When you write the differential at a point, you write f`(x)|_(x = k). What is the correct LaTeX command for the "|" symbol? Is it just a pipe, or is there a separate mathematical symbol for it?

My LaTeX for the above is:

```
\[f^\prime(x)|_{x = 1} = 5\]
```

Is this formally correct?

## Best Answer

This is not a differential, but a derivative; they are different things. Since

f'denotes the derivative off, which is a function of its own, the best notation for the value at 1 isIf you want to use the heavier notation

then

is perfectly good. You may want to define a command for this:

to be used as

or, if you need a larger bar to cover a larger function symbol,

The optional argument can be one of

`\big`

,`\Big`

,`\bigg`

or`\Bigg`

. I wouldn't use an automatically growing bar, because it may give bad results.