## Happenings – 2011 Apr 9

Let’s just pick this up where I left off last Saturday.

The list of things I do not understand continues to grow faster than the shorter list of things I do understand… but at least the shorter list is growing too.

The two mechanics problems – of course – worked out. I still don’t understand how things went wrong, but I know what went wrong.

In the case where Mathematica® was hung up, my second equation was no longer an equation… it had become “True”. In the case where my intermediate equation appeared to be wrong but the answer appeared to be right… for reasons I do not understand, Mathematica ended up with Ra/2 where I expected Ra (the reaction force at point A). In other words, its variable Ra was twice the reaction force – but the reaction force was still correct.

I’ve made a little progress on chemical reaction rates… but not much – I have finished two whole problems!

I’m still thinking about what I will say in my first two posts about group theory.

I really should just polish off two or three posts about elliptical orbits. And the long-delayed second post on projectile motion.

I’m still thinking about compressed sensing… specifically about a signal composed of spikes and sinusoids. The two books I ordered are not particularly helpful for elementary problems – not yet anyway! – but they both look worthwhile.

I’m considering buying MATLAB®… I’ve got an awful lot of books with MATLAB code in them… and it would be really nice to be able to confirm their alleged calculations. Yes, I can do anything I understand using Mathematica – but I’ve got too many MATLAB examples for things I don’t understand!

Oh, I’ve also been thinking about multicollinearity… I woke up dreaming about it this morning, and I think I nailed down the key idea. I’m not the first person who did something right, but then needed some time to appreciate just how right it was. (I think the key to multicollinearity is the matrix product X.v, which rotates the rows of X and shows us precisely how close to zero the last components are. And I decided that two posts ago. It just didn’t sink in.)

This post is going out early (ah, my alter ego the kid hasn’t had his shot at math this morning), so I’ll get some breakfast and then settle down to mathematics.