Reduced Phase Space Quantization and Dirac Observables

In her recent work, Dittrich generalized Rovelli's idea of partial observables to construct Dirac observables for constrained systems to the general case of an arbitrary first class constraint algebra with structure functions rather than structure constants. Here we use this framework and propose a new way for how to implement explicitly a reduced phase space quantization of a given system, at least in principle, without the need to compute the gauge equivalence classes. The degree of practicality of this programme depends on the choice of the partial observables involved. The (multi-fingered) time evolution was shown to correspond to an automorphism on the set of Dirac observables so generated and interesting representations of the latter will be those for which a suitable preferred subgroup is realized unitarily. We sketch how such a programme might look like for General Relativity. We also observe that the ideas by Dittrich can be used in order to generate constraints equivalent to those of the Hamiltonian constraints for General Relativity such that they are spatially diffeomorphism invariant. This has the important consequence that one can now quantize the new Hamiltonian constraints on the partially reduced Hilbert space of spatially diffeomorphism invariant states, just as for the recently proposed Master constraint programme.Comment: 18 pages, no figure

Calculus on surfaces with general closest point functions

The Closest Point Method for solving partial differential equations (PDEs) posed on surfaces was recently introduced by Ruuth and Merriman [J. Comput. Phys. 2008] and successfully applied to a variety of surface PDEs. In this paper we study the theoretical foundations of this method. The main idea is that surface differentials of a surface function can be replaced with Cartesian differentials of its closest point extension, i.e., its composition with a closest point function. We introduce a general class of these closest point functions (a subset of differentiable retractions), show that these are exactly the functions necessary to satisfy the above idea, and give a geometric characterization this class. Finally, we construct some closest point functions and demonstrate their effectiveness numerically on surface PDEs

Turbopump thermodynamic cooling

System for cooling turbopumps used in cryogenic fluid storage facilities is described. Technique uses thermodynamic propellant vent to intercept pump heat at desired conditions. Cooling system uses hydrogen from outside source or residual hydrogen from cryogenic storage tank

Software architectures and Open Source Software: Where can research leverage the most?

Software architectures have been playing a central role in software engineering research for some years now. They are considered of pivotal importance in the success of complex software systems development. However, with the emergence of Open Source Software (OSS) development, a new opportunity for studying architectural issues arises. In this paper, we introduce accepted notions of software architectures (Section 2), discuss some of the known issues in OSS (Section 3), resulting in a set of aspects we consider to be relevant for future research (Section 4)

Sale the seven Cs: Teaching/training aid for the (e-)retail mix

The ‘4Ps’ of the marketing mix have long been popular with students, tutors, trainers and practitioners as a learning and teaching aid. The purpose of this paper is to present an equivalent tool for retail and e-retail: ‘Sale the 7Cs’. The approach is by reference to other authors’ versions of the marketing, retail and e-retail mixes, distilled into a simplified framework: C1 Convenience; C2 Customer value and benefit; C3 Cost to the customer; C4 Computing and category management; C5 Customer franchise; C6 Customer care and service; C7 Communication and customer relationships. This simplified mnemonic is new for (e-)retail. Mini case examples are used to illustrate the applicability. These have a practical value for trainers and educators as specimen answers to activity exercises. Retailers may find the convenient 7Cs structure useful when planning strategies and tactics

Spectroscopy of the enigmatic short-period cataclysmic variable IR Com in an extended low state

We report the occurrence of a deep low state in the eclipsing short-period cataclysmic variable IR Com, lasting more than two years. Spectroscopy obtained in this state shows the system as a detached white dwarf plus low-mass companion, indicating that accretion has practically ceased. The spectral type of the companion derived from the SDSS spectrum is M6-7, somewhat later than expected for the orbital period of IR Com. Its radial velocity amplitude, K_2=419.6+-3.4 km/s, together with the inclination of 75-90deg implies 0.8Msun<Mwd<1.0Msun. We estimate the white dwarf temperature to be ~15000K, and the absence of Zeeman splitting in the Balmer lines rules out magnetic fields in excess of ~5 MG. IR Com still defies an unambiguous classification, in particular the occurrence of a deep, long low state is so far unique among short-period CVs that are not strongly magnetic.Comment: Revised version, MNRAS Letters in pres