Notes. The first two rows are for realistic values of the longitudinal velocity for a slow surface wave on a discontinuous boundary of a cylindrical structure. The third row is for realistic values of the longitudinal velocity around the resonant position for a resonant slow wave in a photospheric pore. The last two lines are examples where M_A begins to be unrealistically high and which show that even then the slopes are not low enough to be in an unstable regime. For coronal conditions we took (v_Ae, v_si, v_se) = (5/2, 1/2, 1/4)v_Ai, whereas for photospheric conditions we took (v_Ae, v_si, v_se) = (1/4, 1/2, 3/4)v_Ai.