There‘s HP calculator guys and TI guys. Around the age of 17 I spent lots of time programming my HP28s calculator in a Forth like language that had symbolic mathematics, lots of ideas from Scheme (closures, functions as first class arguments, recursion). It felt like magic dealing with concepts I hadn’t seen in the C compiler on my Amiga or later in Turbo Pascal. But I saw these concepts later in Mathematica and was familiar.
I had programmed games, complex 3d visualisations (super slow but oh well), and was totally fascinated by what this device could do.
I'm a TI guy, but really like RPN and there is (was?) an excellent RPN package for the TI-89 https://old.reddit.com/r/calculators/comments/fdytes/lars_fr...
An HP 50g was my calculator of choice, and the whole RPN style really rubbed off on me. Plus it had more advanced symbolic algebra capabilities than a ti83 equivalent. I enjoyed learning common lisp, scheme, racket, etc through high school and college and still am fond of them today because of this calculator.
Most if not all high schools and colleges in the US required TI “graphing” calculators for algebra/trig on up. I don’t know if they still do. I never saw this HP28, sounds awesome!
I remember one of my math teachers claiming only TI showed up to the math text book meetings or something like that, so guess what calculator the book recommends...
The rest of the world only has Casio, I think.