Using beamer. Let's say I want to write A+B but with the plus invisible in one slide and visible in the next one. I think I read somewhere that more space is left normally around the + for being a binary operator. It seems to be that A\phantom{+}B leaves only the space of the sign +. How can I get all the space I need.
[Tex/LaTex] Phantom width of binary operator
math-modemath-operators
Related Solutions
The inspection of the math list happens after expansion (and after assignments). It is an extra stage just applicable to math mode that converts the math list into a horizontal list that is then typeset as a normal horizontal list.
So there is no real connection between the macro structure and the math spacing, It does not matter whether the thing to the left of the + has arguments or not, it just matters what it expands to (nothing in your example) so your example is equivalent to
\noindent
\( +x\) \\ % unary: "+x"
\( +x\) \\ % unary: "+x"
\({} +x\) \\ % binary: " + x"
\( +x\) \\ % unary: "+x"
\({} +x\) \\ % binary: " + x"
and as your comments show a binop like + gets binary spacing if it is between two mathord atoms such as {} or x.
Your example (from elsewhere, with \somecommand being a zero-argument macro)
\(\somecommand{} {+} x\)
is
\({} {+} x\)
Here the {+} construct makes a mathord so you get no spacing. My comment that this was probably bad markup was mainly related to the trailing {} after \somecommand it is OK to do that habitually in text mode to avoid dropping spaces but in math mode it's usually has an effect.
\(\somecommand{} {+ x}\)
is
\({} {+ x}\)
so here the math list has two, not three, mathord atoms: {} and +x. In this simple case it doesn't affect the spacing, but the inner expression is a single atom, so {+x} (and not x) would take any superscripts etc, and as it is an inner list linebreaking is suppressed and any white space is forced to its natural width (again not relevant here); basically in math node {...} is a box command like \hbox{....}.
There are in fact two choices for a unary math sign, a mathord or a mathop, it's easy to get an ad hoc mathord by using {+} but it is probably more consistent to declare the operators explicitly.
1 $a-+b$
2 $a-{+}b$
3 $a-\mathord{+}b$
4 $a-\mathop{+}b$
5 $x+y$
6 $x{+}y$
7 $x\mathord{+}y$
8 $x\mathop{+}y$
As you see from (1) if two binop atoms are adjacent the second one effectively turns into a mathord so you get the spacing as in (2) or (3) although arguably as a prefix operator giving it mathop spacing (with a small gap before its argument) is more consistent.
either way you don't want to be filling you document expressions with weird {} constructs and \mathxx primitives, just define
\unaryplus{{+}}
or whatever version you like and then use
a + \unaryplus b
and it will all work out OK.
I'm not convinced this is a good idea but you could do something like
\documentclass{article}
\usepackage{amsmath}
\renewcommand\sb[1]{_\text{$#1$}}
\begin{document}
\[
\sum_{i = 1}^{n} a_{i}
\]
\[
\sum\sb{i = 1}^{n} a_{i}
\]
\end{document}
Here I've redefined the standard command \sb which by default does the same as _, you could go further and redefine _ but history shows that if you do that usually you end up breaking something.
Best Answer
Need to add additional
{}which emulate a binary operator as TeX thinks there is something on either side of the+: