I understand they don't want to issue certificates without verifying the identity of the person taking the course, but what's with the sudden jump from "requiring identity verification" to "requiring payment"?

I'm guessing there are expenses related to verifying someone's identity.

It's not at all like SICP (I've never met a math book that was similar), but it's a wonderful introduction and I don't see it mentioned in this thread yet: Needham's "Visual Complex Analysis".

Heh. I bought SICM expecting it would be "SICP for physics". I started reading and felt out of my depth after just a few pages. But it seemed interesting, so I got a Lagrangian and Hamiltonian mechanics book and started reading that. That was 6 months ago, I'm about halfway through and I'm really enjoying it!

Learning Lagrangian mechanics when all your life you only ever heard about the Newtonian approach is a lot like stumbling upon Scheme after years of programming in C.

I'm looking forward to cracking the cover of SICM once more this summer!

This. Exactly this, and I think it's not a coincidence that learning Scheme was also like this, but for programming.

Today I learned that Americans still swipe their credit and debit cards in a magnetic strip reader.

The original was copy-protected, I believe, which means it cannot be archived in the standard .dsk format (there are some more complicated formats that represent a disk at a lower level, but they are not very well supported by emulators). In a few decades, the only surviving copies of many 80s software programs will be the ones that were cracked and distributed widely. In this way, crackers and pirates acted as inadvertent archivists.

For what it's worth, the pirated version I had (back in the old days) had your name on it (plus a crack group's name and probably BBS phone number). I remember thinking "Hey, that's the Apple Writer guy, what else did he write?" and that's how I found GraFORTH. Bought Leo Brodie's book and had a blast learning the language, then actually implementing small Forth-like languages for the fun of it. There was a Byte book, "Threaded Interpretive Languages" that gave me several epiphanies about programming in general and language design in particular.

I used Apple Writer for years, until eventually my typing speed increased so much that the program couldn't keep up with me anymore when typing long paragraphs (the word wrap algorithm, that was run after each character, apparently took time proportional to the length of the current paragraph, with a big enough constant that my 1 MHz Apple II started dropping keystrokes regularly despite the program having a 32-character buffer). I eventually resorted to disabling word-wrap while typing, and re-enabling it prior to printing (Control-Z, I think it was). Later, I bought a Zip Chip that increased the computer speed to about 3.5 MHz, and the problem disappeared.

I wasn't using Apple Writer by then, because I had discovered another word processor, a weird, Rube Goldberg contraption called Gutenberg Sr. that used double-high-resolution graphics at a time when few programs did, and had a more powerful markup language (troff-inspired, I found out later), could two two-column printing and had some page layout capabilities, and had great support for printer-downloadable fonts, including user-defined ones. The interface was atrocious but the software itself was powerful.

If you make one with two extra keys under the space bar, for use as thumb-activated modifiers, and Cherry blue switches as an option, I will buy one. For now, it's not really bringing anything new to the table, except for those Cherry clears, but I don't care for them.

Titrate is invisible.

Well, it might be the case that you were lacking some assumed prerequisites (don't forget that the book is aimed at MIT undergraduates, who will have some background in math and physics).

But if you cannot relate an exercise to the subject matter you just studied, it might also be the case that you think you understand what you just read, but don't really do. It's easy to read math superficially, honestly believing that one has understood the point, when in fact the "mental image" the authors wished to impart on you has not been grasped.

So you should not ignore the exercises, but work hard at them. They (well, some of them) are supposed to be hard. They are supposed to make you think, not just rehash what you just read. And after all that hard work, hopefully, something will click and you will be ready for the next section.

And then you will feel awesome.

It's this generation's notion of a personal computer. I like it.