[Tex/LaTex] When should \cdot be used to indicate multiplication

best practices

Is this purely personal style, or are the generally accepted guidelines? Clearly, when dealing with numbers either $2 \cdot 2$ or $2 \times 2$ is needed, but within formulae where an absence of an operator implies multiplication, when should a \cdot be used? I tend to use it only sparingly if it aids in grouping. (Of course, barring the cases where it's required, e.g. a dot product.)

Best Answer

This is only useful to avoid ambiguity. Two circumstances come to mind:

Multiplying actual numbers, as you say: 2 \cdot 2 versus 22 is obviously the way to go. More subtly, writing, say, 2 \cdot 3^4 is an improvement over 2 3^4 for the same reason. You might say "of course!" except that sometimes, if you have a pedantic input style (like me) you fool yourself: 2 {13}^2 looks like 213^2.
Function notation. This is the only time in abstract algebra that juxtaposition of symbols means something other than multiplication; it means the "product" of a function acting on its argument instead. So, for example, if you have a function f(x) and you try to multiply it by x + 1, you could write either f(x + 1) (obviously wrong) or f(x) (x + 1) (too many similar braces). Here, writing f \cdot (x + 1) or f(x) \cdot (x + 1) is probably best.

This is a totally anecdotal answer.

Related Solutions

[Tex/LaTex] When should one use \begingroup instead of \bgroup

\bgroup is a synonym for {, which is defined in Plain TeX using \let\bgroup={.

It interacts with TeX's "digestive system" in hairy ways: {s and \bgroups start the same sort of groups, called simple groups, and each can be terminated with either }s or \egroups, since they are the same. But when the TeX digestive system encounters them, they are of different catcodes, so commands that look ahead, e.g., in LaTeX with \section\bgroup Title}, can break this matching.

\begingroup is different. It is a TeX primitive, and it matches a different sort of group that TeX accounts for separately, called "semi-simple groups" (a Knuth joke, I assume). Thus a \begingroup must be terminated by an \endgroup, not a }, and vice versa for \endgroup.

I generally avoid \bgroup, and use \begingroup, but \bgroup could be useful if you are messing about with a nested token list.

[Tex/LaTex] When should we use a blank line

I can tell you what I'd do in these 3 cases:

1.: After \chapter{} and similar command I'd always use a blank line as it makes the document structure clearer. Moreover, "In this chapter ..." is the start of a paragraph.

2.: I wouldn't use a blank line here if both displays belong to the same paragraph. However, I usually wouldn't use two consecutive displays at all; I'd use align* or gather* instead.

3.: In that particular case don't use blank lines as everything is within one paragraph. Speaking of clarity of the document structure, I'd say that

Assume we have    
\[
  \cos x
\]
that satisfies ...

is clear enough. Only use a blank line after the display if the paragraph ends after the display.

4.: As for your extra question about vertical spacing commands: In the context you provide, I'd always surround them with blank lines for clarity of the document structure. I sometimes use them before a display like this; then you mustn't put a blank line:

... some text
\vspace{-1ex}
\[
  ...
\]

I should point out that manual corrections like in this last example are rarely needed, and you should only use them if you're sure that you know what you're doing.

Best Answer

Related Solutions

[Tex/LaTex] When should one use \begingroup instead of \bgroup

[Tex/LaTex] When should we use a blank line

Related Question