Which property is satisfied by a harmonic function f(x,y)f(x, y)f(x,y)?
fxx+fyy=0f_{xx} + f_{yy} = 0fxx+fyy=0
fxx−fyy=0f_{xx} - f_{yy} = 0fxx−fyy=0
fx+fy=0f_x + f_y = 0fx+fy=0
fxx+fyy=1f_{xx} + f_{yy} = 1fxx+fyy=1