关于Extremal Ray的除子型收缩

设X是定义在复数域上的n维光滑射影簇且它的典范除子Kx不是数字有效的,A是X上的一个ample除子。本文详细研究了由Kx (n-k)A确定的关于X的除子型收缩映射（1≤k≤n-1),对其例外休的结构作了比较完整的分类,特别地,当1≤k≤3时,即得到Fujita,Sommese等人的相关结果。     

具有奇素典范指标的一般型三维簇的典范稳定性

主要研究具有奇指标的一般型光滑射影三维簇的典范映射,称φm是典范稳定的,如果对于所有的m≥m(r),φm是双有理的.证明了m(3)=13,m(5)=15,m(7)=18以及m(r)=2r对于所有的r≥11.     

关于高维簇的小收缩映射

设Ｘ是非奇异的ｎ维射影簇，Ａ是Ｘ上的一个Ａｍｐｌｅ除子．本文研究了以Ｋｘ＋（ｎ—ｋ）Ａ为支撑除子的小收缩映射的例外集的结构．     

关于双有理态射分解的一点注记

本文是关于5维簇的双有理态射的一点注记,在非常强的条件下,证明了这种双有理态射可以通过一个blowing-up分解.     

关于高维簇的小收缩映射

设X是非奇异的n维射影簇,A是X上的一个Ample除子.本文研究了以K     

三维光滑复射影簇上全纯曲线的退化性

讨论三维光滑复射影簇X上全纯曲线f:C→X的退化性.设D_1…,D_r为X上处于一般位置的相异的不可约有效除子.假定D_1…,D_r均为nef除子,而且存在正整数n_1…,n_r,c,使得n_in_jn_k(D_i.D_j.D_k)=c对所有i,j,k均成立.如果f的像取不到D_1…,D_r上的点,那么只要r≥11,f必定代数退化,即它的像包含于X的某个代数真子集中.     

双诱导映射(L)集合范畴的幂对象及子对象分类子探讨

引入双诱导映射(L)集合范畴的幂对象及子对象分类子的概念,并分别讨论它们的判别定理,同时给出它们的具体结构.     

超Heisenberg-Virasoro代数的超双导子及其应用

令L为一个超Heisenberg-Virasoro代数,具有一组C-基{L_n,I_n,G_n|n∈Z},满足如下关系式[L_m,L_n]=(m-n)L_m+n,[L_m,I_n]=-nI_m+n,[L_m,G_n]=-nG_m+n和[G_m,G_n]=I_m+n.本文证明了L的所有超反对称超双导子都是内导子.进一步,我们还证明了L上的每个线性超交换映射都具有这样的形式:Ψ(x)=f(x)I₀对于所有x∈L都成立,其中f(x)是从L到C的线性映射.     

一类有理Bezier曲线的类de Casteljau算法及de Casteljau-型子分割

Bezier曲线的一个良好性质是 de Casteljau算法不仅可以用于升阶 ,而且可以用于子分割 .本文主要研究基于有理调配函数的一类有理 Bezier曲线的类 de Casteljau算法及 de Casteljau-型子分割方法 .第一部分从一类有理 Bezier曲线的递推关系出发 ,讨论这一类有理 Bezier曲线的类 de Casteljau算法 .第二部分给出了这一类有理 Bezier曲线的 de Casteljau-型子分割方法 .     

套代数上的σ-双导子和σ-可交换映射

本文证明了当dim 0_+≠1或dim H_-~⊥≠1时,套代数γ(N)上的每一个σ-双导子都是σ-内双导子.作为应用,给出了满足条件f(X)X=σ(X)f(X)的线性映射f的形式.     

On the extremal sets of extremal quasiconformal mappings

The properties of the extremal sets of extremal quasiconformal mappings are discussed. It is proved that if an extremal Beltrami coefficient μ(z) is not uniquely extremal, then there exists an extremal Beltrami coefficient v(z) in its equivalent class and a compact subset E Δ with positive measure such that the essential upper bound of v(z) on E is less than the norm of [μ].     

Disjoint unions of complete minors

Given integers r and s, and n large compared to r and s, we determine the maximum size of a graph of order n having no minor isomorphic to sK_r, the union of s disjoint copies of K_r.The extremal function depends on the relative sizes of r and s. If s is small compared to r the extremal function is essentially independent of s. On the other hand, if s is large compared to r, there is a unique extremal graph ; this assertion is a generalization of the case r=3 which is a classical result of Erd?s and Pósa.     

On characterizations of unique extremality

In this paper, we make some further considerations of the characteristic conditions of (infinitesimally) unique extremality for Beltrami coefficients obtained by Bozin et al., and find some sufficient conditions simpler in form for a Beltrami coefficient μ with nonconstant absolute value to be (infinitesimally) uniquely extremal.     

万有Teichmuller空间中测地线的不唯一性

本文主要讨论万有Ｔｅｉｃｈｍｔｉｌｌｅｒ空间中的测地线问题，并给出了一些测地线不唯一的判断准则．特别地，证明了如下定理：设ｆ是单位圆△到自身上的极值拟共形映射，μ是它的复特征，如果存在常数ｋ．＜｜｜μ｜｜＿∞，使集合｛ｚ∈△：｜μ（Ｚ）｜≤ｋ｝有内点，则在万有Ｔｅｉｃｈｍｕｌｌｅｒ空间中，原点［０］与［μ］之间存在无穷多条测地线．     

New Extremal Type I Codes of Lengths 40, 42, and 44

It is known that it is possible to construct a generator matrix for a self-dual code of length 2n+2 from a generator matrix of a self-dual code of length 2n. With the aid of a computer, we construct new extremal Type I codes of lengths 40, 42, and 44 from extremal self-dual codes of lengths 38, 40, and 42 respectively. Among them are seven extremal Type I codes of length 44 whose weight enumerator is 1+224y ⁸+872y ¹⁰+·. A Type I code of length 44 with this weight enumerator was not known to exist previously.     

Four‐cycles in graphs without a given even cycle

We prove that every bipartite C_2l‐free graph G contains a C₄‐free subgraph H with e(H) ≥ e(G)/(l – 1). The factor 1/(l – 1) is best possible. This implies that ex(n, C_2l) ≤ 2(l – 1)ex(n, {C₄, C_2l}), which settles a special case of a conjecture of Erd?s and Simonovits. © 2004 Wiley Periodicals, Inc. J Graph Theory 48: 147–156, 2005     

Hypersurfaces which are equiaffine extremal and centroaffine extremal

For a centroaffine hypersurface, we use the relations between equiaffine and centroaffine invariants to find many newexamples of equiaffine extremal hypersurfaces. *Supported by NSFC No. 10371013; Joint Research of NSFC and KOSEF; NEU. **Supported by ABRL No. R14-2002-003-01002-0 from KRF; No. F01-2005-000-100024-0 from KOSEF.     

A weighted generalization of Turán's theorem

We obtain a generalization of Turáns theorem for graphs whose edges are assigned integer weights. We also characterize the extremal graphs in certain cases. © 1997 John Wiley & Sons, Inc. J Graph Theory 25: 267–275, 1997     

Some results on extremal vectors and invariant subspaces

In 1996 P. Enflo introduced the concept of extremal vectors and their connection to the Invariant Subspace Problem. The study of backward minimal vectors gives a new method of finding invariant subspaces which is more constructive than the previously known methods. In this article we study the properties and behaviour of extremal vectors, give some new formulas related to backward minimal vectors and improve results from papers by Ansari and Enflo (1998) and Enflo (1998).