The level seemed mismatched to the audience for a "Primer." I clicked on the Fourier series link. Anyone who needs a basic explanation of a Fourier series doesn't know enough mathematics to understand the explanation provided.
Next looked at the Linear Algebra link - another example of making something much harder than it needs to be. Sorry.
Just as a quick note: I'm a counterexample to the assertion that anyone who needs a basic explanation of Fourier Series doesn't know enough maths to understand the primer.
I've got a strong (pure) mathematical background (degree in Maths & Philosophy from Oxford), and am now doing a PhD in CS, but my degree was entirely pure maths, and I never did any applied calculus. Admittedly, this isn't a common background, but one of the joys of writing on the internet is that you can write for whatever audience you choose!
Jeremy: thanks very much for providing these primers! For me, they've been great: there are lots of areas of maths which I've not looked at, and for a while I'd been wanting a good introduction to them which was at the right level for me: i.e. mathematically clear, without having to go into too much background, and these have been great at that.
Did you read up until the part of the Fourier series post that is just calculus? There's some linear algebra stuff at first, but it's just an overview intended for someone who is mathematically inclined.
Next looked at the Linear Algebra link - another example of making something much harder than it needs to be. Sorry.