@MichaelE2 I see Michael, I'm sorry I'm not as of yet familiar with a lot of things here. I will bear that in mind.

I just did. Sorry, still learning how to use the platform in terms of asking questions after years and years of merely lurking around :-)

Thank you so incredibly much! My X matrix is a complete mess but it seems to have worked. I'm very much a Mathematica beginner, I wonder, is Chop needed here because after all the engine does in fact a numerical calculation?

@HansOlo Thanks! I'm on it.

@cvgmt Hello! $O(D,D)$ is as far as I know the split orthogonal group. I haven't myself been able to find much information about it, though I suspect its matrices might be orthogonal. I have now clarified A is symmetric and B is diagonal.