Free public beta until December 4, 2026: every course is open, no card needed. After that, the first chapter of each course stays free and full access is a one-time $39 ($19 for up to 100 founding members chosen for helpful feedback). Your progress is kept. How the beta works

set-builder notation

Describing a set by a rule instead of listing its members.

$\{x \mid x > 3\}$ reads "the set of all $x$ such that $x > 3$ " — a set defined by a defining property rather than an explicit list.