Functional and isoperimetric inequalities for probability measures on H-type groups

We investigate isoperimetric and functional inequalities for probability measures in the sub-elliptic setting and more specifically, on groups of Heisenberg type. The approach we take is based on U-bounds as well as a Laplacian comparison theorem for H-type groups. We derive different forms of functional inequalities (of [Phi]-entropy and F-Sobolev type) and show that they can be equivalently stated as isoperimetric inequalities at the level of sets. Furthermore, we study transportation of measure via Talagrand-type inequalities. The methods used allow us to obtain gradient bounds for the heat semigroup. Finally, we examine some properties of more general operators given in Hormander’s sum of squares form and show that the associated semigroup converges to a probability measure as t → [infinity]

Density of States and Tachyons in Open and Closed String Theory

In this note we reexamine the possibility of constructing stable non-supersymmetric theories that exhibit an exponential density of states. For weakly coupled closed strings there is a general theorem, according to which stable theories with an exponential density of states must exhibit an almost exact cancellation of spacetime bosons and fermions (not necessarily level by level). We extend this result to open strings by showing that if the above cancellation between bosons and fermions does not occur, the open strings do not decouple from a closed string tachyon even in the NCOS scaling limit. We conclude with a brief comment on the proposed generalization of the AdS/CFT correspondence to non-supersymmetric theories.Comment: 11 pages, harvmac, v2: improved discussion, references added, v3: version published in JHE

Singular integrals on Sierpinski gaskets

We construct a class of singular integral operators associated with homogeneous Calder\'{o}n-Zygmund standard kernels on $d$-dimensional, $d <1$, Sierpinski gaskets $E_d$. These operators are bounded in $L^2(\mu_d)$ and their principal values diverge $\mu_d$ almost everywhere, where $\mu_d$ is the natural (d-dimensional) measure on $E_d$

The Space Between Us; Taking Stock, Looking Ahead

The attempt to theorise certain concepts like “the region†or "spatial development" should begin with the conceptualisation of the very basic element of them that is space. In the fields of contemporary economic geography and geographical economy there are two broad perspectives of “spaceâ€, which both have formed the basis of a long standing debate in multiple dimensions (eg. deduction- induction, quantitative- qualitative etc). The first perspective sees space as a container of action. Action is clearly demarcated from space, which has become “neutral†and no dynamic relation exists between them. Regions then can be compared and the measurable elements of action can be analysed and modelled through positivism. Scholars from the second perspective partially reject that way of thinking and tend to emphasise on the role of past and that of embeddedness of action in time-space. These see space as a medium for action. Every region (or locality) here is an existent alterity (historically produced) with its own politics, institutions and culture that cannot be compared with other regions in a positivistic sense, nor best practices can be easily transferred. What matters is the inter-relational action that produces space and at the same time it is influenced by space. Action and space form a duality in time. Instead of the statistical comparative methodology and modelling of the first perspective, what is of importance here, not only in terms of methodology but also in terms of policy making, is the deep relational experience of the researcher/ policy-maker with the space. The paper argues that what is needed is not a demarcation line between those two streams of literature but an interdisciplinary approach that gives an emphasis not on the sum of them but in the synthesis of logics and methodologies in an approach that promotes holisticity.

Market Vs. Planning: The Old Controversy Revisited.

The paper examines the evolution of urban land uses in Greece and USA in the last twenty years and reviews the role of the market forces and the role of the land-use regulations in shaping the current urban form in both countries. The paper proposes a framewrk of coordination between these two elements in order to improve and make more effective urban planning efforts.

Whatever happened to competition in space agency procurement? The case of NASA

Using the U.S. National Aeronautics and Space Administration (NASA) as a case study, this paper examines how conflicting objectives in procurement policies by public space agencies result in anti-competitive procurement. Globally, public sectors have actively encouraged mergers and acquisitions of major contractors at the national level, since the end of the “Cold War”, following largely from the perceived benefits of economies of size. The paper examines the impact the resulting industrial concentration has on the ability of space agencies to follow a pro-competitive procurement policy. Using time series econometric analysis, the paper shows that NASA’s pro-competitive policy is unsuccessful due to a shift, since the mid-1990s, in the share of appropriations in favour of its top contractors.procurement, space industry, space agencies, NASA