## Happenings – 2012 Feb 18

Let’s see.

It’s been a relatively slow week, mathematically. You can see that a group theory post went out last Monday… I had said last Saturday morning that it existed only in my head. Clearly I succeeded in taking it from stage II through stage V… but I didn’t have time for much else. I’m a little disappointed in that, but rather pleased that I finished what I did.

My alter ego the kid looked at control theory… and my alter ego the graduate student is still looking at rings. It was my undergraduate who put out the group theory post. He will be trying to write another group theory post for this Monday.

Although the 2nd and 3rd group theory posts have not matched the success of the 1st one, business is still booming on Mondays and Tuesdays: total hits for the last 3 Mondays have been 530, 437, and 367; total hits for the last 3 Tuesdays has been 327, 331, and 332. Considering that the previous high was 309 in March of 2010, I’m delighted.

I’ve also got 3 books on order.

The kid had also looked at algebraic geometry recently, so I ordered Mumford’s “The Red Book of Varieties and Schemes…” republished with additional material.

Something – catastrophe theory? Korner’s “the pleasures of counting” or “naïve decision theory” – reminded me of something by Edward Lorenz … the Lorenz equations of a simple chaotic system.

And, since I’ve been thinking about Fourier analysis of time series, I ordered a well reviewed book of that title by Bloomfield. It turns out there was a recent 2nd edition.

And with that, it’s time to start writing a group theory post for Monday. I want to talk about normal subgroups. My 1st exposure to them was confusing as hell… but, to completely change the metaphor, I know how to cut the Gordian knot. They can be introduced very simply.

(If you know that a group is called simple if it has no proper normal subgroups, you may have cringed at the previous sentence. I swear that no pun was intended.)