Canonical varieties with no canonical axiomatisation

We give a simple example of a variety of modal algebras that is canonical but cannot be axiomatised by canonical equations or first-order sentences. We then show that the variety of representable relation algebras, although canonical, has no canonical axiomatisation. Indeed, we show that every axiomatisation of these varieties involves infinitely many non-canonical sentences.

Using probabilistic methods of Erdos, we construct an infinite sequence of finite graphs with arbitrarily large chromatic number, such that each is a bounded morphic image of and has no odd cycles of length at most . The inverse limit of the sequence is a graph with no odd cycles, and hence is 2-colourable. It follows that a modal algebra (respectively, a relation algebra) obtained from the satisfies arbitrarily many axioms from a certain axiomatisation of , while its canonical extension satisfies only a bounded number of them. First-order compactness will now establish that has no canonical axiomatisation. A variant of this argument shows that all axiomatisations of these classes have infinitely many non-canonical sentences.

     

概率统计的多元化教学探讨

概率统计是一门具有独特思想方法的数学学科,我们在传统的理论教学基础上,以概率思想教育为主线,加强实践环节的教学和实际操作,注重提升学生在数学文化层面上的认识.从教学情况来看,收到了较好的教学效果.     

Discriminator or dual discriminator?

Varieties are considered with p(x, y, z), a single ternary operation, which acts as a local discriminator or dual discriminator on the subdirectly irreducible elements. If p(x, y, z) is "global", then all subvarieties are finitely based. In the general case a continuum of non-finitely based subvarieties are presented. A graph theoretical picture leads to a variety of groupoids connecting the left-zero and the right-zero semigroups. For this variety some open problems are presented. Received October 7, 1998; accepted in final form October 4, 1999.     

Composition operators on noncommutative Hardy spaces

In this paper we initiate the study of composition operators on the noncommutative Hardy space , which is the Hilbert space of all free holomorphic functions of the form

     

Vector bundles on flag varieties

We study vector bundles on flag varieties over an algebraically closed field k. In the first part, we suppose $G=G_k(d,n)$ $(2\le d\le n-d)$ to be the Grassmannian parameterizing linear subspaces of dimension d in $k^n$, where k is an algebraically closed field of characteristic $p>0$. Let E be a uniform vector bundle over G of rank $r\le d$. We show that E is either a direct sum of line bundles or a twist of the pullback of the universal subbundle $H_d$ or its dual $H_d^{\vee }$ by a series of absolute Frobenius maps. In the second part, splitting properties of vector bundles on general flag varieties $F(d_1,\text{…},d_s)$ in characteristic zero are considered. We prove a structure theorem for bundles over flag varieties which are uniform with respect to the ith component of the manifold of lines in $F(d_1,\text{…},d_s)$. Furthermore, we generalize the Grauert–M$\ddot{\mathrm{u}}$lich–Barth theorem to flag varieties. As a corollary, we show that any strongly uniform i-semistable $(1\le i\le n-1)$ bundle over the complete flag variety splits as a direct sum of special line bundles.     

On the equivalence of certain quasi-Hermitian varieties

By Aguglia et al., new quasi-Hermitian varieties ${\mathrm{ℳ}}_{\alpha ,\beta }$ in $\text{PG}\left(r,q^2\right)$ depending on a pair of parameters $\alpha ,\beta$ from the underlying field $\text{GF}\left(q^2\right)$ have been constructed. In the present paper we study the structure of the lines contained in ${\mathrm{ℳ}}_{\alpha ,\beta }$ and consequently determine the projective equivalence classes of such varieties for $q$ odd and $r=3$. As a byproduct, we also prove that the collinearity graph of ${\mathrm{ℳ}}_{\alpha ,\beta }$ is connected with diameter 3 for $q\equiv 1\left(\mathrm{mod}4\right)$.