The sines in the denominator of Eq. 81b can only be zero for s = Q, and in this case the equation for cos θ n is expressed in Eq. 81a. From the above equations, we note that any angle θ n only depends on Θ n and on the angles {θ j } for j < n. Thus, the focused initial conditions in Eq. 79 (or more generally, Eq. 80) can be used to recursively generate values of θ n based on Eq. 81.