The letters QED (or the unabbreviated “quod erat demonstrandum” [“which is what was to be proved”]) are often used to indicate the end of a formal proof. This longstanding tradition goes back to a style of proof writing, where the culminating sentence of the argument gives a recapitulation of the statement of the theorem; the QED places a stamp of finality on the discussion.
In modern typesetting, the QED has been largely replaced with typographic symbols; typically a solid or hollow rectangle or square is used to demark the end of the proof. (The cynic in me wonders if these just serve as flags for when the reader should take up reading carefully again.)
How does one indicate the end of a proof in a classroom setting? Often I will scribble out a square (or whatever symbol our textbook uses); sometimes I’ve written out “Q.E.D.”. Often I’ll pause, then solicit questions and comments.
But apparently I much more frequently channel Michael Palin.
Today I gave a final exam in Real Analysis II. This group of students has gotten to work with me on proofs for a full year, so they know my quirks and foibles better than most.
On the last page of the final, most of the students ended their last proof with the phrase:
“And there was much rejoicing.”
This isn’t a phrase I’ve consciously chosen to use in class, but it rings true enough as something I’m sure I *have* said on occasion. But if it made this much of an impression on the students, I wonder if I use it all the time, not just occasionally. Hmmm.
Come to think of it, it isn’t such a bad replacement for Quod erat demonstrandum. Captures the feeling of a good solid proof, well-understood. And it is much more evocative than a box.
), and sure enough that works. But I was on Homework Patrol, so I handed it off to TwoPi, and he came up with 
and 
. Walphaing these shows that both work well:
360 