## Background

I reviewed my old post on power means inequalities.

## Problem

At the 71st line of that post, I carelessly put a `\|`

instead of `|`

, causing it to be interpreted as `rVert`

.

As the MathJax tutorial on Math Meta SE pointed out, the
correct $\rm \LaTeX$ syntax for `|`

in `{}`

denoting a set should be
`\mid`

. However, the `|`

in `{}`

doesn’t match the fraction.

```
$$\max\left\{\frac{1}{a_i} \mid i = 1,\dots,k \right\}$$
```

gives

## Goal

I need to change it back to

## Attempt

Following a comment by asmeurer, I tried

```
$$\max\left\{\frac{1}{a_i} \middle| i = 1,\dots,k \right\}$$
```

which gives

The horizontal spacing around `|`

*isn’t* correct.

$\max\left\{ \frac{1}{a_i} \,\middle|\, i = 1,\dots,k \right\} \quad \text{(right)} \\ \max\left\{\frac{1}{a_i} \middle| i = 1,\dots,k \right\} \quad \text{(wrong)}$

## Solution

See the external link of this post by clicking the title of this post.

## Lessons learnt

- Avoid using
`\|`

as`\lVert`

or`\rVert`

. - Use
`\providecommand{\norm}[1]{\lVert#1\rVert}`

for denoting norms in $\rm \LaTeX$ documents. Similar commands can be used for absolute values and sets. - Avoid using fractions in superscripts and subscripts.