The ideal Arbital math page

Think of the best math textbook you've ever read. Why was it good?

Did the author spend time motivating results before diving into details? Did they provide lots of concrete examples of abstract objects? Did they anticipate and answer your questions? ¹

Our ideal Arbital math page is something like a section from an exceptionally clear, fun-to-read textbook. In particular:

• Arbital is not an encyclopedia. There doesn't have to be one-to-one correspondence between Arbital pages and notable mathematical concepts. You can break concepts down into smaller (or larger) chunks than you would on Wikipedia, and there can be multiple explanations of a single topic (see: Arbital's lenses).
• An Arbital page is not an academic paper. Don't be terse for the sake of being terse. Use a conversational style, or whatever style seems most likely to produce understanding in your page's target audience.²

¹ _{Were there footnotes with funny asides?}
² _{Though it's probably best to stick to standard terminology, so that readers do not sow confusion when they go interact with the wider world.}