Three countries and areas take a leading role in the transaction.The top three are the European Union(EU),the Middle East and the USA,the transaction amounts were respectively US$2.32 billion,US$931 billion and US$927...Three countries and areas take a leading role in the transaction.The top three are the European Union(EU),the Middle East and the USA,the transaction amounts were respectively US$2.32 billion,US$931 billion and US$927 million.展开更多
The investigation of U-ample ω-semigroups is initiated. After obtaining some properties of such semigroups, a structure of U-ample ω-semigroups is established. It is proved that a semigroup is a U-ample ω-semigroup...The investigation of U-ample ω-semigroups is initiated. After obtaining some properties of such semigroups, a structure of U-ample ω-semigroups is established. It is proved that a semigroup is a U-ample ω-semigroup if and only if it can be expressed by WBR（T, 0）, namely, the weakly Bruck-Reilly extensions of a monoid T. This result not only extends and amplifies the structure theorem of bisimple inverse ω-semigroups given by N. R. Reilly, but also generalizes the structure theorem of ,-bisimple type A ω-semigroups given by U. Asibong-Ibe in 1985.展开更多
Let X be a smooth projective variety of dimension n,and let E be an ample vector bundle over X.We show that any Schur class of E,lying in the cohomology group of bidegree(n-1,n-1),has a representative which is strictl...Let X be a smooth projective variety of dimension n,and let E be an ample vector bundle over X.We show that any Schur class of E,lying in the cohomology group of bidegree(n-1,n-1),has a representative which is strictly positive in the sense of smooth forms.This conforms the prediction of Griffiths conjecture on the positive polynomials of Chern classes/forms of an ample vector bundle on the form level,and thus strengthens the celebrated positivity results of Fulton and Lazarsfeld(1983)for certain degrees.展开更多
This note gives two examples of surfaces with normal crossing singularities.In the first example the canonical ring is not finitely generated.In the second,the canonical line bundle is not ample but its pull back to t...This note gives two examples of surfaces with normal crossing singularities.In the first example the canonical ring is not finitely generated.In the second,the canonical line bundle is not ample but its pull back to the normalization is ample.The latter answers in the negative a problem left unresolved in Ⅲ.2.6.2 of lments de gometrie algbrique,1961,and raised again by Viehweg.展开更多
Let k be an algebraically closed field, and V be a vector space of dimension n over k. For a set w = （ d →（1）,..., d→ （m）） of sequences of positive integers, denote by Lω the ample line bundle corresponding t...Let k be an algebraically closed field, and V be a vector space of dimension n over k. For a set w = （ d →（1）,..., d→ （m）） of sequences of positive integers, denote by Lω the ample line bundle corresponding to the polarization on the product X = Пi=1 m Flag（V, →n（i）） of flag varieties of type n→（i） determined by ω. We study the SL（V）-linearization of the diagonal action of SL（V） on X with respect to Lω. We give a sufficient and necessary condition on ω such that X ss（Lω） ≠ 0 （resp., Xs（Lω） ≠ 0）. As a consequence, we characterize the SL（V）-ample cone （for the diagonal action of SL（V） on X）,which turns out to be a polyhedral convex cone.展开更多
文摘Three countries and areas take a leading role in the transaction.The top three are the European Union(EU),the Middle East and the USA,the transaction amounts were respectively US$2.32 billion,US$931 billion and US$927 million.
文摘The investigation of U-ample ω-semigroups is initiated. After obtaining some properties of such semigroups, a structure of U-ample ω-semigroups is established. It is proved that a semigroup is a U-ample ω-semigroup if and only if it can be expressed by WBR（T, 0）, namely, the weakly Bruck-Reilly extensions of a monoid T. This result not only extends and amplifies the structure theorem of bisimple inverse ω-semigroups given by N. R. Reilly, but also generalizes the structure theorem of ,-bisimple type A ω-semigroups given by U. Asibong-Ibe in 1985.
基金supported by Tsinghua University Initiative Scientific Research Program(Grant No.2019Z07L02016)National Natural Science Foundation of China(Grant No.11901336)。
文摘Let X be a smooth projective variety of dimension n,and let E be an ample vector bundle over X.We show that any Schur class of E,lying in the cohomology group of bidegree(n-1,n-1),has a representative which is strictly positive in the sense of smooth forms.This conforms the prediction of Griffiths conjecture on the positive polynomials of Chern classes/forms of an ample vector bundle on the form level,and thus strengthens the celebrated positivity results of Fulton and Lazarsfeld(1983)for certain degrees.
基金provided by the NSF under grant number DMS-0500198
文摘This note gives two examples of surfaces with normal crossing singularities.In the first example the canonical ring is not finitely generated.In the second,the canonical line bundle is not ample but its pull back to the normalization is ample.The latter answers in the negative a problem left unresolved in Ⅲ.2.6.2 of lments de gometrie algbrique,1961,and raised again by Viehweg.
文摘Let k be an algebraically closed field, and V be a vector space of dimension n over k. For a set w = （ d →（1）,..., d→ （m）） of sequences of positive integers, denote by Lω the ample line bundle corresponding to the polarization on the product X = Пi=1 m Flag（V, →n（i）） of flag varieties of type n→（i） determined by ω. We study the SL（V）-linearization of the diagonal action of SL（V） on X with respect to Lω. We give a sufficient and necessary condition on ω such that X ss（Lω） ≠ 0 （resp., Xs（Lω） ≠ 0）. As a consequence, we characterize the SL（V）-ample cone （for the diagonal action of SL（V） on X）,which turns out to be a polyhedral convex cone.
