how is it beginner friendly, first paragraph and already an obscure formula for non math people

You will never find a level of "beginner friendly" that suits everyone.

I agree that this is not an ideal start - at least without any further clarification - for beginners but I think it works well for people that already known mathematical notation but not many specifics of linear algebra.

Also, I don't want to be the preacher bringing this into every argument but this is one of the genuinely good uses for AI that I have found. Bringing the beginning of a beginner friendly work down to my level. I can have it explain this if I'm unsure about the specific syntax and it will convey the relevant idea (which is explained in a bit of unnecessary complexity / generality, yes) in simple terms.

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relaxing 2 days ago | parent | prev [-]

I agree. I wouldn’t consider someone who has taken (and remembers) a course in set theory a beginner without some added qualifier.

One of my pet peeves is using mathematical symbols beyond basic arithmetic without introducing them once by name. Trying to figure out what a symbol is and what branch of math it comes from is extremely frustrating.

	▲	CamperBob2 2 days ago \| parent \| next [-]
		I haven't taken any courses in set theory, but it makes perfect sense to me. Once someone tells you that the funny script 'R' means "Real numbers", the funny E means "is an element of", and the vertical \| means "given that," that's pretty much all you need to know to dive in. If those concepts cause difficulty, it probably makes sense to go back down the learning curve a bit before tackling linear algebra. Alternatively, just cut and paste the expression into any LLM and it'll explain what's what.
	▲	solarwindy 2 days ago \| parent \| prev \| next [-]
		VLLMs are incredibly good at decoding math from screenshots, if you’re working from a PDF textbook. ChatGPT especially, and since it’s conversant in LaTeX, it can respond directly in the notation you don’t recognize to break it down for you. It even manages with photos of my handwritten scrawl (mostly).
	▲	schoen 2 days ago \| parent \| prev [-]
		There are a handful of textbooks that have a nice appendix that defines each symbol (or maybe sometimes tells you where you can go to learn more about the topic represented by the symbol!). That way they can presume that most readers are already familiar but still be helpful to those who aren't.