Define a star "*" - to become a dot

Question

When writing in mathmode, is it possible to define a * to become \cdot, in the main document?

As it makes it easier when writing equations to make a star.

Answer 1

\documentclass{article}

% \DeclareMathSymbol{\cdot}{\mathbin}{symbols}{"01} % from fontmath.ltx
\DeclareMathSymbol{*}{\mathbin}{symbols}{"01}
\begin{document}

$ x * y $

\end{document}

Answer 2

Either

\mathcode`\*=\number\cdot

or

\mathcode`\*=\number\circ

or

\mathcode`\*=\number\bullet

(The \number isn't really necessary, but it might help for abnormal versions of \circ or \cdot or \bullet.)

Answer 3

As I pointed out in a comment, redefining the use of * in the wrong way will prevent you from using star versions of math-mode macros (see partial list below*). For example, making * active would be the wrong way. However, David C and Don both note that their approaches do not suffer this problem, and I was negligent for not noting the distinction.

While the use of star macros in math mode may be rare, it occurs often enough to warrant avoiding an approach that would use an active *. Even if you use David's approach, you lose the use of the typeset asterisk in math mode, unless you save it beforehand into a named macro.

An alternative that does not mess with * in any way would be to redefine \*. This is a plain-Tex macro known as a "discretionary multiplication sign". I've never used it myself, but the idea is that you can place it between multiplied math terms. Normally, it does nothing, but if the line breaking wants to happen between the two terms, a symbol (equivalent to \times) is inserted at the line end (in text size). It acts like a mathematical hyphen, but with \times instead of -. If you, like me, never use such a construct, then I think it would be safer to redefine \* rather than *.

\documentclass{article}
\renewcommand\*{\cdot}
\begin{document}
$ x \* y $
\end{document}

*Examples of star macros include things like \tag*, \ref*, \\*, \matrix* (and all its variants), \operatorname*, \alignat*, just to get started.

Answer 4

For the sake of completness, version for unicode-math:

\documentclass{article}
\usepackage{unicode-math}
\AtBeginDocument{\Umathcode`\* = 2 0 "22C5}
\begin{document}
$x*y$
\end{document}