I've been working on a history project peripherally for months, I'm just recording a few notes here as I still haven't gotten to the bottom of it. Because I haven't arrived at the answer to my research yet, the following will ramble on a bit, but I wanted to capture my notes so far. You see, my old electrical engineering courses keep creeping into my quantum classes and vice versa. It's not that it's just the same math, it's also the same notation. The ultimate answer to all of this may be that both subjects pulled their notation from pure mathematics. The latest inspiration for really looking into this came up as I was studying for my quantum midterm yesterday. I came across the following integral in Merzbacher that I felt certain I'd seen before (picture 1). Merzbacher certainly felt it should be familiar since not a bit of explanation was given for its execution. When I got home, I pulled out my EE systems engineering book, "Discrete ...