[Tex/LaTex] What does `\mathrel{}` do (with the empty parameter group)

math-mode

I know basically what \mathrel does when it's given a parameter: it typesets its parameter as though it were a mathematical relation for the purposes of spacing.

However, I have seen in several places the usage \mathrel{}, which seems to be treating the empty group as a relation.
This doesn't square with how I've been told \mathrel is supposed to work; is there something else going on here?

In short, what does \mathrel (and, presumably, \mathbin, \mathop, etc.) do when its parameter is an empty group?

(I have seen this other question, but it didn't make sense to me and seemed to be not quite the same as mine.)

Best Answer

In math mode {} makes an empty math atom (not just an empty group as in text mode) so \mathrel{} makes an empty atom that gets relation spacing.

Compare

$a {} b$ to $ a \mathrel{} b$

Related Solutions

[Tex/LaTex] What does \right. do

It produces an "invisible" delimiter that is used to match a corresponding \left. You can use it when you need a delimiter only on one side of the expression. lshort also describes it.

An example of such usage would be typesetting a multi-case function definition: with a figure brace on the left and nothing on the right.

\documentclass{article}

\usepackage{amstext}

\begin{document}

\[
  |x| = \left\{ \begin{array}{rl}
       x, & \text{if $x \geq 0$} \\
      -x, & \text{otherwise} 
    \end{array} \right.
\]

\end{document}

enter image description here

(NOTE: This is only an example and has improper spacing. In real life you would use the cases environment for this.)

[Tex/LaTex] the difference between \mathbin vs. \mathrel

From the name, \mathbin modifies the spacing around something so that it adheres to that of a binary operator, while \mathrel modifies the spacing to denote that of a binary relation. Here is an elementary approach at showcasing the difference:

tables

\documentclass{article}
\begin{document}
\begin{tabular}{clc}
  \multicolumn{3}{c}{Relations} \\[5pt]
  \LaTeX & Typeset & width \\ \hline
  \verb|$x=x$| & $x=x$ & \setbox0=\hbox{$x=x$} \the\wd0 \\
  \verb|$x\mathbin{=}x$| & $x\mathbin{=}x$ & \setbox0=\hbox{$x\mathbin{=}x$} \the\wd0 \\
  \verb|$x\mathrel{=}x$| & $x\mathrel{=}x$ & \setbox0=\hbox{$x\mathrel{=}x$} \the\wd0 \\[10pt]
  \multicolumn{3}{c}{Binary operators} \\[5pt]
  \LaTeX & Typeset & width \\ \hline
  \verb|$x+x$| & $x+x$ & \setbox0=\hbox{$x+x$} \the\wd0 \\
  \verb|$x\mathbin{+}x$| & $x\mathbin{+}x$ & \setbox0=\hbox{$x\mathbin{+}x$} \the\wd0 \\
  \verb|$x\mathrel{+}x$| & $x\mathrel{+}x$ & \setbox0=\hbox{$x\mathrel{+}x$} \the\wd0
\end{tabular}
\end{document}

Note the spacing around + matching that of \mathbin, while the spacing around = matches that of \mathrel.

From the TeXBook (Chapter 17: More about Math, p 154-):

Every math character is given an identifying code number between 0 and 4095, obtained by adding 256 times the family number to the position number. This is easily expressed in hexadecimal notation, using one hexadecimal digit for the family and two for the character; for example, \hex{24A} stands for character \hex{4A} in family 2. Each character is also assigned to one of eight classes, numbered 0 to 7, as follows:

Class 0: Ordinary (eg., /)
Class 1: Large operator (eg., \sum)
Class 2: Binary operation (eg., +)
Class 3: Relation (eg., =)
Class 4: Opening (eg., ()
Class 5: Closing (eg., ))
Class 6: Punctuation (eg., ,)
Class 7: Variable family (eg., x)

Classes 0 to 6 tell what "part of speech" the character belongs to, in math-printing language; class 7 is a special case [...]. The class number is multiplied by 4096 and added to the character number, and this is the same as making it the leading digit of a four-digit hexadecimal number.
...
TeX associates classes with subformulas as well as with individual characters. Thus, for example, you can treat a complex construction as if it were a binary operation or a relation, etc., if you want to. The commands \mathord, \mathop, \mathbin, \mathrel, \mathopen, \mathclose, and \mathpunct are used for this purpose; each of them is followed either by a single character or by a subformula in braces. For example, \mathopen\mathchar"1234 is equivalent to \mathchar"4234, because \mathopen forces class 4 (opening). In the formula $G\mathbin:H$ , the colon is treated as a binary operation.
...
There's also an eighth classification, \mathinner, which is not normally used for individual symbols; fractions and \left...\right constructions are treated as "inner" subformulas, which means that they will be surrounded by additional space in certain circumstances. All other subformulas are generally treated as ordinary symbols, whether they are formed by \overline or \hbox or \vcenter or by simply being enclosed in braces. Thus, \mathord isn't really a necessary part of the TeX language; instead of typing $1\mathord,234$ you can get the same effect from $1{,}234$ .

Best Answer

Related Solutions

[Tex/LaTex] What does \right. do

[Tex/LaTex] the difference between \mathbin vs. \mathrel

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