A New Algorithm for Circulant Rational Covariance Extension and   Applications to Finite-interval Smoothing
  The partial stochastic realization of periodic processes from finite covariance data has recently been solved by Lindquist and Picci based on convex optimization of a generalized entropy functional. The meaning and the role of this criterion have an unclear origin. In this paper we propose a solution based on a nonlinear generalization of the classical Yule-Walker type equations and on a new iterative algorithm which is shown to converge to the same (unique) solution of the variational problem. This provides a conceptual link to the variational principles and at the same time yields a robust algorithm which can for example be successfully applied to finite-interval smoothing problems providing a simpler procedure if compared with the classical Riccati-based calculations. 
Giorgio Picci and Bin Zhu
stat.ME math.ST stat.TH