TITLE: What does this joke mean?
QUESTION [15 upvotes]: I saw this written on a blackboard in the math department building the other day:
Gas Law: $PV=nRT$
Ideal Gas Law: $(P)(V)=(n)(R)(T)$
I know the ideal gas law is something from chemistry, but I'm assuming this is meant to be some sort of joke involving math. Any ideas?

REPLY [14 votes]: In a ring, "$(a)$" is common notation for the principal ideal generated by $a$. (See https://en.wikipedia.org/wiki/Principal_ideal.)