Surfaces built out of measure theory, so that a minimiser exists before anyone knows it is smooth. Covers Hausdorff measure and rectifiability, the area and coarea formulas, sets of finite perimeter, currents and varifolds, and why regularity holds up to dimension eight and fails after it.
Free to start · adaptive placement finds your level · reviews timed to your own forgetting.
Every idea is taught with motivation and a worked example before the drills, and an FSRS spaced-repetition engine schedules each review for the day its model predicts you would forget it. A short placement check finds what you already know, so you start Geometric Measure Theory exactly where it's useful.