A common notation used for information divergence is `D(P||Q)`

. I hope to get something like `D(S||Q)`

where `S=||P||/2`

. I wonder if it is possible to display it nicely ? In such a way that the middle double bar has the right height. I tried

```
$D \left( \frac{\|P\|}{2} \right| \left| Q \right)$
```

The above do not work as the middle bar are not of the same vertical height but its closest to what I hope to achieved. Using just `\|`

instead of `\right| \left|`

for the middle double bar results in a short middle double bar.

Does anyone has a good idea on how to achieve that. Thanks

Else, I could just create a new notation.

— Update —

I managed to find out how to do it with this useful operator `\middle`

```
$D \left( \frac{\|P\|}{2} \middle\| Q \right)$
```

Hope someone else may find this useful.

## Best Answer

This double bar should be treated as a binary relation, so it will be distinguishable from other usages, such as a norm.

The simplest way to cope with the problem is to use

`mathtools`

facilities:Note that number 1 is wrong in two respects: there is an unwanted space between the "D" and the left parenthesis; the two bars are too near the symbols, making for ambiguity.