Which of the following functions f(x,y)f(x, y)f(x,y) is harmonic, satisfying ∇2f=0\nabla^2 f = 0∇2f=0?
f(x,y)=x2+y2f(x, y) = x^2 + y^2f(x,y)=x2+y2
f(x,y)=x2−y2f(x, y) = x^2 - y^2f(x,y)=x2−y2
f(x,y)=ex+eyf(x, y) = e^x + e^yf(x,y)=ex+ey
f(x,y)=xy2f(x, y) = xy^2f(x,y)=xy2