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

sigma sign

The capital Greek S, $\Sigma$ , meaning "add these up"; $\sum_{i=1}^{n} x_i$ adds items 1 through $n$ .

Examples

$\sum_{i=1}^{3} x_i = 3 + 5 + 10 = 18$ .