So again, you’ll have to sorta relearn – I mean, it’s not a complete relearning, but you’ll have to relearn what it means that way. Now, you might – you can just as well assume that A is A transpose. In other words, if you have A34 and A43, these are the two contributions from I equals three, J equals four and I equals four, J equals three. You can see that these numbers are the same. So I can pull them out and make it A34 plus A43. And I might as well replace both of those with the average of the two. It doesn’t change anything. So in matrix language, you write it this way. You say that X transpose AX – and let’s do a quick calculations this first. Let’s take X transpose AX, and that is a scaler. That’s a scaler. ...

See the whole transcript at Introduction to Linear Dynamical Systems - Lecture 16