# Left pointing vector accent

I'd like to typeset $$\vec{\square}$$ but with the arrow pointing left, where $$\square$$ stands for an arbitrary letter from the Latin alphabet. I've tried to implement the solution described here, however it uses the \reflectbox command, which MathJax doesn't support. How can a left pointing vector accent be implemented in MathJax?

• Related from earlier this year, How to rotate text by 180 and/or mirror it in Math.SE. But the idea below seems much less work. Commented Sep 9, 2022 at 23:33
• @hardmath: Thanks! I posted an alternative answer below based on the post you linked to. Commented Sep 11, 2022 at 1:01
• Voting to migrate this to TeX Stack Exchange. Commented Sep 13, 2022 at 20:10
• Does this answer your question? MathJax basic tutorial and quick reference
– Surb
Commented Sep 14, 2022 at 17:56

$$\newcommand{\cev}[1]{\stackrel{\tiny\leftarrow}{\mathbf{#1}}}$$ In general, you can put one thing on top of another using \stackrel{topthing}{bottomthing}. It's natural to use \leftarrow as the top thing, but it will look too large unless you modify it: \stackrel{\leftarrow}{\mathbf{v}} gives $$\stackrel{\leftarrow}{\mathbf{v}}$$. You can make a symbol a lot smaller with \tiny: \stackrel{\tiny\leftarrow}{\mathbf{v}} gives $$\stackrel{\tiny\leftarrow}{\mathbf{v}}$$ which has an arrow about the same size as the one produced by \vec{\mathbf{v}}: $$\vec{\mathbf{v}}$$ although a bit less bold. I don't know a heavier left arrow symbol.

To avoid typing out the full command every time, you can do \newcommand{\cev}[1]{\stackrel{\tiny\leftarrow}{\mathbf{#1}}} at the top of your answer. Then \cev{u} will produce $$\cev{u}$$

• Alas, I think the scaling depends on the MathJax renderer. Here's what I see using the default "HTML-CSS" renderer: i.sstatic.net/tUOCr.png And here's what I (and, I suspect, you) see if I switch to the "Common HTML" renderer: i.sstatic.net/KmFce.png (Screenshots taken on Firefox 104.0.2 / macOS 12.5.1; behavior on Chrome 105.0.5195.125 and Safari 15.6.1 is pretty much the same.) Evan Aad's answer works great for me on both renderers, though. Commented Sep 16, 2022 at 18:05

Here's an alternative solution to Matthew's, based on this answer, which I was made aware of thanks to hardmath's comment.

\newcommand{\cev}[1]{\style{display:inline-block; transform:scale(-1,1)}{\vec{\style{display:inline-block; transform:scale(-1,1)}{#1}}}} $$\newcommand{\cev}[1]{\style{display:inline-block; transform:scale(-1,1)}{\vec{\style{display:inline-block; transform:scale(-1,1)}{#1}}}}$$

Using this command, \cev{y} produces $$\cev{y}$$. For comparison, \vec{y} produces $$\vec{y}$$, and Matthew's code produces $$\stackrel{\tiny\leftarrow}{y}$$.

One advantage of my code over Matthew's is that with my code the arrow stretches automatically when more than one letter is involved. Thus, \cev{xy} produces $$\cev{xy}$$, whereas Matthew's code produces $$\stackrel{\tiny\leftarrow}{xy}$$. For comparison, \vec{xy} produces $$\vec{xy}$$.

A standard command exists for this: \overleftarrow{PQ} for $$\overleftarrow{PQ}$$.

In fact, \vec is defined to be a shorthand for \overrightarrow.

• I don't think your second sentence can be correct - \vec{v} gives $\vec{v}$ which has a very different size arrow to \overrightarrow{v} which makes $\overrightarrow{v}$ Commented Sep 14, 2022 at 10:00
• @MatthewTowers My fault. \vec is a (simple) math accent while \overrightarrow is an extensible accent. Commented Sep 14, 2022 at 10:09