Comments on: Adventures in Terminology

By: Kenny Easwaran

Kenny Easwaran — Thu, 16 Aug 2007 07:14:38 +0000

It’s amazing that topologists still use that terminology rather than \Sigma_1 and \Pi_1, which generalize nicely, and are easy to remember because of the relations between addition and union, and multiplication and product. Although the lowercase sigma also means countable unions in \sigma-algebra and the like.

By: John Armstrong

John Armstrong — Tue, 14 Aug 2007 21:09:55 +0000

Actually “the G deltas” would be a pretty good band name…

I think they both would. I can see a battle of the bands: the G-Deltas against the F-Sigmas.

By: Walt

Walt — Tue, 14 Aug 2007 20:27:41 +0000

I studied French in high school, so I remember the F one. I know no German other than the German words for number, space, and (thanks to the opera Wozzeck) blood, plus one curse word, so I pretty much have to memorize that one. Maybe somebody should write a song. Actually “the G deltas” would be a pretty good band name…

By: John Armstrong

John Armstrong — Tue, 14 Aug 2007 14:06:19 +0000

$latex G_\delta$ is “gebiet durchschnitt” (German: “open intersection”), and $latex F_\sigma$ is “fermé somme” (French: “closed sum”).

By: Pedro Terán

Pedro Terán — Tue, 14 Aug 2007 08:50:23 +0000

I believe the rationale for the notation is that “G” and “F” are the first letters of “open” and “closed” in German (or the words used at that time, or in any case the standard notation for “open” and “closed” in German, which is still widely used today), and similarly “d” and “s” for “intersection” and “union”.

Maybe someone can confirm or correct that.

But certainly it *is* painful to remember.