Topology of the Semigroup of Singular Endomorphisms

The topological interpretations of some of the algebraic properties of the semigroup Sn of singular endomorphisms of an n-dimensional vector space over K are discussed here. Since Sn is known to be an idempotent generated regular semigroup, we pay more attention to the topological properties of the set En of idempotents in Sn. The local structure of En is shown to be that of a C^infinity-manifold and of a finite-dimensional vector bundle over the Grassmann manifolds. The topology of the biorder relations and sandwich sets are also discussed.     

Finitely Presented, Coherent, and Ultrasimplicial Ordered Abelian Groups

We study notions such as finite presentability and coherence, for partially ordered abelian groups and vector spaces. Typical results are the following: (i) A partially ordered abelian group G is finitely presented if and only if G is finitely generated as a group, G⁺ is well-founded as a partially ordered set, and the set of minimal elements of G⁺\ {0} is finite. (ii) Torison-free, finitely presented partially ordered abelian groups can be represented as subgroups of some Zⁿ, with a finitely generated submonoid of (Z⁺)ⁿ as positive cone. (iii) Every unperforated, finitely presented partially ordered abelian group is Archimedean. Further, we establish connections with interpolation. In particular, we prove that a divisible dimension group G is a directed union of simplicial subgroups if and only if every finite subset of G is contained into a finitely presented ordered subgroup.     

An Edge-Isoperimetric Problem for Powers of the Petersen Graph

In this paper we introduce a new order on the set of n-dimensional tuples and prove that this order preserves nestedness in the edge isoperimetric problem for the graph Pⁿ, defined as the nth cartesian power of the well-known Petersen graph. The cutwidth and wirelength of Pⁿ are also derived. These results are then generalized for the cartesian product of Pⁿ and the m-dimensional binary hypercube.     

L p Norms of Eigenfunctions in the Completely Integrable Case

The eigenfunctions e^iál,x? e^{i\langle\lambda,x\rangle} of the Laplacian on a flat torus have uniformly bounded L^p norms. In this article, we prove that for every other quantum integrable Laplacian, the L^p norms of the joint eigenfunctions blow up at least at the rate || j_k || L^p 3 C(e)l_k^{[(p-2)/(4p)]-e} \| \varphi_k \| L^{p} \geq C(\epsilon)\lambda_{k}^{{p-2\over4p}-\epsilon} when p > 2. This gives a quantitative refinement of our recent result [TZ1] that some sequence of eigenfunctions must blow up in L^p unless (M,g) is flat. The better result in this paper is based on mass estimates of eigenfunctions near singular leaves of the Liouville foliation.     

The Monoid of the Random Graph

We investigate properties of the endomorphism monoid of the countable random graph R. We show that End(R) is not regular and is not generated by its idempotents. The Rees order on the idempotents of End(R) has 2^N0 many minimal elements. We also prove that the order type of Q is embeddable in the Rees order of End(R).     

Gorenstein injective dimension and Tor-depth of modules

The main aim of this paper is to obtain a dual result to the now well known Auslander-Bridger formula for G-dimension. We will show that if R is a complete Cohen-Macaulay ring with residue field k, and M is a non-injective h-divisible Ext-finite R-module of finite Gorenstein injective dimension such that for each i 3 1i \geq 1 Extⁱ (E,M) = 0 for all indecomposable injective R-modules E 1 E(k)E \neq E(k), then the depth of the ring is equal to the sum of the Gorenstein injective dimension and Tor-depth of M. As a consequence, we get that this formula holds over a d-dimensional Gorenstein local ring for every nonzero cosyzygy of a finitely generated R-module and thus in particular each such n^th cosyzygy has its Tor-depth equal to the depth of the ring whenever n 3 dn \geq d.     

On the Resolution of the Heat Equation with Discontinuous Coefficients

We give in this work some results about the existence and uniqueness with optimal regularity for solutions of a parabolic equation in nondivergence form in L^q(0,T;L^p(Omega)) where 1 < p,q < infinity in two cases. We use Lamberton's results (cf. [9]) in the first case and Dore-Venni's results (cf. [6]) in the second case.     

One Non-Relativistic Particle Coupled to a Photon Field

We investigate the ground state energy of an electron coupled to a photon field. First, we regard the self-energy of a “free” electron, which we describe by the Pauli-Fierz Hamiltonian. We show that, in the case of small values of the coupling constant a \alpha , the leading order term is represented by 2ap^-1(L - ln[1 + L]) 2\alpha\pi^{-1}(\Lambda - \textrm{ln}[1 + \Lambda]) .¶ Next we put the electron in the field of an arbitrary external potential V , such that the corresponding Schrödinger operator p² + V has at least one eigenvalue, and show that by coupling to the radiation field the binding energy increases, at least for small enough values of the coupling constant a \alpha . Moreover, we provide concrete numbers for a \alpha , the ultraviolet cut-off, and the radiative correction for which our procedure works.     

Multiple positive solutions for a nonlinear Schr?dinger equation

We are interested in positive entire solutions of the nonlinear Schrödinger equation -Du+(la(x)+1)u = u^p-\Delta u+(\lambda a(x)+1)u = u^p where a ≤ 0 has a potential well and p > 1 is subcritical. Using variational methods we prove the existence of multiple positive solutions which localize near the potential well int(a^-1(0)) for l\lambda large.     

On the Eigenfunction Expansion of a Piecewise Smooth Function

For eigenfunction expansions of piecewise smooth functionsassociated with an elliptic operator of order m in R² theconvergence is proved.     

Precise Asymptotic Formulas for Semilinear Eigenvalue Problems

. We consider the nonlinear Sturm-Liouville problem¶¶-u"(t) = | u(t) | ^p-1u(t) - lu(t), t ? I :=(0,1), u(0) = u(1) = 0 -u'(t) = \mid u(t)\mid^{p-1}u(t) - \lambda u(t), t \in I :=(0,1), u(0) = u(1) = 0 ,¶¶ where p > 1 and l ? R \lambda \in {\bf R} is an eigenvalue parameter. To investigate the global L²-bifurcation phenomena, we establish asymptotic formulas for the n-th bifurcation branch l = l_n (a) \lambda = \lambda_n (\alpha) with precise remainder term, where a \alpha is the L² norm of the eigenfunction associated with l \lambda .     

POWERS OF OPERATORS DOMINATED BY STRICTLY SINGULAR OPERATORS

It is proved that every positive operator R on a Banach latticeE dominated by a strictly singular operator T:E E satisfiesthat the R⁴ is strictly singular. Moreover, if E is order continuousthen the R² is already strictly singular.     

Minimal and Maximal Semigroups Related to Causal Symmetric Spaces

Let G/H be an irreducible globally hyperbolic semisimple symmetric space, and let S ³ G be a subsemigroup containing H not isolated in S. We show that if S^o p 0 then there are H-invariant minimal and maximal cones Cmin ³ Cmax in the tangent space at the origin such that H exp Cmin ³ S ³ HZK(a)expCmax. A double coset decomposition of the group G in terms of Cartan subspaces and the group H is proved. We also discuss the case where G/H is of Cayley type.     

A Relation between Hausdorff Dimension and a Condition on Time Sets for the Image by the Brownian Sheet to Possess Interior Points

We prove that if W_{N, d} is a Brownian sheet mapping to R^d and E is a set in (0, )^N of Hausdorff dimensiongreater than , then for almost every rotation about a point x and translation _x such that _x(E) (0, )^N, the set _x(E) is such that almost surely W(E) containsinterior points. The techniques are adapted from Kahane andRosen and generalize to higher dimensional time and range.     

On the Error Term for the Fourth Moment of the Riemann Zeta-Function

Let E₂(T) denote the error term in the asymptotic formula for^T₀|(+it)|⁴dt. It is proved that there exist constants A>0,B>1 such that for TT₀>0 every interval [T, BT] containspoints T₁, T₂ for which and that ^T₀|E₂(t)|^adt>>T^1+(a/2) for any fixed a1. Theseresults complement earlier results of Motohashi and Ivi that^T₀E₂(t)dt<<T^3/2 and that ^T₀E²₂(t)dt<<T²log^CT forsome C>0. Omega-results analogous to the above ones are obtainedalso for the error term in the asymptotic formula for the Laplacetransform of |(+it)|⁴.     

The Canonical Decomposition of the Poset of a Hammock

In the Auslander-Reiten quiver of a representation-directedalgebra several hammocks occur naturally; they begin at theprojective cover of a simple module E and end in the correspondinginjective hull. It is known that hammocks are Auslander-Reitenquivers of posets, so there is a poset corresponding to eachsimple module; it describes the set of modules having E as acomposition factor. In this paper we show that this poset Sdecomposes canonically into a coideal S⁺ and an ideal S^–which can easily be described by vectorspace-categories correspondingto a one-point extension or a one-point coextension, respectively.In addition, we describe the simple modules for which S⁺ andS^– are not comparable, and also those for which S⁺ S^–. We also show how to use the results in order to prove for certainposets that they do not occur as posets corresponding to simplemodules.     

Beck, H and Leech Coextensions

J. Leech studied certain coextensions ES , of a monoid S and Ch. Wells studied the relationship between Leech coextensions and those studied in a general categorical setting by J. Beck. In this paper it is completed some of the Wells results and it is given a classification of the Leech coextensions. Specifically, Wells shows that a Leech coextension is a Beck coextension if its corresponding functor is "centralizing" (a generalization of abelian-valued). Our Theorem 1.1 provides a technical condition under which Beck coextensions are actually abelian. This technical condition implies other properties as well. Corollary 1.8 provides three natural conditions under any of which the conditions of 1.1 are satisfied. Also it is provided an example of a non-abelian Beck coextension. Table 1 shows the general classification and Table 2 shows the reduced set of possibilities when the semigroup is a regular monoid. Stronger results are provided for coextensions with S an inverse monoid, and for extensions of groups by monoids.     

On products of quadratic twists and ranks of elliptic curves over large fields

In this paper, we give examples of elliptic curves E/K overa number field K satisfying the property that there exist P₁,P₂ K[t] such that the twists and are of positiverank over K(t). As a consequence of this result on twists, weshow that for those elliptic curves E/K, and for each , the rank of E over the fixed field (K^ab) under is infinite, where K^ab is the maximal abelian extension ofK.     

Properties of Removable Singularities for Hardy Spaces of Analytic Functions

Removable singularities for Hardy spaces H^p() = {f Hol(): |f|^p u in for some harmonic u}, 0 < p < are studied. A setE = is a weakly removable singularity for H^p(\E) if H^p(\E) Hol(), and a strongly removable singularity for H^p(\E) if H^p(\E)= H^p(). The two types of singularities coincide for compactE, and weak removability is independent of the domain . The paper looks at differences between weak and strong removability,the domain dependence of strong removability, and when removabilityis preserved under unions. In particular, a domain and a setE that is weakly removable for all H^p, but not strongly removablefor any H^p(\E), 0 < p < , are found. It is easy to show that if E is weakly removable for H^p(\E)and q > p, then E is also weakly removable for H^q(\E). Itis shown that the corresponding implication for strong removabilityholds if and only if q/p is an integer. Finally, the theory of Hardy space capacities is extended, anda comparison is made with the similar situation for weightedBergman spaces.     

Basis Problem for Turbulent Actions II: c0-Equalities

Let (Xⁿ,dⁿ) be a sequence of finite metric spaces of uniformlybounded diameter. An equivalence relation D on the product defined by if and only if is a c₀-equality.A systematic study is made of c₀-equalities and Borel reductionsbetween them. Necessary and sufficient conditions, expressedin terms of combinatorial properties of metrics d_n, are obtainedfor a c₀-equality to be effectively reducible to the isomorphismrelation of countable structures. It is proved that every Borelequivalence relation E reducible to a c₀-equality D either reducesa c₀-equality D' additively reducible to D, or is Borel-reducibleto the equality relation on countable sets of reals. An appropriatelydefined sequence of metrics provides a c₀-equality which isa turbulent orbit equivalence relation with no minimum turbulentequivalence relation reducible to it. This answers a questionof Hjorth. It is also shown that, whenever E is an F-equivalencerelation and D is a c₀-equality, every Borel equivalence relationreducible to both D and to E has to be essentially countable.2000 Mathematics Subject Classification: 03E15.