69,128 research outputs found
Unitary null energy condition violation in cosmologies
A non-singular cosmological bounce in the Einstein frame can only take place
if the Null Energy Condition (NEC) is violated. We explore situations where a
single scalar field drives the NEC violation and derive the constraints imposed
by demanding tree level unitarity on a cosmological background. We then focus
on the explicit constraints that arise in P(X) theories and show that
constraints from perturbative unitarity make it impossible for the NEC
violation to occur within the region of validity of the effective field theory
without also involving irrelevant operators that arise at a higher scale that
would enter from integrating out more massive degrees of freedom. Within the
context of P(X) theories we show that including such operators allows for a
bounce that does not manifestly violate tree level unitarity, but at the price
of either imposing a shift symmetry or involving technically unnatural small
operator coefficients within the low-energy effective field theory.Comment: 35 pages, 1 figur
Metrics for generalized persistence modules
We consider the question of defining interleaving metrics on generalized
persistence modules over arbitrary preordered sets. Our constructions are
functorial, which implies a form of stability for these metrics. We describe a
large class of examples, inverse-image persistence modules, which occur
whenever a topological space is mapped to a metric space. Several standard
theories of persistence and their stability can be described in this framework.
This includes the classical case of sublevelset persistent homology. We
introduce a distinction between `soft' and `hard' stability theorems. While our
treatment is direct and elementary, the approach can be explained abstractly in
terms of monoidal functors.Comment: Final version; no changes from previous version. Published online Oct
2014 in Foundations of Computational Mathematics. Print version to appea
Null boundary controllability of a 1-dimensional heat equation with an internal point mass
We consider a linear hybrid system composed by two rods of equal length
connected by a point mass. We show that the system is null controllable with
Dirichlet and Neumann controls. The results are based on a careful spectral
spectral analysis together with the moment method.Comment: 12 pages, typos corrected, added references, matches version to be
submitted to Systems and Control Letter
Reconciling the Returns to Education in Off-FarmWage Employment in Rural China
Previous studies have found that the returns to education in rural China are far lower than estimates for other developing economies. In this paper, we seek to determine why previous estimates are so low and provide estimates of what we believe are more accurate measures of the returns. Whereas estimates for the early 1990s average 2.3 percent, we find an average return of 6.4 percent. Furthermore, we find even higher returns among younger people, migrants,and for post-primary education. The paper demonstrates that, although part of the difference between our estimate and previous estimates can be attributed to increasing returns during the 1990s, a larger part of the difference is due to the nature of the data and the methodological approaches used by other authors.
Household Investment through migration in Rural China
In this paper, we strive to better understand how household investment is affected by participation in migration in rural China. After we describe investment patterns across different regions of rural China, we use a theoretical model to describe a relationship between migration and investment and to generate hypotheses about the relationship consistent with our descriptive findings. We test the hypotheses using household data collected in rural China in 2000 and find that in poorer areas migration increases consumptive investment by nearly 20 percent. We find no evidence of a link between migration and productive investment.China, migration, development, household investment, dynamic panel data
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