A Narrow Margin

A puzzle game where you prove small theorems, one card at a time.

Every problem is a theorem with its proof missing. Play move cards, or type the idea in your own words, and the proof is typeset on the page as you go. Tempting cards can be wrong turns, and those get crossed out in red pen. Each step asks a short question before it goes in.

Most theorems have two or three different proofs to find, and every finished proof gets a mark out of 10.

62 problems in six chapters (number theory, inequalities, sums, combinatorics, algebra, games and puzzles), plus Drills: the same arguments with new numbers every time.

Plays in the browser, on desktop or phone. Your progress is saved in your browser.