Bounded perturbation resilience of projected scaled gradient methods

We investigate projected scaled gradient (PSG) methods for convex minimization problems. These methods perform a descent step along a diagonally scaled gradient direction followed by a feasibility regaining step via orthogonal projection onto the constraint set. This constitutes a generalized algorithmic structure that encompasses as special cases the gradient projection method, the projected Newton method, the projected Landweber-type methods and the generalized Expectation-Maximization (EM)-type methods. We prove the convergence of the PSG methods in the presence of bounded perturbations. This resilience to bounded perturbations is relevant to the ability to apply the recently developed superiorization methodology to PSG methods, in particular to the EM algorithm.Comment: Computational Optimization and Applications, accepted for publicatio

An Optimal Control Derivation of Nonlinear Smoothing Equations

The purpose of this paper is to review and highlight some connections between the problem of nonlinear smoothing and optimal control of the Liouville equation. The latter has been an active area of recent research interest owing to work in mean-field games and optimal transportation theory. The nonlinear smoothing problem is considered here for continuous-time Markov processes. The observation process is modeled as a nonlinear function of a hidden state with an additive Gaussian measurement noise. A variational formulation is described based upon the relative entropy formula introduced by Newton and Mitter. The resulting optimal control problem is formulated on the space of probability distributions. The Hamilton's equation of the optimal control are related to the Zakai equation of nonlinear smoothing via the log transformation. The overall procedure is shown to generalize the classical Mortensen's minimum energy estimator for the linear Gaussian problem.Comment: 7 pages, 0 figures, under peer reviewin

Land Rental Markets in the Process of Rural Structural Transformation: Productivity and Equity Impacts from China

While the importance of land rental for overall economic development has long been recognized in theory, empirical evidence on the productivity and equity impact of such markets and the extent to which they realize their potential has been scant. Representative data from China's nine most important agricultural provinces illustrate the impact of rental markets on households' economic strategies, their welfare, and productivity of land use at the plot level. While there are positive impacts in each of these dimensions, transaction costs constrain participation by many producers, thus preventing rental markets from attaining their full potential. Factors that increase transaction costs are identified, together with a rough estimate of the productivity- and equity- impact of removing them.Land Economics/Use,

Securing property rights in transition: Lessons from implementation of China's rural land contracting law

While recent studies point towards the importance of institutions, in particular secure property rights, as key determinants of economic growth, options to foster institutional change to make property rights more secure are not well understood. Data from 800 villages all over China on the effectiveness with which a law aiming to increase households' tenure security -at the expense of local leaders' powers to reallocate or expropriate land without adequate compensation- provides an opportunity to identify such factors. Using illegal land reallocations and low compensation payments for expropriated land to identify lack of effective institutional change, we find that the impact of property rights reform is contingent on the institutional constraints imposed on leaders' power by democratic institutions and a clear legal framework, households' knowledge of the law, and to some extent presence of land certificates.Land Economics/Use,

THE EFFECT OF LOST EXPORTS ON U.S. BEEF PRICES

Since the discovery of Bovine Spongiform Encephalopathy (BSE) in the United States in December 2003, U.S. beef exports have declined approximately 85 percent. A number of countries, including Japan and Korea (the top export markets for U.S. beef), have banned imports of beef from the United States, while U.S. exports to other important markets, such as Mexico and Canada, have been well below previous levels. Domestic demand in the United States was not significantly affected by the BSE discovery, but the effect of decreased beef exports on U.S. price is significant. This study examines the effect of exports and other supply and demand factors on U.S. meat prices, and estimates the effect of the drop in exports on U.S. beef and cattle prices. Results indicate that if all other factors remain the same, the drop in exports results in a $0.22 per pound reduction in retail beef prices and a$0.04 per pound reduction in the slaughter steer price. Prices in 2004 remained relatively high, however, possibly due to a decline in U.S. production and strong domestic demand.Bovine Spongiform Encephalopathy, beef, pork, chicken, exports, International Relations/Trade,