Collections

Mathematical and computational problems that reward both proof and implementation.

A quiet archive of mathematical and programming challenges.

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

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.

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