It is actually linear. a,b,c,d are the unknowns and x1 and x2 are the inputs.

A bit confusing since normally x1,x2 denote the unknowns.

Unless you like econometrics: then X are data, betas and gammas and sigmas are unknown (oh my!).

Jargon is fun, unless I want to communicate across fields :/.

Well today I learned. I never knew that. Stupid mathematics and its inconsistent notation.

It makes sense when you look at historical development, but there is no reason to learn separate notations now without learning all sides :).

Not if you are doing econometrics/statistics.

He's solving for abcd, which is linear in powers of x. I'm not going to argue for what's the correct solution, but it's certainly not that mess.