Let D⊂R2D\subset\mathbb R^2D⊂R2 be connected and open and let f:D→Rf:D\to \mathbb Rf:D→R be of class C2C^2C2. Then the Laplacian of fff is given by the sums of the second partial derivatives Δf(z)=fxx(z)+fyy(z).\Delta f(z) = f_{xx}(z) + f_{yy}(z).Δf(z)=fxx(z)+fyy(z).
Wikidata ID: Q203484