Harmonic functions past the classical tier: subharmonicity and Perron's method, capacity and energy, which boundary points a solution can reach, the fine topology, and the discrete and Brownian pictures of the same theory.
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 Potential Theory exactly where it's useful.