As another Cartesian example, number X20 denotes the Green's function in the semi-infinite body ( 0 < x < ∞ ∞ ) with a Neumann (type 2) boundary at x = 0. Here X denotes the Cartesian coordinate, 2 denotes the type 2 boundary condition at x = 0 and 0 denotes the zeroth type boundary condition (boundedness) at x = ∞ ∞ . The boundary value problem for the X20 Green's function is given by