I have found undefined notation to be the bane of my existence when trying to teach myself a field of math. How do you Google mathematical notation? It can be a giant and opaque barrier to deepening one's knowledge when studying the subject on one's own.
Maybe this will help: https://detexify.kirelabs.org/classify.html Then you can e.g. use the name of the TeX command for the symbol to Google its meaning (Stackexchanges are often full of questions looking for the TeX command for a symbol, so you could use these to infer in the other direction)
Thanks--this is most helpful! :)
Strongly agree. It's very difficult to work out what a symbol means if you've never seen it before.
In saying that, in general I've found it's not too bad. Mostly greeks and the occasional symbol like sigma along with some set notation.
In the Wikipedia page you linked, I immediately encounter several terms that I do not know the meaning of. I must either click through or search separately to fill in my own gaps.
Your criticism is not totally disagreeable, but we have to wonder where to draw the line. Is a "Hacker's Guide to Numerical Analysis" obligated to hold your hand all the way through basic notation? The notation that you mention in your comment are the "Hello world" of mathematics.