8,907 research outputs found
Projective manifolds of sectional genus three as zero loci of sections of ample vector bundles
No full textLet E be an ample vector bundle of rank r \geq 2 on a smooth complex projective variety X of dimension n such that there exists a global section of E whose zero locus Z is a smooth subvariety of dimension n-r \geq 2 of X. Let H be an ample line bundle on X such that the restriction H_Z of H to Z is very ample. Triplets (X,E,H) with g(Z,H_Z)=3 are classified, where g(Z,H_Z) is the sectional genus of (Z,H_Z)
Ample vector bundles with zero loci having a bielliptic curve section of low degree
No full textLet E be an ample vector bundle of rank r \geq 2 on a smooth complex projective variety X of dimension n such that there exists a global section of E whose zero locus Z is a smooth subvariety of dimension n-r \geq 3 of X. Let H be an ample line bundle on X such that its restriction H_Z to Z is very ample. Triplets (X,E,H) are classified under the assumption that (Z,H_Z) has a smooth bielliptic curve section of genus \geq 3 with H^{n-r}c_r(E) \leq 8
Ample vector bundles and Bordiga surfaces
No full textLet X be a smooth complex projective variety and let Z be a smooth surface which is the zero locus of an ample vector bundle E of rank dim(X)-2 \geq 2 on X. Let H be an ample line bundle on X, whose restriction H_Z to Z is a very ample line bundle and assume that
(Z,H_Z) is a Bordiga surface, i.e., a rational surface having (P^2,O(4)) as its minimal adjunction theoretic reduction. Triplets (X,E,H) as above are discussed and classifie
Ample vector bundles with sections vanishing on submanifolds of sectional genus three
No full textLet E be an ample vector bundle of rank r geq 2 on a smooth complex projective variety X of dimension n such that there exists a global section of E whose zero locus Z is a smooth subvariety of dimension n-r geq 2 of X. Let H be an ample line bundle on X such that |H| defines an embedding of Z. The triplets (X,E,H) such that (Z,H_Z) has sectional genus g(Z,H_Z) = 3 are classified. This is the first step towards the classification of ample vector bundles of rank n-1 on X of curve genus three
Ample vector bundles with zero loci having a bielliptic curve section
No full textLet X be a smooth complex projective variety and let Z ⊂ X be a smooth submanifold of dimension ≥ 2, which is the zero locus of a section of an ample vector bundle E of rank dimX − dimZ ≥ 2 on X. Let H be an ample line bundle on X whose restriction HZ to Z is very ample. Triplets (X, E, H) as above are studied and classified under the assumption thatZ is a projective manifold of high degree with respect to HZ, admitting a curve section which is a double cover of an elliptic curve
Ample vector bundle characterizations of projective bundles and quadric fibrations over curves
No full textLet E be an ample vector bundle of rank r \geq 2 on a smooth complex projective n-fold X having a section whose zero locus, Z, is a smooth submanifold of the expected dimension n-r \geq 3. Pairs (X,E) as above are classified in the following cases: a) Z is a projective bundle over a smooth curve of positive genus, b) (Z,H_Z) is a scroll over a smooth curve B, and c) (Z,H_Z) is a quadric fibration over B, for some ample nine bundle H on X
Nitrate Contamination of Groundwater and Soil Management
No full textThe Japanese Government set the environmental quality standard for nitrate (NO3) in groundwater at 10 mg N L1 in 1998, based on a level considered acceptable for avoiding infant methemoglobinemia. In 1998, 6.3% of groundwater in Japan contained NO3 exceeding 10 mg L¡¦, with agriculture regarded to be a primary source of the NO3 (Environmental Agency, Japan, 1999). This paper aims to define the mechanisms of NO3 contamination of groundwater associated with soil management in arable land.
The author gives an overview of the relation between nitrogen (N) fertilization and groundwater contamination. First of all, the utilization efficiency of N fertilizers for outdoor cultivation of vegetables is usually 50% or less (Nishio, 2001; Vance, 2001). Although N fertilizer is essential for crop production, excessive N could leach out of arable soils and eventually cause NO3 contamination of groundwater. However, conversely, excessive N is necessary as insurance in some cases, such as when there is heavy rainfall immediately after fertilization. It should be also noted that some vegetables physiologically require a high content of N in soil even at harvest.
Nitrate leaching from different fertilizers was monitored for 7 years and the data were evaluated using an N and water balance equation (Maeda et al., 2003). Excessive N from chemical fertilizers caused substantial NO3 leaching, while compost application was promising to achieve high yields and low N leaching during a few years but led to the same level of NO3 leaching as that in the plots subjected to chemical fertilizer application over longer periods of time. Thus, it is of importance to predict the N mineralization rates both for manure and for soil under natural conditions. Experimental results of this kind can provide full information on N dynamics in fields for policy decisions or regulations to reduce NO3 leaching while maintaining crop yields. Likewise, we must consider other influencing factors such as soil types, climatic conditions, and cropping systems for this purpose
Double covers of Del Pezzo manifolds and bielliptic curve sections
No full textLet X be a smooth complex projective variety, let (W,H) be a Del Pezzo manifold, and let p:X \to W be a double covering. Then the very ampleness of p^*H is investigated. This procedure gives several new examples of polarized manifolds possessing bielliptic curves as their curve sections
Geometrically ruled surfaces as zero loci of ample vector bundles
No full textLet E be an ample vector bundle of rank n-2 on a complex projective manifold X of dimension n, having a section whose zero locus is a smooth surface Z. Pairs (X,E) as above are classified under the assumption that Z is a P^1-bundle over a smooth curve. We also prove that X has negative Kodaira dimension if, in a similar setting, Z is a birationally ruled surface
Adjoint bundles of ample and spanned vector bundles on algebraic surfaces
No full textLet E be an ample and spanned vector bundle of rank r \geq 2 on a smooth complex propjective surface X and set L = \det E. We show that K_X+L is spanned except for (X,E)=(P^2, O(1)^{\oplus 2}) and we describe all pairs (X,E) for which K_X+det E fais to be very ample
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