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Search Results: 1 - 10 of 472384 matches for " Miguel A. Barja "
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Generalized Clifford-Severi Inequality and the Volume of Irregular Varieties
Miguel A. Barja
Mathematics , 2013, DOI: 10.1215/00127094-2871306
Abstract: We give a sharp lower bound for the selfintersection of a nef line bundle $L$ on an irregular variety $X$ in terms of its continuous global sections and the Albanese dimension of $X$, which we call the Generalized Clifford-Severi inequality. We also extend the result to nef vector bundles and give a slope inequality for fibred irregular varieties. As a byproduct we obtain a lower bound for the volume of irregular varieties; when $X$ is of maximal Albanese dimension the bound is $Vol(X) \geq 2 n! \chi\omega_X$ and it is sharp.
Numerical bounds of canonical varieties
Miguel A. Barja
Mathematics , 1999,
Abstract: We study lower bounds for the self-intersection of the canonical divisor of "canonical varieties" (i.e. varieties whose canonical linear system gives a birational map). We give some improvements for the known results in the case of surfaces and new bounds for the case of canonical threefolds.
Lower bounds of the slope of fibred threefolds
Miguel A. Barja
Mathematics , 1999,
Abstract: We study from a geographical point of view fibrations of threefolds over smooth curves, such that the general fibre is of general type. We prove the non-negativity of certain relative invariants under general hypotheses and give lower bounds for the self-interssection of the relative canonical divisor of the fibration, depending on other relative invariants. We also study the influence of the relative irregularity on these bounds. A more detailed study of the lowest cases of the bounds is given.
Stability conditions and positivity of invariants of fibrations
Miguel A. Barja,Lidia Stoppino
Mathematics , 2012,
Abstract: We study three methods that prove the positivity of a natural numerical invariant associated to $1-$parameter families of polarized varieties. All these methods involve different stability conditions. In dimension 2 we prove that there is a natural connection between them, related to a yet another stability condition, the linear stability. Finally we make some speculations and prove new results in higher dimension.
Slopes of trigonal fibred surfaces and of higher dimensional fibrations
Miguel A. Barja,Lidia Stoppino
Mathematics , 2008,
Abstract: We give lower bounds for the slope of higher dimensional fibrations over curves under conditions of GIT-semistability of the fibres, using a generalization of a method of Cornalba and Harris. With the same method we establish a sharp lower bound for the slope of trigonal fibrations of even genus and general Maroni invariant; in particular this result proves a conjecture due to Harris and Stankova-Frenkel.
A Note on a Conjecture of Xiao
Miguel A. Barja,Francesco Zucconi
Mathematics , 1999,
Abstract: Let f:S ->B be a relatively minimal fibred surface. In this note we give a partial affirmative answer to a conjecture of Xiao, proving that the direct image of the relative dualizing sheaf of $f$ is ample when the slope of the fibration is less than 4, if the general fibre of $f$ is non-hyperelliptic or the genus of the fibre or of the base curve is low.
On the Slope of Fibred Surfaces
Miguel A. Barja,Francesco Zucconi
Mathematics , 1999,
Abstract: Given a relatively minimal non locally trivial fibred surface f: S->B, the slope of the fibration is a numerical invariant associated to the fibration. In this paper we explore how properties of the general fibre of $f$ and global properties of S influence on the lower bound of the slope. First of all we obtain lower bounds of the slope when the general fibre is a double cover. We also obtain a lower bound depending as an increasing function on the relative irregularity of the fibration, extending previous results of Xiao. We construct several families of examples to check the assimptotical sharpness of our bounds.
Surfaces on the Severi line
Miguel ángel Barja,Rita Pardini,Lidia Stoppino
Mathematics , 2015,
Abstract: Let S be a minimal complex surface of general type and of maximal Albanese dimension; by the Severi inequality one has $K^2_S\geq 4\chi(\mathcal O_S)$. We prove that the equality $K^2_S=4\chi(\mathcal O_S)$ holds if and only if $q(S):= h^1(\mathcal O_S)=2$ and the canonical model of $S$ is a double cover of the Albanese surface branched on an ample divisor with at most negligible singularities.
Stability and singularities of relative hypersurfaces
M. A. Barja,L. Stoppino
Mathematics , 2014, DOI: 10.1093/imrn/rnv158
Abstract: We study relative hypersurfaces over curves, and prove an instability condition for the fibres. This gives an upper bound on the log canonical threshold of the relative hypersurface. We compare these results with the information that can be derived from Nakayama's Zariski decomposition of effective divisors on relative projective bundles.
Linear stability of projected canonical curves with applications to the slope of fibred surfaces
M. A. Barja,L. Stoppino
Mathematics , 2006,
Abstract: Let f :S\to B be a non locally trivial fibred surface. We prove a lower bound for the slope of f depending increasingly from the relative irregularity of f and the Clifford index of the general fibres.
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