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When n=2, PQf depend on x. This is Ricatti's Eq. It can also be transformed to 2nd-order linear ODE. Seehttps://en.m.wikipedia.org/wiki/Riccati_equation
Yes, it can be done for the cases that PQf are constants. If they are functions of x, cannot solved by separation. Thks a lot for your reminding. I walked too quickly in simplified form.