“…In [2], the PO integral over a linear triangular surface is reduced to a line integral over the intersection of the linear triangle and the plane formed by the propagation direction of the incident plane wave and the observation direction, called k r -plane, then it is concluded that the closed-form expression to the line integral should be a triangle function since the domain of the PO integral is a linear triangle. This approach is also extended to NURBS surfaces [8], quadratic triangular patches [9], different source and observation configurations [3,4], even applied to time domain integral equations [10]. However, the closed-form expression presented in [2] can be obtained mathematically following the steps applied in [9] for exact evaluation of the PO integral for quadratic triangles.…”