Computing the invariants that `topology` names: homology by hand from a triangulation, covering spaces as a Galois correspondence, and the stable theory above the first course.
Free to start · adaptive placement finds your level · reviews timed so it stays learned.
Every idea is taught with motivation and a worked example before the drills, and an FSRS spaced-repetition engine schedules each review for the moment just before you'd forget it. A short placement check finds what you already know, so you start Algebraic Topology exactly where it's useful.