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

partial differential equation

An equation relating an unknown function of two or more variables to some of its partial derivatives.

A partial differential equation relates an unknown function $u$ of two or more independent variables to one or more of $u$ 's partial derivatives.