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

natural numbers

The counting numbers: 1, 2, 3, … — symbol $\mathbb{N}$ .

$\mathbb{N} = \{1, 2, 3, \dots\}$ — the positive whole numbers used for counting.