20,579 research outputs found
Generalized CoK\"ahler Geometry and an Application to Generalized K\"ahler Structures
In this paper we define the notion of a generalized coK\"ahler structure and
prove that the product of generalized contact metric
manifolds , , where
is endowed with the product generalized complex structure induced from
and , is generalized K\"ahler if and only if are generalized coK\"ahler structures. We also prove that
products of generalized coK\"ahler and generalized K\"ahler manifolds admit a
generalized coK\"ahler structure. We use these product constructions to give
nontrivial examples of generalized coK\"ahler structures. Finally, we show the
analogs of these theorems hold in the setting of twisted generalized
geometries. We use these theorems to construct new examples of twisted
generalized K\"ahler structures on manifolds that do not admit a classical
K\"ahler structure and we give examples of twisted generalized coK\"ahler
structures on manifolds which do not admit a classical coK\"ahler structure.Comment: Final print version. To appear in Journal of Geometry and Physic
Fibrations and stable generalized complex structures
A generalized complex structure is called stable if its defining
anticanonical section vanishes transversally, on a codimension-two submanifold.
Alternatively, it is a zero elliptic residue symplectic structure in the
elliptic tangent bundle associated to this submanifold. We develop
Gompf-Thurston symplectic techniques adapted to Lie algebroids, and use these
to construct stable generalized complex structures out of log-symplectic
structures. In particular we introduce the notion of a boundary Lefschetz
fibration for this purpose and describe how they can be obtained from genus one
Lefschetz fibrations over the disk.Comment: 35 pages, 2 figure
Fitting quotients of finitely presented abelian-by-nilpotent groups
We show that every finitely generated nilpotent group of class 2 occurs as
the quotient of a finitely presented abelian-by-nilpotent group by its largest
nilpotent normal subgroup.Comment: This second version takes into account the suggestions by the
referee; 11 page
Year-class formation in Pacific herring (Clupea pallasi) estimated from spawning-date distributions of juveniles in San Francisco Bay, California
Inter and intra-annual variation in year-class strength was analyzed for San Francisco Bay Pacific herring (Clupea pallasi) by using otoliths of juveniles. Juvenile herring
were collected from March through June in 1999 and 2000 and otoliths from subsamples of these collections were aged by daily otolith increment analysis. The composition of the year classes in 1999 and 2000 were determined by back-calculating the birth date distribution for surviving juvenile herring. In 2000, 729% more juveniles were captured than in 1999, even though an estimated 12% fewer
eggs were spawned in 2000. Spawning-date distributions show that survival for the 2000 year class was exceptionally good for a short (approximately 1 month) period of spawning,
resulting in a large abundance of juvenile recruits. Analysis of age at size shows that growth rate increased
significantly as the spawning season progressed both in 1999 and 2000. However, only in 2000 were the bulk of surviving juveniles a product of the fast growth period. In the two years examined, year-class strength was not predicted by the estimated number of eggs spawned, but rather appeared to depend on survival of eggs or larvae (or both) through the juvenile stage. Fast growth through the larval stage may have little effect on year-class strength if mortality during the egg stage is high and few larvae are available
Breaking of axial symmetry in excited heavy nuclei as identified in Giant Dipole Resonance data
A recent theoretical prediction of a breaking of axial symmetry in quasi all
heavy nuclei is confronted to a new critical analysis of photon strength
functions of nuclei in the valley of stability. For the photon strength in the
isovector giant dipole resonance (IVGDR) regime a parameterization of GDR
shapes by the sum of three Lorentzians (TLO) is extrapolated to energies below
and above the IVGDR. The impact of non-GDR modes adding to the low energy slope
of photon strength is discussed including recent data on photon scattering and
other radiative processes. These are shown to be concentrated in energy regions
where various model calculations predict intermediate collective strength; thus
they are obviously separate from the IVGDR tail. The triple Lorentzian (TLO)
ansatz for giant dipole resonances is normalized in accordance to the dipole
sum rule. The nuclear droplet model with surface dissipation accounts well for
positions and widths without local, nuclide specific, parameters. Very few and
only global parameters are needed when a breaking of axial symmetry already in
the valley of stability is admitted and hence a reliable prediction for
electric dipole strength functions also outside of it is expected.Comment: 21 pages, 21 figures, PACS: 26.50.+x, 25.20.Dc,27.60.+j Accepted by
Europ. Phys. Journal
The Causal Effect of Studying on Academic Performance
Despite the large amount of attention that has been paid recently to understanding the determinants of educational outcomes, knowledge of the causal effect of the most fundamental input in the education production function - students' study time and effort - has remained virtually non-existent. In this paper, we examine the causal effect of studying on grade performance using an Instrumental Variable estimator. Our approach takes advantage of a unique natural experiment and is possible because we have collected unique longitudinal data that provides detailed information about all aspects of this experiment. Important for understanding the potential impact of a wide array of education policies, the results suggest that human capital accumulation is far from predetermined at the time of college entrance.
The Effect of Credit Constraints on the College Drop-Out Decision A Direct Approach Using a New Panel Study
A serious difficulty in determining the importance of credit constraints in education arises because standard data sources do not provide a direct way of identifying which students are credit constrained. This has forced researchers to adopt a variety of indirect approaches. This paper differentiates itself from previous work by taking a direct approach for providing evidence about this issue which is made possible by unique longitudinal data that have been collected specifically for this type of purpose. Our results suggest that, while credit constraints likely play an important role in the drop-out decisions of some students, the large majority of attrition of students from low income families should be primarily attributed to reasons other than credit constraints.
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