Equation Icon  u (x,y,z ) = u (x,z)u (y, z) with nm n( m)1 ∕2 ( )1∕2 ( )n ∕2 ( √- ) ( ) (140 ) un (x,z) = (2)1∕4 -n1--- -q0 q0∗ q∗(z) Hn -2x- exp − i kx2 . π 2 n!w0 q(z) q0 q(z) w(z) 2q(z)