Bradley M. Bell James V. Burke Gianluigi Pillonetto An inequality constrained nonlinear Kalman-Bucy smoother by interior point likelihood maximization. 25-33 2009 45 Automatica 1 http://dx.doi.org/10.1016/j.automatica.2008.05.029 db/journals/automatica/automatica45.html#BellBP09