Multi-expression display equation: and or comma? [closed]

Example:

In display equations that contain two separate expressions in a single line, which we would normally read off verbally with an "and" conjunction, does it make sense to use a comma or an "and" in typesetting them?

If we follow the rule that we should treat math as part of the text, I lead myself to feel that we would use "and" although I think I've seen the comma more often. I tried looking at Knuth and Chicago Manual of Style, but couldn't quite find the style answer.

• This question seems to more properly belong on math.stackexchange, since it's about style and not really about LaTeX presentation. – barbara beeton Dec 26 '19 at 20:37
• @barbarabeeton - I tried to give my answer a (La)TeX angle by mentioning \quad as the preferred spacing amount for display-math material. – Mico Dec 26 '19 at 20:43

Just imagine that you were using inline math instead of display math to express the claim. Then, and would doubtless be the correct conjunction. What's correct for inline math should be correct for display math as well. For display math, though, you should probably insert \quad instead of ordinary inter-word space on either side of the word and.

\documentclass{article}
\begin{document}
Inline math:

The variables $x$ and $y$ obey the following constraints: $x+y<1$ and $x-y=2$.

Display math:

The variables $x$ and $y$ obey the following constraints:
$x+y<1 \quad\mbox{and}\quad x-y=2.$
\end{document}

• Great! Sorry, why use \mbox and not \text for the "and"? – AimForClarity Dec 26 '19 at 20:12
• @AimForClarity I'd go for \text; it's slightly less efficient, but semantics are important. – egreg Dec 26 '19 at 20:31
• @AimForClarity In Addition, if you have thee formulas, I would set no space between a formula and a comma like x<1,\quad x+y<1 \quad\mbox{and}\quad x-y=2. (display mode). This is implied by Micos answer. – CampanIgnis Dec 27 '19 at 0:27
• @AimForClarity I am typing my notes for 20 years and can tell you: use \; before commas and periods, even if some are yelling at you. It is way better to let them yell than having a reader confuse $x_i,\quad \dots$ with $x_{i^\prime}\quad \dots$ and so on. And type differential d's upright. Those who read the results will appreciate it. – user194703 Dec 27 '19 at 1:07
• @Schrödinger'scat Touché! In addition, I forgot a Harvard comma. – CampanIgnis Dec 27 '19 at 13:45