I stumbled upon a problem with the Mac OSX Preview program today. The print dialog hides the current page number when it pops up. This is a big problem when you want to print a single page, and the page numbers on the PDF do not correspond to the actual page numbers.
The two screenshots, with and without the print dialog open:
I don’t like it so much.
The cross product is defined as
. It’s generally used to find a vector in 
that is orthogonal to two other vectors.
Firstly, the cross product does not generalize cleanly to dimensions greater than three. There are a lot of cool numerical tricks used to solve differential equations and it’d be nice if we could make use of them to solve more complicated problems. Fortunately, for the orthogonalization problem, there are far better schemes.
Secondly, the vector (0,0,0) is really a point and represents a singularity in the system. The cross product of a vector with this point will map onto another point (which is again (0,0,0)). I’m not really sure whether this should be handled as a special case in your numerical code.
That is all.
After about two years of inactivity, I’ve decided to start maintaining a blog again.
My previous blog was maintained with the help of Emacs using software I wrote. While it served most of my needs, it lacked two key features that I think most blogs need: comments and category support. Instead of adding support for these features, I have instead moved to Wordpress.
These are interesting times in my life, and it would be good to document them. As always, the focus is on the fundamentals.
