Well, I noticed! It stood out pretty clearly, I'd say. I really don't think that the intended audience of this sort of ground-up summary is going to be comfort

Well, I noticed! It stood out pretty clearly, I'd say. I really don't think that the intended audience of this sort of ground-up summary is going to be comfortable with \mathbb{R}^{m\times n} notation all over the place. Yes, you explain what the notation means, but there's zero chance that a reader who didn't already know that notation will become fluent in it right away: they're going to be flipping back to the definition every time. That might be okay if you're trying to teach all of math, but a person reading "Linear Algebra in Four Pages" presumably neither wants nor needs that full skill set. (I'd say that {obj|obj description} notation would make the text even more opaque to novices, no matter how cleanly introduced.) Why not just make a less precise reference to a table of "numbers" (which, as an added bonus, would leave the door open to complex matrices without having to start from scratch)?