[Tex/LaTex] \abs produces “undefined control structure” error

Question

I'm getting an Undefined control structure error when I use \abs, even for a simple term like \abs{x}.

I'm using pdflatex and the amsmath package in math mode $ $. I've also tried deleting .aux files.

Do you have any advice?

Answer 1

Not every conceivable mathematical operator has been defined in advance. So you neeed to define it. Here is my usual method:

\usepackage{mathtools}
\DeclarePairedDelimiter{\abs}{\lvert}{\rvert}

Then \abs{x} = \lvert x \rvert, \abs*{x}=\left\lvert x \right\rvert and say \abs[\big]{x} = \bigl\lvert x \bigr\rvert.

In recent projects I've extended it a bit with

\usepackage{mathtools,etoolbox}
\DeclarePairedDelimiterX{\abs}[1]{\lvert}{\rvert}{\ifblank{#1}{{}\cdot{}}{#1}}

Then one can write \abs{}: \mathbb{R}\to[0,\infty) and \abs{} will added a centered dot to indicate an empty argument.

Addition: Here's an example of why it's not always a good idea to autoscale: