Another one of those things that I’ve had to re-derive 100 times, and so I am posting here for future reference.
Take a quadratic equation for a vector .
You want to convert this into the form
What are , , and ?
First, assume is symmetric. Then we have, in decreasing order of obviousness,
Now, suppose is non-symmetric. (Of course, still being symmetric is okay, but you would be using a needlessly complicated formula…) Then,