A function is harmonic if ∇2f=∂2f∂x2+∂2f∂y2=0\nabla^2 f = \frac{\partial^2 f}{\partial x^2} + \frac{\partial^2 f}{\partial y^2} = 0∇2f=∂x2∂2f+∂y2∂2f=0. Which of the following is harmonic?
f(x,y)=x2+y2f(x,y) = x^2 + y^2f(x,y)=x2+y2
f(x,y)=excosyf(x,y) = e^x \cos yf(x,y)=excosy
f(x,y)=x3yf(x,y) = x^3 yf(x,y)=x3y
f(x,y)=x2y2f(x,y) = x^2 y^2f(x,y)=x2y2