Technique training for college math competitions (Putnam-style). Not a topic dump — six transferable problem-solving weapons: invariants and monovariants (find the quantity that never changes, or only moves one way), the pigeonhole principle at competition strength (prove existence by counting), telescoping and collapsing sums, generating functions as a problem-solving tool, the competition inequalities (AM–GM and Cauchy–Schwarz) with their all-important equality cases, and functional equations by clever substitution.
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 Putnam & College Problem-Solving exactly where it's useful.