LGTM

Absolutely agree, lowk a programmer must've done it else I don't think I've met a mathmematician (or maybe, not crazy enough ones) that ever preferred redefining variables (esp in pedagogical material!!!!)

What do you want to learn PDEs for? It's not my strong point, but I've heard high praise for Partial Differential Equations by Stanley J. Farlow. I found it useful in my undergrad, else I think I could have died in analytical mechanics