Probably the weirdest math I’ve ever encountered is the Frobenius method and more generally power series methods of solving differential equations. I remembered encountering Taylor and Maclaurin series in Calc I, and thinking “oh, that’s a cute trick.”

A decade later, I’m casually working my through an introductory QM textbook, and realize that somebody had actually put the stuff to good use. It felt like power series methods are literally just tricking the hard differential equations around single electron 3D atoms into giving up their secrets.