I am pondering about comma rules/conventions in mathematical writing. Consider the following example:
We prove that the variables
$A, \; B, \; C$
are equal. Hence the equations
$A = B, \; B = C, \; A = C$
When typing a document, the mathematical text roughly looks like that when the mathematics is put into display style. The above is equivalent to
We prove that the variables $A$, $B$, and $C$ are equal. Hence the equations $A = B$, $B = C$, and $A = C$ are valid.
It seems to be customary that in the transition from "mathematics in normal text" to "mathematics in text with displayed mathematics" enumerations are put down without using "and" before the last item. The reason is that normal text in displayed equations looks awkward.
Question: do you retain the commas in the enumerations when there is a line break? I presume you would always do because the line break is just formatting, and without commas you obviously have ambiguities.
Question: when do you put commas after the displayed enumeration? My guess is that the usage of commas in the "original" text is authorative.
I have seen all options in writing, including mixed style throughout the document.