有限集上的复合函数

有限集上的复合函数徐学文（华中师大数学系４３００８７００）在中学数学中，人们常见的函数大多数是定义在无限集上．对于定义在有限集上的函数考虑较少．本文将探讨定义在有限集上的函数的性质，为数学竞赛的命题研究提供一定的参考．本文约定下述记号：ｆ１（Ｘ）＝ｆ...     

偏序集上的Mobius反演公式（续）

偏序集上的Ｍｏｂｉｕｓ反演公式（续）万哲先（中国科学院）５局部有限偏序集上的关联代数和推广的Ｍｏｂｉｕｓ反演公式局部有限偏序集Ｐ上的Ｍｏｂｉｕｓ函数可以看作是Ｐ上的关联代数中的元素，现在先给出关联代数的定义．定义７设Ｐ是局部有限偏序集，Ｒ是有单位元１...     

实函数连续点集的结构

在弄清实函数连续点集结构的基础上，证明了不存在定义在R^n上的实函数f，使得f在有理点集Q^n上连续，而在无理点集(Q^n)上间断．并且得到：设(X，d)为具有σ有限开集的距离空间，E为X中的Gδ型集，则存在定义在X上的实函数f，使它在E上连续，而在F上间断．     

平面有限点集中的最大面积六边形

若平面上的有限点集构成凸多边形的顶点集,则称此有限点集处于凸位置令P表示平面上处于凸位置的有限点集,研究了P的子集所确定的凸六边形的面积与CH(P)面积比值的最大值问题.     

问题183三角迷题

观点1空集既不是有限集也不是无限集．理由：全日制普通高级中学教科书必修（人民教育出版社2003年版）中,先后定义了有限集、无限集和空集,行文上是并列关系。由此可以认为空集既不是有限集也不是无限集．     

带Morita对偶的偏序集环

设Ｒ是有１的结合环，Ｉ是任意偏序集，ＲＩ是Ｒ上Ｉ的偏序集环．本文考虑了带对偶的偏序集环，得到：ＲＩ带Ｍｏｒｉｔａ对偶当且仅当Ｒ带Ｍｏｒｉｔａ对偶．推广了已有的在Ｒ是有限偏序集时的有关结果     

群的一个Domain结构

群可以通过在其上赋予循环群拓扑构造群的Domain模型，从而实现群的可计算性。群上的循环群偏序集及其对偶都是连续的偏序集，这个连续偏序集是代数Domain的充分必要条件是群的单位元是代数元。如果在pseudo-有限的群G及其循环群偏序集的对偶￡^op[G]上分别赋予循环群拓扑和Scott拓扑，那么恒同函数是连续的。     

6元素集合上T0拓扑总数的计算

文中证明了有限预序集与有限偏序集的一些性质,并基于有限集上的拓扑和其上预序的一一对应关系,利用这些性质通过对极小元和极大元个数进行分类讨论,以一种有别于计算机算法而又容易理解的计算方法得出6元素集合上的T₀拓扑总数为130023.     

命题逻辑系统Ln^＊中公式关于有限理论的∑r-真度理论

将模糊命题逻辑中的∑-a-重言式理论与计量逻辑学中的真度理论相结合，在模糊命题逻辑系统Ln^＊中引入了公式集相对于有限理论的∑r-模糊真度理论，讨论了其中的主要性质。并利用真度关系：τr（A）＋τr（A→B）≤1＋τr（B）在模糊命题逻辑系统Ln^＊中的公式集F（S）上引入相对于有限理论的 Г-伪距离概念，从而为在模糊命题逻辑系统Ln^＊中建立相对于有限理论的近似推理框架奠定了基础。     

积分凸性及其应用

该文在Banach空间中通过向量值函数的Bochner积分引进集合与泛函的积分凸性以及集合的积分端点等概念. 文章主要证明有限维凸集、开凸集和闭凸集均是积分凸集，下半连续凸泛函与开凸集上的上半连续凸泛函均是积分凸的, 非空紧集具有积分端点, 对紧凸集来说其积分端点集与端点集一致, 最后给出积分凸性在最优化理论方面的两个应用.     

Cyclic Covers over Strongly Liftable Schemes

A smooth scheme $X$ over a field $k$ of positive characteristic is said to be strongly liftable over $W_2(k)$, if $X$ and all prime divisors on $X$ can be lifted simultaneously over $W_2(k)$. In this paper, the author gives a criterion for those cyclic covers over strongly liftable schemes that are still strongly liftable. As a corollary, cyclic covers over projective spaces of dimension at least three are strongly liftable over $W_2(k)$.     

Remarks on Uniformly Finitely Precomplete Positive Equivalences

The paper contains some observations on e-complete, precomplete, and uniformly finitely precomplete r. e. equivalence relations. Among these are a construction of a uniformly finitely precomplete r. e. equivalence which is neither e- nor precomplete, an extension of Lachlan's theorem that all precomplete r. e. equivalences are isomorphic, and a characterization of sets of fixed points of endomorphisms of uniformly finitely precomplete r. e. equivalences. Mathematics Subject Classification: 03D45.     

On liftable and weakly liftable modules

Let T be a Noetherian ring and f a nonzerodivisor on T. We study concrete necessary and sufficient conditions for a module over R=T/(f) to be weakly liftable to T, in the sense of Auslander, Ding, and Solberg. We focus on cyclic modules and obtain various positive and negative results on the lifting and weak lifting problems. For a module over T we define the loci for certain properties: liftable, weakly liftable, having finite projective dimension and study their relationships.     

On Finite Linear Systems Containing Strict Inequalities

This paper deals with linear systems containing finitely many weak and/or strict inequalities, whose solution sets are referred to as evenly convex polyhedral sets. The classical Motzkin theorem states that every (closed and convex) polyhedron is the Minkowski sum of a convex hull of finitely many points and a finitely generated cone. In this sense, similar representations for evenly convex polyhedra have been recently given by using the standard version for classical polyhedra. In this work, we provide a new dual tool that completely characterizes finite linear systems containing strict inequalities and it constitutes the key for obtaining a generalization of Motzkin theorem for evenly convex polyhedra.     

Uniformly Perfect Analytic and Conformal Attractor Sets

Conditions are given which imply that analytic iterated functionsystems (IFSs) in the complex plane C have uniformly perfectattractor sets. In particular, it is shown that the attractorset of a finitely generated conformal IFS is uniformly perfectwhen it contains two or more points. Also, an example is givenof a finitely generated analytic attractor set which is notuniformly perfect.     

Characterization of the closedness of the sum of two shift-invariant spaces

We first present a formula for the supremum cosine angle between two closed subspaces of a separable Hilbert space under the assumption that the ‘generators’ form frames for the subspaces. We then characterize the conditions that the sum of two, not necessarily finitely generated, shift-invariant subspaces of L²(Rd) be closed. If the fibers of the generating sets of the shift-invariant subspaces form frames for the fiber spaces a.e., which is satisfied if the shift-invariant subspaces are finitely generated or if the shifts of the generating sets form frames for the respective subspaces, then the characterization is given in terms of the norms of possibly infinite matrices. In particular, if the shift-invariant subspaces are finitely generated, then the characterization is given wholly in terms of the norms of finite matrices.     

Varieties whose finitely generated members are free

We prove that a variety of algebras whose finitely generated members are free must be definitionally equivalent to the variety of sets, the variety of pointed sets, a variety of vector spaces over a division ring, or a variety of affine vector spaces over a division ring.     

Weakly left localizable rings

A new class of rings, the class of weakly left localizable rings, is introduced. A ring R is called weakly left localizable if each non-nilpotent element of R is invertible in some left localization S^?1R of the ring R. Explicit criteria are given for a ring to be a weakly left localizable ring provided the ring has only finitely many maximal left denominator sets (eg, this is the case for all left Noetherian rings). It is proved that a ring with finitely many maximal left denominator sets that satisfies some natural conditions is a weakly left localizable ring iff its left quotient ring is a direct product of finitely many local rings such that their radicals are nil ideals.     

On the Euler characteristic of finite unions of convex sets

The Euler characteristic plays an important role in many subjects of discrete and continuous mathematics. For noncompact spaces, its homological definition, being a homotopy invariant, seems not as important as its role for compact spaces. However, its combinatorial definition, as a finitely additive measure, proves to be more applicable in the study of singular spaces such as semialgebraic sets, finitely subanalytic sets, etc. We introduce an interesting integral by means of which the combinatorial Euler characteristic can be defined without the necessity of decomposition and extension as in the traditional treatment for polyhedra and finite unions of compact convex sets. Since finite unions of closed convex sets cannot be obtained by cutting convex sets as in the polyhedral case, a separate treatment of the Euler characteristic for functions generated by indicator functions of closed convex sets and relatively open convex sets is necessary, and this forms the content of this paper.