Polyakov loop correlators from D0-brane interactions in bosonic string theory

In this paper we re-derive the effective Nambu-Goto theory result for the Polyakov loop correlator, starting from the free bosonic string and using a covariant quantization. The boundary conditions are those of an open string attached to two D0-branes at spatial distance R, in a target space with compact euclidean time. The one-loop free energy contains topologically distinct sectors corresponding to multiple covers of the cylinder in target space bordered by the Polyakov loops. The sector that winds once reproduces exactly the Nambu-Goto partition function. In our approach, the world-sheet duality between the open and closed channel is most evident and allows for an explicit interpretation of the free energy in terms of tree level exchange of closed strings between boundary states. Our treatment is fully consistent only in d=26; extension to generic d may be justified for large R, and is supported by Montecarlo data. At shorter scales, consistency and Montecarlo data seem to suggest the necessity of taking into account the Liouville mode of Polyakov's formulation.Comment: 17 pages, 4 figures, minor corrections, a few references added, version accepted for publication in JHE

On Equivalence and Cores for Incomplete Databases in Open and Closed Worlds

Data exchange heavily relies on the notion of incomplete database instances. Several semantics for such instances have been proposed and include open (OWA), closed (CWA), and open-closed (OCWA) world. For all these semantics important questions are: whether one incomplete instance semantically implies another; when two are semantically equivalent; and whether a smaller or smallest semantically equivalent instance exists. For OWA and CWA these questions are fully answered. For several variants of OCWA, however, they remain open. In this work we adress these questions for Closed Powerset semantics and the OCWA semantics of [Leonid Libkin and Cristina Sirangelo, 2011]. We define a new OCWA semantics, called OCWA*, in terms of homomorphic covers that subsumes both semantics, and characterize semantic implication and equivalence in terms of such covers. This characterization yields a guess-and-check algorithm to decide equivalence, and shows that the problem is NP-complete. For the minimization problem we show that for several common notions of minimality there is in general no unique minimal equivalent instance for Closed Powerset semantics, and consequently not for the more expressive OCWA* either. However, for Closed Powerset semantics we show that one can find, for any incomplete database, a unique finite set of its subinstances which are subinstances (up to renaming of nulls) of all instances semantically equivalent to the original incomplete one. We study properties of this set, and extend the analysis to OCWA*

Data exchange and schema mappings in open and closed worlds

In the study of data exchange one usually assumes an open-world semantics, making it possible to extend instances of target schemas. An alternative closed-world semantics only moves ‘as much data as needed’ from the source to the target to satisfy constraints of a schema mapping. It avoids some of the problems exhibited by the open-world semantics, but limits the expressivity of schema mappings. Here we propose a mixed approach: one can designate different attributes of target schemas as open or closed, to combine the additional expressivity of the open-world semantics with the better behavior of query answering in closed worlds. We define such schema mappings, and show that they cover a large space of data exchange solutions with two extremes being the known open and closed-world semantics. We investigate the problems of query answering and schema mapping composition, and prove two trichotomy theorems, classifying their complexity based on the number of open attributes. We find conditions under which schema mappings compose, extending known results to a wide range of closed-world mappings. We also provide results for restricted classes of queries and mappings guaranteeing lower complexity

Control Variates for Monte Carlo-Pricing of Three-Asset Spread Options with Application in the Energy Markets

The purpose of this paper is to compare a collection of control variates for Monte Carlo-valuation of spread options on three assets with a view towards energy markets and to lay a foundation for continued research on control variates, e.g. combinations of control variates and adaption for quasi-Monte Carlo. The paper builds upon previous research on option pricing using Monte Carlo-simulation and closed form approximations. The use of underlying assets, underlying spread, call option, exchange options and delta hedge as control variates are tested in both a parametric study to test the impact of every input parameter, and a real-world scenario using data from the Dutch energy markets. In the real-world scenario the exchange option outperforms the other control variates in most cases. Uneven results for the exchange option in the parametric study leads to the conclusion that the use of delta hedge as control variate is the best performing based on the test results

Government expenditures and equilibrium real exchange rates

Economists have long investigated theoretically and empirically the relationship between government spending and equilibrium real exchange rates. As Frenkel and Razin (1996) summarize for a small open economy, government expenditures (financed by lump-sum taxes) influence real exchange rates via a resource-withdrawal channel and a consumption-tilting channel. Recent theoretical and empirical studies, such as Froot and Rogoff (1991), Rogoff (1992), De Gregorio, Giovannini, and Krueger (1994), De Gregorio, Giovannini, and Wolf (1994), De Gregorio and Wolf (1994), and Chinn and Johnston (1996), have focused only upon the effects of government spending through the resource-withdrawal channel. Extending Frenkel and Razin (1996), this paper generates closed-form theoretical solutions for the relationships among the real exchange rate, relative per capita private consumption, relative per capita government consumption, and relative per capita tradables and nontradables production in a two-country general equilibrium model. Using relative price level, private and government per capita consumption, and relative productivity data from the Summers and Heston (1991) Penn World Tables and OECD (1 996) data for a sample of OECD countries relative to the United States, we estimate the model\u27 s structural equations. The results suggest that government expenditures influence equilibrium real exchange rates approximately equally via the resource-withdrawal and consumption-tilting channels. Moreover, the results imply that government spending and private consumption are complements in utility

The chase procedure and its applications

The goal of this thesis is not only to introduce and present new chase-based algorithms, but also to investigate the differences between the main existing chase procedures. In order to achieve this, first we will investigate and do a clear delimitation between the existing chase algorithms based on their termination criteria. This will give a better picture of which chase algorithm can be used for different dependency classes. Next, we will investigate the data exchange, data repair and data correspondence problems and show how the chase algorithm can be used to characterize different types of solutions. For the later two problems, we will also investigate the data complexity of solution-existence and solution-check problems. Further, we will introduce a new chase based algorithm which computes representative solutions under constructible models, a new closed world semantics. This new semantics is, in our view, appropriate to be used as a closed world semantics in data exchange. We will also show that the conditional table computed by this chase algorithm can help to get both possible and certain answers for general queries. And finally, we will investigate strong representation systems and strong data exchange representation system. We will prove, by introducing a new chase based algorithm, that mappings specified by source-to-target second order dependencies and target richly acyclic TGD’s are strong data exchange representation systems for the class of first order queries