equation icon  PS (x) = - -q-U (x,x')@ar - ---q--U (x,x'')@aradv + -q-U;a(x,x') + q-U;a'(x,x')ua'@au a 2r2 2radv2 2r 2r --q-- '' --q-- '' a'' 1- ' 1- '' + 2r U;a(x,x ) + 2r U;a''(x,x )u @av + 2 qV (x, x)@au - 2qV (x,x )@av ad integral vv adv - 1q \~/ aV (x,z)dt, (413) 2 u