Paean to a Pen
I'm a slow learner, so when I work through technical stuff I usually write out and at least try to derive all the equations. My favorite pen for that kind of work is the Pilot Precise V5. It writes a nice precise 0.5 mm line, dries almost instantly, never skips, lasts a long time, and it's dirt cheap (a dozen for $13.28 from Amazon).
I've used a lot of pens, some of them nearly two orders of magnitude more expensive, but I've never found one better suited to my needs (writing equations with lots of symbols, Greek letters, and subscripts.) There is a retractable and refillable version, but I usually use the stick.
A true triumph of technology. I got about 200 pages of equations and notes out of one pen.