Unify em dashes to spaced version

[danielzgtg]

Em dashes were annoyingly randomly spaced and unspaced. Edit: For mathematical writing, the spaced version was requested as clearer.

[mikeshulman]

I know that most style guides recommend no spaces around em dashes, but I think there's an argument that in mathematical writing with inline math spaces are preferable, since an em dash adjacent to inline math without a space might be mistaken for part of the formula. I think we might even have had an informal convention of using spaces around em dashes. How many instances are there of em dashes without spaces?

[danielzgtg]

There are 26 em dashes without spaces – forming 22.2% of the em dashes:

$ grep --include=\*.tex -ro '[^ ]---[^ ]' | wc -l
26
$ grep --include=\*.tex -ro ' --- ' | wc -l
90
$ grep --include=\*.tex -ro '^---[^ ]' | wc -l
0
$ grep --include=\*.tex -ro '^--- ' | wc -l
1

So if unspaced em dashes are ruled out, @mikeshulman, what should we convert the em dashes to?

• Spaced en dashes (Standard British English, and similar width to unspaced em dashes)
• Spaced em dashes (traditionally considered ugly but increasingly common such as on Quanta Magazine)

[mikeshulman]

Well, we used spaced em dashes in the rest of the book, so I'd be inclined to standardize on that.

[danielzgtg]

spaced em dashes in the rest of the book, so I'd be inclined to standardize on that.

I redid this to standardize on spaced em dashes as requested. The branch name is unchanged to avoid creating a new PR.

[mikeshulman]

Thanks!