What 'best approximation' means and why the norm is part of the question, Bernstein polynomials and Korovkin's theorem, the Runge phenomenon and the Lebesgue constant, equioscillation and the Remez algorithm, Jackson and Bernstein inequalities, and the constructive side from Hermite and Bezier through Pade, acceleration and spectral accuracy.
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 Approximation Theory exactly where it's useful.