Protected: a subgeometry of D_3-dual polar space

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row sums of character tables

If you ever worked with finite groups, chances are you stared at group character tables, e.g. ones in Atlas of Finite Groups, or ones produced by a CA system, such as GAP. Each row stores a character of an irreducible representation of a group columns correspond to the values of on representatives of the conjugacy classes of The following must be known, but so far I haven’t found any references.

Proposition. is a nonnegative integer, namely
it is the multiplicity of in the character of acting on itself by conjugation.

Indeed, note that as it counts the order of the centraliser in of an element in . Then using the First Orthogonality Relation of characters on and completes the proof: as claimed.

Update (21.04.09) It turns out to be well-known: see e.g. Solomon, Louis, “On the sum of the elements in the character table of a finite group”. Proc. AMS 12(1961) 962–963.

The below conjecture was actually made by Richard L. Roth in “On the conjugating representation of a finite group”. Pacific J. Math. 36(1971), 515-521.

And it was disproved by Edward Formanek in “The conjugation representation and fusionless extensions”. Proc. AMS 30(1971), 73–74.

Question. When does It does happen, e.g. take a nontrivial 1-dimensional representation of an abelian group (as an abelian group acts trivially on itself, only trivial character will occur in ).
More generally, if is an irreducible character such that for then does not occur in and so Is the latter also only if? (Here denotes the centre of )

Update (21.04.09) See the 3rd reference above for a negative answer to this.

e-learning week

it’s here again. We are supposed to put all the lectures for this online, using a pretty usless tool that allows one to record a very low-resolution video of your face, voice-over, and synchronise with your powerpoint slides (or perhaps some other slide format is supported too, I didn’t investigate). The thing runs on Windows only, and the university pays to the company that developed that oh so wonderful product quite a bit, I gather…

As I can’t skip it this time (I was lucky to schedule midterms precisely for such a week last year ) I’ll just shoot a video of myself giving lecture at a whiteboard, as I did 2 years ago…

PS. And while I was shooting the video using iSight camera and Powerbook G4, the harddisk in the latter just died…