Attainability in Repeated Games with Vector Payoffs

We introduce the concept of attainable sets of payoffs in two-player repeated games with vector payoffs. A set of payoff vectors is called {\em attainable} if player 1 can ensure that there is a finite horizon $T$ such that after time $T$ the distance between the set and the cumulative payoff is arbitrarily small, regardless of what strategy player 2 is using. This paper focuses on the case where the attainable set consists of one payoff vector. In this case the vector is called an attainable vector. We study properties of the set of attainable vectors, and characterize when a specific vector is attainable and when every vector is attainable.Comment: 28 pages, 2 figures, conference version at NetGCoop 201

Measuring the Contributions of Motives and Perceived Barriers to Active Aging

General motivation to be active and general perceived barriers to activity may help to explain the overall activity patterns of older persons in the second half of the life course. We report on a project designed to develop and refine measures of motivation and perceived barriers that can be used to examine the relationships between activity motivation, activity barriers, and several forms of actual activities. Four specific activities were considered: working, volunteering, exercising, and taking classes. An opportunity sample of 192 middle-aged and older persons from eastern Massachusetts responded to a questionnaire concerned with motivation, perceived barriers, and activities. Reliable measures of both general and activity-specific motivation and perceived barriers to activity were developed. The measures were examined for construct validity purposes. This analysis demonstrates that general activity motivation and perceived barriers are helpful in understanding patterns of activity among older people

Elder Activities: Patterns, Motives, and Interpretation. Massachusetts Lifestyles Study III

We are engaged in a continuing effort to understand the scope of elder activity and the implications of activity for well-being. We are particularly interested in the forces that encourage or inhibit multiple forms of activity, the ways in which participation in some activities influences participation in other activities, and the cumulative implications of activities for well being. Our interest in multiple forms of activities sets us somewhat apart from Gerontologists who focus on single forms of activity such as volunteering, taking classes, caring for grandchildren, working, or exercising

Ground States of Two-Dimensional Polyampholytes

We perform an exact enumeration study of polymers formed from a (quenched) random sequence of charged monomers $\pm q_0$, restricted to a 2-dimensional square lattice. Monomers interact via a logarithmic (Coulomb) interaction. We study the ground state properties of the polymers as a function of their excess charge $Q$ for all possible charge sequences up to a polymer length N=18. We find that the ground state of the neutral ensemble is compact and its energy extensive and self-averaging. The addition of small excess charge causes an expansion of the ground state with the monomer density depending only on $Q$. In an annealed ensemble the ground state is fully stretched for any excess charge $Q>0$.Comment: 6 pages, 6 eps figures, RevTex, Submitted to Phys. Rev.

Functional Nanoscale Organization of Signaling Molecules Downstream of the T Cell Antigen Receptor

Receptor-regulated cellular signaling often is mediated by formation of transient, heterogeneous protein complexes of undefined structure. We used single and two-color photoactivated localization microscopy to study complexes downstream of the T cell antigen receptor (TCR) in single-molecule detail at the plasma membrane of intact T cells. The kinase ZAP-70 distributed completely with the TCRζ chain and both partially mixed with the adaptor LAT in activated cells, thus showing localized activation of LAT by TCR-coupled ZAP-70. In resting and activated cells, LAT primarily resided in nanoscale clusters as small as dimers whose formation depended on protein-protein and protein-lipid interactions. Surprisingly, the adaptor SLP-76 localized to the periphery of LAT clusters. This nanoscale structure depended on polymerized actin and its disruption affected TCR-dependent cell function. These results extend our understanding of the mechanism of T cell activation and the formation and organization of TCR-mediated signaling complexes, findings also relevant to other receptor systems. © 2011 Elsevier Inc

The Galactic Center Black Hole Laboratory

The super-massive 4 million solar mass black hole Sagittarius~A* (SgrA*) shows flare emission from the millimeter to the X-ray domain. A detailed analysis of the infrared light curves allows us to address the accretion phenomenon in a statistical way. The analysis shows that the near-infrared flare amplitudes are dominated by a single state power law, with the low states in SgrA* limited by confusion through the unresolved stellar background. There are several dusty objects in the immediate vicinity of SgrA*. The source G2/DSO is one of them. Its nature is unclear. It may be comparable to similar stellar dusty sources in the region or may consist predominantly of gas and dust. In this case a particularly enhanced accretion activity onto SgrA* may be expected in the near future. Here the interpretation of recent data and ongoing observations are discussed.Comment: 30 pages - 7 figures - accepted for publication by Springer's "Fundamental Theories of Physics" series; summarizing GC contributions of 2 conferences: 'Equations of Motion in Relativistic Gravity' at the Physikzentrum Bad Honnef, Bad Honnef, Germany, (Feb. 17-23, 2013) and the COST MP0905 'The Galactic Center Black Hole Laboratory' Granada, Spain (Nov. 19 - 22, 2013

Star Formation and Dynamics in the Galactic Centre

The centre of our Galaxy is one of the most studied and yet enigmatic places in the Universe. At a distance of about 8 kpc from our Sun, the Galactic centre (GC) is the ideal environment to study the extreme processes that take place in the vicinity of a supermassive black hole (SMBH). Despite the hostile environment, several tens of early-type stars populate the central parsec of our Galaxy. A fraction of them lie in a thin ring with mild eccentricity and inner radius ~0.04 pc, while the S-stars, i.e. the ~30 stars closest to the SMBH (<0.04 pc), have randomly oriented and highly eccentric orbits. The formation of such early-type stars has been a puzzle for a long time: molecular clouds should be tidally disrupted by the SMBH before they can fragment into stars. We review the main scenarios proposed to explain the formation and the dynamical evolution of the early-type stars in the GC. In particular, we discuss the most popular in situ scenarios (accretion disc fragmentation and molecular cloud disruption) and migration scenarios (star cluster inspiral and Hills mechanism). We focus on the most pressing challenges that must be faced to shed light on the process of star formation in the vicinity of a SMBH.Comment: 68 pages, 35 figures; invited review chapter, to be published in expanded form in Haardt, F., Gorini, V., Moschella, U. and Treves, A., 'Astrophysical Black Holes'. Lecture Notes in Physics. Springer 201