Non-perturbative string connections

We discuss the duality between two type I compactifications to four dimensions and an heterotic construction with spontaneous breaking of the N=4 supersymmetry to N=2. This duality allows us to gain insight into the non-perturbative properties of these models. Through the analysis of the gravitational corrections, we then investigate the connections between four-dimensional, N=2 M-theory vacua constructed as orbifolds of type II, heterotic, and type I strings.Comment: 7 pages, latex, to appear in the proceedings of the TMR meeting on Quantum aspects of gauge theories, supersymmetry and unification, Paris, 1--7 September 199

Type IIB orbifolds with D5-branes and their string duals

We consider Z2, freely acting orbifolds of the type IIB string with 16 parallel D5-branes. When the string is compactified on T2 X T4 and the D5-branes are wrapped on T2, these systems possess N=2 supersymmetry, originating from the spontaneous, partial breaking of N=4. Extended supersymmetry allows us to investigate the duality with certain heterotic and type I constructions, and to obtain informations about their non-perturbative regime.Comment: latex, 20 pages, no figures, revised version to be published on Nuclear Physics

A Bayesian approach to analyze regional elasticities

This paper presents a bayesian approach to analyze regional elasticity distributions with a regular translog cost function. It is known that a proper statistical analysis concerning elasticities can be performed only with the bayesian approach. Morover we can take advantage of this methodology to form reasonable priors using national data. This way we can produce sounder inferences without much elicition by the analyst. To compare results, this approach is applied to a cost function for the main regions in Italy with a diffuse prior too. Price and substitution elasticities are derived conditional on factor shares or covariates. The low posterior probability than inequality constraints hold with an noninformative prior shows how bayesian methods can be fruitfully employed to assess regional elasticities with a proper prior obtained from national data.

String-String triality for d=4, Z_2 orbifolds

We investigate the perturbative and non-perturbative correspondence of a class of four dimensional dual string constructions with N=4 and N=2 supersymmetry, obtained as Z_2 or Z_2 x Z_2 orbifolds of the type II, heterotic and type I string. In particular, we discuss the heterotic and type I dual of all the symmetric Z_2 x Z_2 orbifolds of the type II string, classified in hep-th/9901123. .Comment: latex, 50 pages, figures, final published versio

Classification of the N=2, Z2 X Z2-symmetric type II orbifolds and their type II asymmetric duals

Using free world-sheet fermions, we construct and classify all the N=2, Z2 X Z2 four-dimensional orbifolds of the type IIA/B strings for which the orbifold projections act symmetrically on the left and right movers. We study the deformations of these models out of the fermionic point, deriving the partition functions at a generic point in the moduli of the internal torus T6=T2 X T2 X T2. We investigate some of their perturbative and non-perturbative dualities and construct new dual pairs of type IIA/type II asymmetric orbifolds, which are related non-perturbatively and allow us to gain insight into some of the non-perturbative properties of the type IIA/B strings in four dimensions. In particular, we consider some of the (non-)perturbative gravitational corrections.Comment: Latex, 47 pages, no figure