# Green’s identity

∇h: gradient of h, ∇h=grad(h)=h_x·i + h_y·j (h_x: partial derivative of x; i, j are unit vector)

2h (or ∆ = ∇2): laplacian operator, ∇2h=∇·∇h=hxx+hyy(the first ∇ is the divergence, the second ∇ is gradient,∇2h is the sum of the non-mixed seconde order derivative )

∂h/∂n=∇h·n:the normal/external derivative, n is the unit vector

(f*v)=fv+f*(v)