Colliding waves on a string in AdS3_3

This paper is concerned with the classical motion of a string in global AdS$_3$. The initially static string stretches between two antipodal points on the boundary circle. Both endpoints are perturbed which creates cusps at a steady rate. The cusps propagate towards the interior where they collide. The behavior of the string depends on the strength of forcing. Three qualitatively different phases can be distinguished: transparent, gray, and black. The transparent region is analogous to a standing wave. In the black phase, there is a horizon on the worldsheet and cusps never reach the other endpoint. The string keeps folding and its length grows linearly over time. In the gray phase, the string still grows linearly. However, cusps do cross to the other side. The transparent and gray regions are separated by a transition point where a logarithmic accumulation of cusps is numerically observed. A simple model reproduces the qualitative behavior of the string in the three phases.Comment: 9 pages, 16 figure

Holography without translational symmetry

We propose massive gravity as a holographic framework for describing a class of strongly interacting quantum field theories with broken translational symmetry. Bulk gravitons are assumed to have a Lorentz-breaking mass term as a substitute for spatial inhomogeneities. This breaks momentum-conservation in the boundary field theory. At finite chemical potential, the gravity duals are charged black holes in asymptotically anti-de Sitter spacetime. The conductivity in these systems generally exhibits a Drude peak that approaches a delta function in the massless gravity limit. Furthermore, the optical conductivity shows an emergent scaling law: $|\sigma(\omega)| \approx {A \over \omega^{\alpha}} + B$. This result is consistent with that found earlier by Horowitz, Santos, and Tong who introduced an explicit inhomogeneous lattice into the system.Comment: 8 pages, 3 figures; v2: minor correction

Strongly polynomial algorithm for a class of minimum-cost flow problems with separable convex objectives

A well-studied nonlinear extension of the minimum-cost flow problem is to minimize the objective $\sum_{ij\in E} C_{ij}(f_{ij})$ over feasible flows $f$, where on every arc $ij$ of the network, $C_{ij}$ is a convex function. We give a strongly polynomial algorithm for the case when all $C_{ij}$'s are convex quadratic functions, settling an open problem raised e.g. by Hochbaum [1994]. We also give strongly polynomial algorithms for computing market equilibria in Fisher markets with linear utilities and with spending constraint utilities, that can be formulated in this framework (see Shmyrev [2009], Devanur et al. [2011]). For the latter class this resolves an open question raised by Vazirani [2010]. The running time is $O(m^4\log m)$ for quadratic costs, $O(n^4+n^2(m+n\log n)\log n)$ for Fisher's markets with linear utilities and $O(mn^3 +m^2(m+n\log n)\log m)$ for spending constraint utilities. All these algorithms are presented in a common framework that addresses the general problem setting. Whereas it is impossible to give a strongly polynomial algorithm for the general problem even in an approximate sense (see Hochbaum [1994]), we show that assuming the existence of certain black-box oracles, one can give an algorithm using a strongly polynomial number of arithmetic operations and oracle calls only. The particular algorithms can be derived by implementing these oracles in the respective settings

Monetary Policy, Interest Rate Rules, and Inflation Targeting: Some Basic Equivalences

Policymakers increasingly view short-term nominal interest rates as the main instrument of monetary policy, often in conjunction with some inflation target. Interest rates on short-term indexed government debt (i.e., a real interest rate) have also been used as policy instruments. To understand the pros and cons of different policy rules and instruments, this paper derives some basic equivalences among different policy rules. It is shown that, under certain conditions, the following three rules are exactly equivalent: (i) a 'k-percent' money growth rule; (ii) a nominal interest rate rule combined with an inflation target; and (iii) a real interest rate rule combined with an inflation target. These policy rules, however, become increasingly complex: the first rule requires no feedback mechanism; the second rule requires responding to the inflation gap; while the third rule involves responding to both the inflation gap and the output gap. It is also shown that policy rules which respond to the output gap may avoid a deflationary adjustment.

Inflation stabilization in chronic inflation countries: The empirical evidence

There is a rich history of stabilizations in chronic inflation countries, which spans more than three decades. This provides a unique opportunity to identify the main patterns of adjustment and examine econometrically some of the main features of disinflation in chronic inflation countries. Our study is based on 17 stabilization plans from 1964 to the present in 7 countries: Argentina, Brazil, the Dominican Republic, Israel, Mexico, Peru, and Uruguay. Twelve of these programs were based on the use of the exchange rate as the nominal anchor, while 5 were based on the use of a monetary aggregate. In addition to reviewing the main "stylized facts" for these 17 stabilization plans we resort to different econometric exercises and thus are able to provide some rigorous econometric basis for several key features of disinflation in chronic inflation countries.inflation stabilization exchange rates mone crises consumption boom

Intertemporal consumption substitution and inflation stabilization:An empirical investigation

Exchange rate based inflation stabilization programs in developing countries often lead to an initial consumption boom followed by an eventual recession. To explain such phenomenon, theoretical models have focused on the role of intertemporal consumption substitution in response to temporary reductions in nominal interest rates. This paper assesses the empirical relevance of such mechanism for six high-inflation developing countries that have gone through repeated stabilization attempts. A simple monetary model is used to obtain estimates of the intertemporal elasticity of substitution, and dynamics simulations are carried out to test the predictive power of the model. The analysis concludes that, in several cases, temporary shocks appeared to have played a key role in generating a consumption boom.inflation consumption boom stabilization interest rates

Environmentally Compatible Structures (ECS) - Introduction into the theory of ECS

p. 723-731The growth of population, industry, megacities, car traffic, intensity of building industry, etc. with consequent health threatening environmental pollution, became a most urgent global problem number one of the development and survival of the mankind. "To ensure protection and restoration of clean environment for future generations, a scientifically based internationally accepted unified environmental strategy is necessary. No doubt, this calls for introducing university subjects, scientific research and corresponding university text books in the field of environmentally compatible design and technology of buildings. Scientists, university staff members together with outstanding specialists in this field have to find and suggest the best acceptable means to unify the fundamentals on how to ensure environmental protection in structural engineering and architecture and identify the role of university education in this attempt. The concept of the text book on ECS, is meant as a contribution to this strategy.Vegh, L.; Vegh, P. (2009). Environmentally Compatible Structures (ECS) - Introduction into the theory of ECS. Editorial Universitat Politècnica de València. http://hdl.handle.net/10251/673

Approximating Minimum-Cost k-Node Connected Subgraphs via Independence-Free Graphs

We present a 6-approximation algorithm for the minimum-cost $k$-node connected spanning subgraph problem, assuming that the number of nodes is at least $k^3(k-1)+k$. We apply a combinatorial preprocessing, based on the Frank-Tardos algorithm for $k$-outconnectivity, to transform any input into an instance such that the iterative rounding method gives a 2-approximation guarantee. This is the first constant-factor approximation algorithm even in the asymptotic setting of the problem, that is, the restriction to instances where the number of nodes is lower bounded by a function of $k$.Comment: 20 pages, 1 figure, 28 reference