ een operator op $\mathbb{R}^{m} \times \mathbb{R}^{m} \times \cdots \times \mathbb{R}^{m}$ ($M$ maal), die lineair is in elk van de operandi met als waarde
\end{defi}
\end{document}
I used Fr\'echet; but loading inputenc with the appropriate encoding, you can simply write Fréchet.
By the way, for definitions it's also common to use use the roman (upright) font instead of italics; in this case, you can change the definition of the mydefi style to:
\newtheoremstyle{mydefi}
{\topskip}{\topskip}
{}{}
{\bfseries}{}
{\newline}{}
I would also like to suggest you (quoting cmhughes's valuable comment) to have a look at some introductory material (the Not so short guide to LaTeX (available in seveal languages) and the amsmath package documentation, for example); in your first few documents, always build up your code slowly, compiling frequently- this will help you find where the errors are.
The following minimal example replicates the problem:
\documentclass{article}
\usepackage{tikz}
\usepackage{preview}
\begin{document}
This is the main text.
\end{document}
It's obviously a bad interaction (incompatibility) between tikz and preview. Either drop the loading of preview, or load it with the [active] package option.
Best Answer
You could define
\calby yourself, such as for example thearticleclass does. In theminimalclass it's undefined, and this way you can fix it: