Collections

Mathematical and computational problems that reward both proof and implementation.

Diagrams, constructions, and the small observations that make a proof click.

Strategies, invariants, boards, and puzzles where the rules hide the real problem.

Problems that make intuition stumble before a clean model restores order.

Monthly challenges and elegant problems from IBM Research, including Ponder This.

Mathematical, logical, and computational puzzles published by Jane Street.

Interesting integer sequences, recurrences, patterns, and the ideas they encode.