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Thank you, sommerfee.

The following
\documentclass{article}
\begin{document}
\newcommand{\NN}[1]{\sqrt{\sin^2 {#1}-CC}}
$$ A = \frac{\sin\alpha\cos a}{\NN{x}} $$
\let\NN\undefined
$$ A = \frac{\sin\alpha\cos a}{\NN{x}} $$
\end{document}

fails, as desired, with the message
! Undefined control ...

When writing messy equations, I frequently use \newcommand to reduce typing, reduce errors, and improve consistency. Here is an example from a current paper:

\newcommand{\NN}[1]{\sqrt{\sin^2 {#1}-CC}}

I would like to "undefine" the macro at the end of the section where it is used. Then, if it is ...

Trying to use the x11names option for xcolor in a beamer presentation, getting an "option clash" error. Example below. What is going on?

Danke/gracias/merci/thanks for your help.

This is the code.
\documentclass{beamer}
\usepackage[x11names]{xcolor}
\begin{document}
begin{frame}
This is a test ...

Recently I used LaTex to create an abstract booklet for a mini-conference. For each entry I tried to use a format like:
\begin{samepage}
\section*{Presenters's name}
Presenter's Affiliation\\
Presentation Title\\[2ex]
Abstract
\end{samepage}
but there were often page breaks between the Title and ...

Thanks, that works too.

It works - thanks, Stefan!
Here is a minimal example in the context I was looking for:
\documentclass{beamer}

\makeatletter
\newcommand{\Tw}{\strip@pt\textwidth}
\newcommand{\Th}{\strip@pt\textheight}
\newcommand{\Thw}{\strip@pt\dimexpr 0.5\textwidth\relax}
\newcommand{\Thh}{\strip@pt\dimexpr 0.5 ...

Here is the context: In beamer presentations, I often include slides which display a number of color pictures from various sources, in various formats (jpg, png, pdf). I use the LaTeX picture() environment because it gives fine-grained control over exactly where and in what order the graphics should ...

Thanks, Stefan and localghost for the welcome, and for the pointers to documentation. "texdoc xcolor" gave me the answers I needed. If anyone
else finds this thread, the answer to my specific question is yes, \definecolor supports several color systems besides rgb, including hsb and cmyk.

Specific question: In the LaTeX beamer \definecolor command, what color spaces can be specified? I have looked through dozens of examples and have found two types: either "\definecolor{rgb}<rgb-triplet>", or "\definecolor{gray}<graylevel>". Is it possible to use, for example "\definecolor{hsv ...