Sino the Times: three spoken drama productions on the Beijing stage

Today's modern theatre in Beijing shows new talents and directions as well as problems that are part of the uncertainties of Chinese society — in what may be the most intriguing transitional period in Chinese history

Making Clean Energy with a Kerr Black Hole: a Tokamak Model for Gamma-Ray Bursts

In this paper we present a model for making clean energy with a Kerr black hole. Consider a Kerr black hole with a dense plasma torus spinning around it. A toroidal electric current flows on the surface of the torus, which generates a poloidal magnetic field outside the torus. On the surface of the tours the magnetic field is parallel to the surface. The closed magnetic field lines winding around the torus compress and confine the plasma in the torus, as in the case of tokamaks. Though it is unclear if such a model is stable, we look into the consequences if the model is stable. If the magnetic field is strong enough, the baryonic contamination from the plasma in the torus is greatly suppressed by the magnetic confinement and a clean magnetosphere of electron-positron pairs is built up around the black hole. Since there are no open magnetic field lines threading the torus and no accretion, the power of the torus is zero. If some magnetic field lines threading the black hole are open and connect with loads, clean energy can be extracted from the Kerr black hole by the Blandford-Znajek mechanism. The model may be relevant to gamma-ray bursts. The energy in the Poynting flux produced by the Blandford-Znajek mechanism is converted into the kinetic energy of the electron-positron pairs in the magnetosphere around the black hole, which generates two oppositely directed jets of electron-positron pairs with super-high bulk Lorentz factors. The jets collide and interact with the interstellar medium, which may produce gamma-ray bursts and the afterglows.Comment: 14 pages, 1 figure, accepted by Ap

A Combinatorial Formula for Certain Elements of Upper Cluster Algebras

We develop an elementary formula for certain non-trivial elements of upper cluster algebras. These elements have positive coefficients. We show that when the cluster algebra is acyclic these elements form a basis. Using this formula, we show that each non-acyclic skew-symmetric cluster algebra of rank 3 is properly contained in its upper cluster algebra

Absolutely Continuous Representations and a Kaplansky Density Theorem for Free Semigroup Algebras

We introduce notions of absolutely continuous functionals and representations on the non-commutative disk algebra $A_n$. Absolutely continuous functionals are used to help identify the type L part of the free semigroup algebra associated to a $*$-extendible representation $\sigma$. A $*$-extendible representation of $A_n$ is ``regular'' if the absolutely continuous part coincides with the type L part. All known examples are regular. Absolutely continuous functionals are intimately related to maps which intertwine a given $*$-extendible representation with the left regular representation. A simple application of these ideas extends reflexivity and hyper-reflexivity results. Moreover the use of absolute continuity is a crucial device for establishing a density theorem which states that the unit ball of $\sigma(A_n)$ is weak-$*$ dense in the unit ball of the associated free semigroup algebra if and only if $\sigma$ is regular. We provide some explicit constructions related to the density theorem for specific representations. A notion of singular functionals is also defined, and every functional decomposes in a canonical way into the sum of its absolutely continuous and singular parts.Comment: 26 pages, prepared with LATeX2e, submitted to Journal of Functional Analysi

Magneto-Centrifugal Launching of Jets from Accretion Disks. I: Cold Axisymmetric Flows

The magneto-centrifugal model for jet formation is studied by time-dependent simulations reaching steady state in a cold gas with negligible fluid pressure, in an axisymmetric geometry, using a modification of the Zeus3D code adapted to parallel computers. The number of boundary conditions imposed at the coronal base takes into account the existence of the fast and Alfvenic critical surfaces, avoiding over-determination of the flow. The size and shape of the computational box is chosen to include these critical surfaces, reducing the influence of the outer boundary conditions. As there is a region, near the origin, where the inclination of field lines to the axis is too small to drive a centrifugal wind, we inject a thin, axial jet, expected to form electromagnetically near black holes. Acceleration and collimation appear for wide generic conditions. A reference run is shown in detail, with a wind leaving the computational volume in the axial direction with a poloidal velocity equal to 4 times the poloidal Alfven speed, collimated inside 11 degrees. Finally, the critical surfaces, fieldlines, thrust, energy, torque and mass discharge of the outgoing wind are shown for simulations with various profiles of mass and magnetic flux at the base of the corona.Comment: 27 pages, including 10 figures and 2 tables. To appear in ApJ (Dec 1999). Revised version clarifies the abstract, section 3.2.4, conclusions and appendix, adds a simulation to section 4.2, and updates the reference