ORDINARY DIFFERENTIAL EQUATIONS
https://users.math.msu.edu/users/zwang/ode.pdfThe equations in examples (c) and (d) are called partial di erential equations (PDE), since the unknown function depends on two or more independent variables, t, x, y, and zin these examples, and their partial derivatives appear in the equations. The order of a di erential equation is the highest derivative order that appears in the