One of the last steps do does is finding an unitary transformation matrix that transforms the (by that time diagonalized) K and W matrices into a form that is not diagonal but has equal diagonal entries. This unitary transformation matrix is not unique and therefore the optimal filter that is found is not unique. Two different methods to find this matrix are implemented. Default is the method of Hwang  that is guaranteed to terminate in a finite number of steps. The method of Mullis and Roberts  is iterative and in some cases can need much computing time before convergence, but sometimes more elegant filter structures are found. This method is chosen if the variable itmax is chosen greater than zero. This variable then specifies the maximum number of iterations allowed for this method. If no convergence takes place within the specified number of iterations, an error message is issued.