4

When I write the following:

\documentclass[12pt,a4paper]{article} 
\begin{document}  

$$
-1, -2, -3, \dots \| -1 \|, \| -2 \|, \| -3 \|
$$

\end{document}  

It produces the following: Problem

As you can see, the negative signs are more closely coupled with the constants in the first three numbers, but this fails to be the case when surrounded by norm signs, which I find aesthetically displeasing. How can I fix this?

  • 3
    Don't use \| on the wild. \| on the wild is an ord atom, you need to ensure that the first \| gets open atom, and the last one close atom. Use \lVert and \rVert, or \bigl\| and \bigr\| (or any other size, \Big(l|r), \bigg(l|r), \Bigg(l|r)); or use \DeclarePairedDelimiter (from mathtools) to define \norm{-1} or \norm[\Big]{-2} to get different sizes. – Manuel Feb 25 '16 at 2:09
  • Thanks, that fixes it. However, this problem doesn't arise when using ( and ), that is to say, I don't need to use \left( and \right) to avoid this issue. I'm curious why I have to use \lVert and \rVert here. – cemulate Feb 25 '16 at 2:17
  • 3
    the parentheses are defined at the outset to be open and close atoms, as are other "naturally" paired delimiters. the vert bars have no "natural" orientation, so that must be indicated specifically when they are used as delimiters. – barbara beeton Feb 25 '16 at 3:15
  • See Why is \[\] preferable to $$? – Werner Feb 25 '16 at 7:02
4

As you've discovered, the symbols generated by \vert (equivalently: |) and \Vert (equivalently: \|) have status "math-ordinary". Hence, TeX interprets the - symbol as a binary operator, since the - symbol is sandwiched between two symbols (\Vert and a numeral) with status "math-ordinary". (This is, of course, the correct default behavior for expressions such as $a-b$.) To get TeX to treat the - symbol as a unary operator, it's advisable to use \lVert and \rVert, which have status "math-open" and "math-close", respectively, instead of just \Vert.

Better still, define a LaTeX macro called, say, \norm, which automatically uses the correct math status values for the opening and closing fences. In the code below, the macro \norm is set up in such a way that \norm* is defined as well; the latter lets the size of the "fences" grow automatically, as needed.

enter image description here

\documentclass[12pt,a4paper]{article} 
\usepackage{mathtools} % for '\DeclarePairedDelimiter' macro
\DeclarePairedDelimiter{\abs}{\lvert}{\rvert}
\DeclarePairedDelimiter{\norm}{\lVert}{\rVert}

\begin{document}  

Original form:

$
-1, -2, -3, \dots, \| -1 \|, \| -2 \|, \| -3 \|
$

\medskip
Better:

$
-1, -2, -3, \dots, \lVert -1 \rVert, \lVert -2 \rVert, \lVert -3 \rVert
$

\medskip
Best:

$
-1, -2, -3, \dots, \norm{-1}, \norm{-2}, \norm{-3}
$

\end{document}  
| improve this answer | |
2

The minus sign sees the left delimiter as the first argument of a subtraction operation and sets the space as such. Enclosing the - in braces forces it to treat the minus as a unary operator (a negation on the following number). (Likewise, I could enclose the whole number in braces as \| {-1} \|)

\documentclass{article} 
\begin{document}  

\[
  -1, -2, -3, \dots \| {-}1 \|, \| {-}2 \|, \| {-}3 \|
\]

\end{document}  

enter image description here

| improve this answer | |

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