8,234 research outputs found
Conifold Transitions and Mirror Symmetries
Recent work initiated by Strominger has lead to a consistent physical
interpretation of certain types of transitions between different string vacua.
These transitions, discovered several years ago, involve singular conifold
configurations which connect distinct Calabi-Yau manifolds. In this paper we
discuss a number of aspects of conifold transitions pertinent to both
worldsheet and spacetime mirror symmetry. It is shown that the mirror transform
based on fractional transformations allows an extension of the mirror map to
conifold boundary points of the moduli space of weighted Calabi-Yau manifolds.
The conifold points encountered in the mirror context are not amenable to an
analysis via the original splitting constructions. We describe the first
examples of such nonsplitting conifold transitions, which turn out to connect
the known web of Calabi-Yau spaces to new regions of the collective moduli
space. We then generalize the splitting conifold transition to weighted
manifolds and describe a class of connections between the webs of ordinary and
weighted projective Calabi-Yau spaces. Combining these two constructions we
find evidence for a dual analog of conifold transitions in heterotic N2
compactifications on K3T and in particular describe the first
conifold transition of a Calabi-Yau manifold whose heterotic dual has been
identified by Kachru and Vafa. We furthermore present a special type of
conifold transition which, when applied to certain classes of Calabi-Yau K3
fibrations, preserves the fiber structure.Comment: 23 page
Modelling Spatial Regimes in Farms Technologies
We exploit the information derived from geographical coordinates to
endogenously identify spatial regimes in technologies that are the result of a
variety of complex, dynamic interactions among site-specific environmental
variables and farmer decision making about technology, which are often not
observed at the farm level. Controlling for unobserved heterogeneity is a
fundamental challenge in empirical research, as failing to do so can produce
model misspecification and preclude causal inference. In this article, we adopt
a two-step procedure to deal with unobserved spatial heterogeneity, while
accounting for spatial dependence in a cross-sectional setting. The first step
of the procedure takes explicitly unobserved spatial heterogeneity into account
to endogenously identify subsets of farms that follow a similar local
production econometric model, i.e. spatial production regimes. The second step
consists in the specification of a spatial autoregressive model with
autoregressive disturbances and spatial regimes. The method is applied to two
regional samples of olive growing farms in Italy. The main finding is that the
identification of spatial regimes can help drawing a more detailed picture of
the production environment and provide more accurate information to guide
extension services and policy makers
Exclusion Statistics in Conformal Field Theory -- generalized fermions and spinons for level-1 WZW theories
We systematically study the exclusion statistics for quasi-particles for
Conformal Field Theory spectra by employing a method based on recursion
relations for truncated spectra. Our examples include generalized fermions in
c<1 unitary minimal models, Z_k parafermions, and spinons for the su(n)_1,
so(n)_1 and sp(2n)_1 Wess-Zumino-Witten models. For some of the latter examples
we present explicit expressions for finitized affine characters and for the
N-spinon decomposition of affine characters.Comment: LaTeX, 49 pages, some further clarifications added, refs added and
updated. Nucl. Phys. B, to be publishe
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