I suspect the chapter titles are simply mathematic expressions of increasing complexity which could indicate the increasing complexity of Harry's thought proceses. Am I right? I do know that the title of this chapter breaks down to (X-2)*(X-3) but I don't see that signifying anything in particular.
Author's response
The complexity isn't important just the expressions. The breakdown is actually the title of chapter six though because I'm going to finish up his pre Hogwarts workings and it's the second part of chapter five. Go back to red jacobson's review and the author's response should shed some light. The titles should make more sense in relation to the chapters' contents.