# How to make a very large forward slash in math mode?

this is the result I'd like to achieve:

I tried various commands to make the forward slash larger, for instance I tried the \left. (...) \right / or composing multiple \mathlarger commands but I couldn't get the desired result, with both of them the forward slash is bigger than normal but not even close to be large enough.

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{amssymb, amsmath, amsthm}

\title{Big forwardslash}
\author{Niccolò Della Rocca}
\date{July 2021}

\begin{document}

$\left. V\otimes_{\mathbb C}W= \mathbb C\left[V\times W\right]\middle\slash \text{Span} \begin{Bmatrix} (\underline{v}+\underline{v}',\underline{w}) - (\underline{v},\underline{w}) - (\underline{v}',\underline{w}) \\ (\underline{v},\underline{w}+\underline{w}') - (\underline{v},\underline{w}) - (\underline{v},\underline{w}')\\ a(\underline{v},\underline{w}) - (a\underline{v},\underline{w})\\ a(\underline{v},\underline{w}) - (\underline{v},a\underline{w})\\ a\in\mathbb C,\:\underline{v}\in V,\:\underline{w}\in W \end{Bmatrix} \right.$

\end{document}

• You can use \middle\slash or \middle\fracslash in unicode-math, between \left. and \right. to get an extensible slash. This doesn’t work with every font out there, but it does with most of them. Jul 31, 2021 at 0:26
• @Davislor: Thanks for your answer. Unfortunately I tried that already and it's still too small. If needed I can attach a picture of what I get with that. Jul 31, 2021 at 0:38
• A MWE would help, I think. Thanks! Jul 31, 2021 at 0:39
• So please do share the code you’ve tried before. Jul 31, 2021 at 0:41
• @Davislor: Added an MWE Jul 31, 2021 at 0:47

You can do it, but should you? Do you really think this is a good formula to display? I really think it isn't, for several reasons:

1. it's very big and difficult to read;
2. it's not the way tensor products are defined (it is one way, but one never uses it, relying instead on the universal property).

Anyway, here's a way to do it.

First of all:

1. \mathbb C is not proper syntax; use \mathbb{C};
2. better yet, define your own command;
3. underlining symbols for vectors is very old-fashioned markup;
4. better defining an abstract command that can be modified without chasing the document for \underline;
5. \text{Span} is wrong syntax: try it in the statement of a theorem;
6. use \DeclareMathOperator.

Now the promised code:

\documentclass{article}
\usepackage{amssymb, amsmath, amsthm}
\usepackage{l3draw}

\newcommand{\CC}{\mathbb{C}}
\newcommand{\vct}[1]{\underline{#1}}
\DeclareMathOperator{\Span}{Span}

\ExplSyntaxOn
\NewDocumentCommand{\reallybigslash}{O{2em}mm}
{% #1 = width (default 2em)
% #2 = left part
% #3 = right part
#2\!
\begin{gathered}
\bafforasta_reallybigslash:nnn { #1 } { #2 } { #3 }
\end{gathered}
\!#3
}

\cs_new_protected:Nn \bafforasta_reallybigslash:nnn
{
\hbox_set:Nn \l_tmpa_box { $\displaystyle#2$ }
\hbox_set:Nn \l_tmpb_box { $\displaystyle#3$ }
\dim_set:Nn \l_tmpa_dim
{
\dim_max:nn { \box_ht:N \l_tmpa_box + \box_dp:N \l_tmpa_box }
{ \box_ht:N \l_tmpb_box + \box_dp:N \l_tmpb_box }
}
\draw_begin:
\draw_linewidth:n { 0.75pt }
\draw_path_moveto:n { 0pt , 0pt }
\draw_path_lineto:n { #1 , \l_tmpa_dim }
\draw_path_use_clear:n { stroke }
\draw_end:
}

\ExplSyntaxOff

\title{Big forwardslash}
\author{Niccolò Della Rocca}
\date{July 2021}

\begin{document}

$\reallybigslash{ V\otimes_{\CC}W= \CC[V\times W] }{ \Span\left\{ \begin{matrix} (\vct{v}+\vct{v}',\vct{w}) - (\vct{v},\vct{w}) - (\vct{v}',\vct{w}) \\ (\vct{v},\vct{w}+\vct{w}') - (\vct{v},\vct{w}) - (\vct{v},\vct{w}')\\ a(\vct{v},\vct{w}) - (a\vct{v},\vct{w})\\ a(\vct{v},\vct{w}) - (\vct{v},a\vct{w})\\ a\in\CC,\vct{v}\in V,\vct{w}\in W \end{matrix} \right\} }$

\end{document}


The \reallybigslash command has an optional argument to set the slash's width. For instance, with

\reallybigslash[1em]{...}{...}


(where the dots stand for the same as before) you get

• Thanks for your answer, it did the trick as expected. About the point 2 of your first list, in our classes we saw both this definition and that based on the universal properties so I thought it was a good idea to include it in my notes, even though I agree the latter is much easier to understand. Jul 31, 2021 at 11:18

The unicode-math package supports auto-scaling \middle\slash and \middle\fracslash (if the math font does).

\documentclass{article}
\usepackage{unicode-math}

\title{Big forwardslash}
\author{Niccolò Della Rocca}
\date{July 2021}

\begin{document}

$\left. \left. V\otimes_{\mathbb C}W= \mathbb C[V\times W\right]\middle\slash \text{Span} \begin{Bmatrix} (\underline{v}+\underline{v}',\underline{w}) - (\underline{v},\underline{w}) - (\underline{v}',\underline{w}) \\ (\underline{v},\underline{w}+\underline{w}') - (\underline{v},\underline{w}) - (\underline{v},\underline{w}')\\ a(\underline{v},\underline{w}) - (a\underline{v},\underline{w})\\ a(\underline{v},\underline{w}) - (\underline{v},a\underline{w})\\ a\in\mathbb C,\:\underline{v}\in V,\:\underline{w}\in W \end{Bmatrix} \right.$

\end{document}


This requires LuaLaTeX or XeLaTeX.

• Yeah that does it, but unfortunately importing unicode-math causes other things I'd like to keep to get messed up. For instance the distance between subscript and superscript changed making some lines unnecessarily high and also \mathscr font changed. Any workaround? Jul 31, 2021 at 1:03
• @Bafforasta You could try loading a different font. By default, unicode-math sets \mathscr to the same font as \mathcal, but you can change that by, e.g., \setmathfont{NewCMMath-Book} followed by \setmathfont[range={scr,bfscr}, StylisticSet=1, Scale=MatchUppercase]{STIX Two Math}. Jul 31, 2021 at 1:53