Revisiting revisitation in computer interaction: organic bookmark management.

According to Milic-Frayling et al. (2004), there are two general ways of user browsing i.e. search (finding a website where the user has never visited before) and revisitation (returning to a website where the user has visited in the past). The issue of search is relevant to search engine technology, whilst revisitation concerns web usage and browser history mechanisms. The support for revisitation is normally through a set of functional built-in icons e.g. History, Back, Forward and Bookmarks. Nevertheless, for returning web users, they normally find it is easier and faster to re-launch an online search again, rather than spending time to find a particular web site from their personal bookmark and history records. Tauscher and Greenberg (1997) showed that revisiting web pages forms up to 58% of the recurrence rate of web browsing. Cockburn and McKenzie (2001) also stated that 81% of web pages have been previously visited by the user. According to Obendorf et al. (2007), revisitation can be divided into four classifications based on time: short-term (72.6% revisits within an hour), medium-term (12% revisits within a day and 7.8% revisits within a week), and long-term (7.6% revisits longer than a week)

Improved speed estimation in sensorless PM brushless AC drives

The application of flux-observer-based sensorless control to permanent-magnet brushless AC motor drives is described. Current methods of speed estimation are assessed, both theoretically and experimentally, and an improved method, which combines the best features of methods in which speed is derived from the differential of rotor position and from the ratio of the electromotive force to excitation flux linkage, is proposed. Its performance is verified experimentally

Sensorless flux-weakening control of permanent-magnet brushless machines using third harmonic back EMF

The sensorless control of brushless machines by detecting the third harmonic back electromotive force is a relatively simple and potentially low-cost technique. However, its application has been reported only for brushless dc motors operating under normal commutation. In this paper, the utility of the method for the sensorless control of both brushless dc and ac motors, including operation in the flux-weakening mode, is demonstrated

Ferromagnetic ground state of an orbital degenerate electronic model for transition-metal oxides: exact solution and physical mechanism

We present an exact ground state solution of a one-dimensional electronic model for transition-metal oxides in the strong coupling limit. The model contains doubly degenerated orbit for itinerant electrons and the Hund coupling between the itinerant electrons and localized spins. The ground state is proven to be a full ferromagnet for any density of electrons. Our model provides a rigorous example for metallic ferromagnetism in narrow band systems. The physical mechanism for ferromagnetism and its relevance to high-dimensional systems, like R$_{1-x}$X$_x$MnO$_3$, are discussed. Due to the orbital degeneracy of itinerant electrons, the superexchange coupling can be ferromagnetic rather than antiferromagnetic in the one-band case.Comment: 4 page, no figure To appear in Phys. Rev. B, (January 1, 1999

An effective Hamiltonian for an extended Kondo lattice model and a possible origin of charge ordering in half-doped manganites

An effective Hamiltonian is derived in the case of the strong Hund coupling and on-site Coulomb interaction by means of a projective perturbation approach. A physical mechanism for charge ordering in half-doped manganites (R_{0.5}X_{0.5}MnO_3) is proposed. The virtual process of electron hopping results in antiferromagnetic superexchange and a repulsive interaction, which may drive electrons to form a Wigner lattice. The phase diagram of the ground state of the model is presented at half doping. In the case of formation of Wigner lattice, we prove that spins of electrons are aligned ferromagnetically as well as that the localized spin background is antiferromagnetic. The influence of the on-site Coulomb interaction is also discussed.Comment: 6 pages ReTex with two figures To appear in Phys. Rev. B 59, (June 1, 1999