The quotient field as a torsion-free covering module

R will denote a commutative integral domain with quotient fieldQ. A torsion-free cover of a moduleM is a torsion-free moduleF and anR-epimorphism σ:F→M such that given any torsion-free moduleG and λ∈Hom _R (G, M) there exists μ∈Hom _R (G,F) such that σμ=λ. It is known that ifM is a maximal ideal ofR, R→R/M is a torsion-free cover if and only ifR is a maximal valuation ring. LetE denote the injective hull ofR/M thenR→R/M extends to a homomorphismQ→E. We give necessary and sufficient conditions forQ→E to be a torsion-free cover.     

Suppression of instability in rotatory hydromagnetic convection

Recently discovered hydrodynamic instability [1], in a simple Bénard configuration in the parameter regime under the action of a nonadverse temperature gradient, is shown to be suppressed by the simultaneous action of a uniform rotation and a uniform magnetic field both acting parallel to gravity for oscillatory perturbations whenever (Qσ ₁/π² + J/π⁴) > 1 and the effective Rayleigh numberR(1 -T ₀α₂) is dominated by either 27π⁴(1 + l/σ₁/4 or 27π⁴/2 according as σ₁ ≥1 or σ₁ ≤ 1 respectively. HereT ₀is the temperature of the lower boundary while α₂ is the coefficient of specific heat at constant volume due to temperature variation and σ₁,R,Q andJ respectively denote the magnetic Prandtl number, the Rayleigh number, the Chandrasekhar number and the Taylor number.     

Lifting chains of prime ideals to paravaluation rings

Résumé  SoitR ⊂T une extension des anneaux commutatifs et soit {P _α :α ∈I} une cha?ne croissante des idéaux premiers deR (I étant un ensemble totalement ordonné, peut-être infini). Alors il existe un anneau de paravaluationV deT et une cha?ne {Q _α} des idéaux premiers deV de sorte queR ⊂V etQ _α ∩R =P _α pour toutα ∈I. Tout d’abord, on établit le cas spécial dans lequelT est un corps; dans ce cas, on trouve en effet un tel anneau de valuationV deT. Ensuite, l’assertion ci-dessus pour le cas général découle comme conséquence. Dans le cas général, on peut aussi remplacer le mot “paravaluation” avec le mot “valuation” siR est un anneau de Marot etT est son anneau total de fractions.      

On the finiteness of a field-theoretic invariant for commutative rings

If T is a (commutative unital) ring extension of a ring R, then Λ(T /R) is defined to be the supremum of the lengths of chains of intermediate fields between R _P /P R _P and T _Q /QT _Q , where Q varies over Spec(T) and P:= Q ∩ R. The invariant σ(R):= sup Λ(T/R), where T varies over all the overrings of R. It is proved that if Λ(S/R)< ∞ for all rings S between R and T, then (R, T) is an INC-pair; and that if (R, T) is an INC-pair such that T is a finite-type R-algebra, then Λ(T/R)< ∞. Consequently, if R is a domain with σ(R) < ∞, then the integral closure of R is a Prüfer domain; and if R is a Noetherian G-domain, then σ(R) < ∞, with examples showing that σ(R) can be any given non-negative integer. Other examples include that of a onedimensional Noetherian locally pseudo-valuation domain R with σ(R)=∞.     

Symplectic embeddings of polydisks

If P and P ^′ are symplectic polydisks of radii R ₁≤...≤R _n and R ₁ ^′≤...≤R _n ^′, respectively, then we prove that P symplectically embeds in P ^′ provided that C(n)R ₁≤R ₁ ^′ and C(n)R ₁...R _n ≤R ₁ ^′...R _n ^′. Up to a constant factor, these conditions are optimal.     

Going-down pairs of commutative rings

Résumé  D'après D. E. Dobbs, Houston J. Math. 23 (1997), 1–11, nous disons que l'anneau (commutatif)A est un anneau-“going-down” siA/P est un domaine-“going-down” pour chaque idéal premier deA. Etant donné une extension,R⊆T, nous disons que (R, T) est une paire d'anneaux-“going-down” (respectivement, une paire “going-down”) siS est un anneau-“going-down” pour chaque anneau tels queR⊆S⊆T (resp., si “going-down” est satisfait par chaque extension d'anneauxA⊆B tels queR⊆A⊆B⊆T). On montre que siR est un anneau de la dimension 0 (au sens de Krull), alors (R, T) est une paire d'anneaux-“going-down” si et seulement sitr.deg. _R/(P∩R) T/P≤1 pour chaque idéal premier minimalP deT. Des résultats partiels sont obtenus quandR n'est pas de dimension 0. En outre, si (R, T) est une paire d'anneaux-“going-down” tel queT ait un seul idéal premier minimal, alors (R, T) est une paire “going-down”. Des résultats dans l'esprit ci-dessus sont également obtenus pour quelques autres types de paires.

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This paper is taken from the author's doctoral dissertation of May 2000, written under the direction of Professor David E. Dobbs of the University of Tennessee, Knoxville.     

Topological entropy zero and asymptotic pairs

Let T be a continuous map on a compact metric space (X, d). A pair of distinct points x, y ∈ X is asymptotic if lim _n→∞ d(T ⁿ x, T ⁿ y) = 0. We prove the following four conditions to be equivalent: 1. h _top(T) = 0; 2. (X, T) has a (topological) extension (Y,S) which has no asymptotic pairs; 3. (X, T) has a topological extension (Y ′, S′) via a factor map that collapses all asymptotic pairs; 4. (X, T) has a symbolic extension (i.e., with (Y ′, S′) being a subshift) via a map that collapses asymptotic pairs. The maximal factors (of a given system (X, T)) corresponding to the above properties do not need to coincide.     

Extensions completes d'une theorie forcing complete

We give two examples of a universal theoryT such that the forcing companionT ^F (as defined by A. Robinson for infinite forcing) has some model which is not the elementary equivalent of a generic model ofT. Our examples answer in a negative way a question posed by E. Fisher and A. Robinson. In the first example, forT ^F, there is an extensionT′ which is complete and forcing comcomplete (that is, (T′) ^F=T′) not generated by a generic model ofT. In the second exampleT ^F has a complete extensionT′ which is not forcing complete.      

AnR danalogue of Valentine’s theorem on 3-convex sets

This paper deals with anR ^danalogue of a theorem of Valentine which states that a closed 3-convex setS in the plane is decomposable into 3 or fewer closed convex sets. In Valentine’s proof, the points of local nonconvexity ofS are treated as vertices of a polygonP contained in the kernel ofS, yielding a decomposition ofS into 2 or 3 convex sets, depending on whetherP has an even or odd number of edges. Thus the decomposition actually depends onc(P′), the chromatic number of the polytopeP′ dual toP. A natural analogue of this result is the following theorem: LetS be a closed subset ofR ^d, and letQ denote the set of points of local nonconvexity ofS. We require thatQ be contained in the kernel ofS and thatQ coincide with the set of points in the union of all the (d − 2)-dimensional faces of somed-dimensional polytopeP. ThenS is decomposable intoc(P′) closed convex sets.     

A non-Bernoulli skew product which is loosely Bernoulli

LetT: Y→Y be the Bernoulli two shift with independent generatorQ={Q ₀,Q ₁} and letS: X→X be a measure preserving bijection. If (S, X) is ergodic then the skew product onX×Y defined by {fx339-1} is aK-automorphism. IfŜ is also Bernoulli we sayS is pre-Bernoulli. J. Feldman showed that ifS is pre-Bernoulli thenS must be loosely Bernoulli. We construct an example to show the converse is false, i.e. anS that is loosely Bernoulli but not pre-Bernoulli.     

Preservation of the injectivity of the Gauss map for general projections

Let X be a smooth irreducible quasi-projective variety of dimension n in P ^N with N ≥ 2n + 2. Let γ be its Gauss map, let be the embedding obtained from the general projection in P ^N and let γ′ be its Gauss map. We say that the general projection preserves the injectivity of the Gauss map if γ(Q) ≠ γ(Q′) implies γ′(Q) ≠ γ′ (Q′). We prove that this property holds in the following cases: N≥ 3n + 1; N ≥ 3n with n ≥ 2; N ≥ 3n−1 with n ≥ 4 and X does not contain a linear (n−1)-space. In case N = 3n−1 and X does contain a linear (n−1)-space (such smooth varieties exist) then the general projection does not preserver the injectivity of the Gauss map. This shows that there does not exist a straightforward kind of Bertini theorem for properties related to the Gauss map. The author is affiliated with the University at Leuven as a research fellow. This paper belongs to the FWO-project G.0318.06.     

Lower bounds at infinity of solutions of partial differential equations in the exterior of a proper cone

An extension of a classical theorem of Rellich to the exterior of a closed proper convex cone is proved: Let Γ be a closed convex proper cone inR ⁿ and −Γ′ be the antipodes of the dual cone of Γ. Let be a partial differential operator with constant coefficients inR ⁿ, whereQ(ζ)≠0 onR ⁿ−iΓ′ andP _i is an irreducible polynomial with real coefficients. Assume that the closure of each connected component of the set {ζ∈R ⁿ−iΓ′;P _j(ζ)=0, gradP _j(ζ)≠0} contains some real point on which gradP _j≠0 and gradP _j∉Γ∪(−Γ). LetC be an open cone inR ⁿ−Γ containing both normal directions at some such point, and intersecting each normal plane of every manifold contained in {ξ∈R ⁿ;P(ξ)=0}. Ifu∈ℒ′∩L _loc ² (R ⁿ−Γ) and the support ofP(−i∂/∂x)u is contained in Γ, then the condition implies that the support ofu is contained in Γ.     

Some Comments on Projective Quadrics Subordinate to Pseudo-Hermitian Spaces

We study in some detail the structure of the projective quadric Q′ obtained by taking the quotient of the isotropic cone in a standard pseudo-hermitian space H _p,q with respect to the positive real numbers \mathbb R⁺{\mathbb R^{+}} and, further, by taking the quotient [(Q)\tilde] = Q￠/U(1){\tilde Q = Q^\prime /U(1)}. The case of signature (1, 1) serves as an illustration. Q̃ is studied as a compactification of \mathbb R ×H_p-1,q-1{\mathbb R \times H_{p-1,q-1}}     

On Fixed Edges and Edge-Reconstruction of Series-Parallel Networks

 An edge e of a graph G is said to be a fixed edge if G−e+e ^′≅G implies that e ^′=e, and a forced edge if G−e+e ^′ is an edge-reconstruction of G implies that e ^′=e. In this paper, we use the method of excludable configurations to investigate the fixed edges and the forced edges of series-parallel networks. It is proved that all series-parallel networks contain fixed edges except P ₃∨K ₁ and P ₄∨K ₁, and that all series-parallel networks are edge-reconstructible. Received: December 22, 1997 Final version received: July 21, 1999     

The ring as a torsion-free cover

LetR be an integral domain andI a non-zero ideal ofR. The canonical mapR→R/I is called atorsion-free cover ofR/I if everyR-homomorphism from a torsion-freeR-module intoR/I can be factored throughR. The main result of this paper is thatR→R/I is a torsion-free cover if and only ifR is complete in theR-topology andI is an ideal of injective dimension 1. In this caseI is contained in the Jacobson radical ofR. And if Λ is the endomorphism ring ofI, then Λ is a quasi-local domain. IfI is a flatR-module, thenQ→Q/Λ is a torsion-free cover, whereQ is the quotient field ofR. And thenQ/Λ is an indecomposable injectiveR (and Λ) module. Special results are obtained ifR is a Noetherian domain or a Prüfer domain.     

Symmetric designs with bruck subdesigns

IfP is a finite projective plane of ordern with a proper subplaneQ of orderm which is not a Baer subplane, then a theorem of Bruck [Trans. AMS 78(1955), 464–481] asserts thatn≧m ²+m. If the equalityn=m ²+m were to occur thenP would be of composite order andQ should be called a Bruck subplane. It can be shown that if a projective planeP contains a Bruck subplaneQ, then in factP contains a designQ′ which has the parameters of the lines in a three dimensional projective geometry of orderm. A well known scheme of Bruck suggests using such aQ′ to constructP. Bruck’s theorem readily extends to symmetric designs [Kantor, Trans. AMS 146 (1969), 1–28], hence the concept of a Bruck subdesign. This paper develops the analoque ofQ′ and shows (by example) that the analogous construction scheme can be used to find symmetric designs.     

Approximation du temps local des processus gaussiens stationnaires par régularisation des trajectoires

Résumé Soit X un processus gaussien stationnaire non dérivable. Nous étudions le nombre de passages en zéro du processus régularisé par convolution. Sous des hypothèses peu restrictives sur X, cette variable convenablement normalisée, converge au sens de L ² quand la taille du filtre tend vers zéro. Lorsque X admet un temps local continu, la limite obtenue est le temps local.

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Summary Let {X(t)} be a stationary non differentiable Gaussian process and let ϕ_ɛ(u=ɛ⁻¹ ϕ(u/ɛ) be an approximate identity. Setting X _ɛ(t)=X*ϕ_ɛ(t) and letting N _ɛ(T) be the number of zeros of X _ɛ in the interval [0, T] it is shown that under weak technical conditions there are constants C(ɛ) so that C(ɛ) N _ɛ(T) converges in L ² as ɛ→0. When X admits a continuous local time, the limit is the local time L(0, T) at zero of X(t).

     

Derived Equivalences of Triangular Matrix Rings Arising from Extensions of Tilting Modules

A triangular matrix ring Λ is defined by a triplet (R,S,M) where R and S are rings and _R M _S is an S-R-bimodule. In the main theorem of this paper we show that if T _S is a tilting S-module, then under certain homological conditions on the S-module M _S , one can extend T _S to a tilting complex over Λ inducing a derived equivalence between Λ and another triangular matrix ring specified by (S′, R, M′), where the ring S′ and the R-S′-bimodule M′ depend only on M and T _S , and S′ is derived equivalent to S. Note that no conditions on the ring R are needed. These conditions are satisfied when S is an Artin algebra of finite global dimension and M _S is finitely generated. In this case, (S′,R,M′) = (S, R, DM) where D is the duality on the category of finitely generated S-modules. They are also satisfied when S is arbitrary, M _S has a finite projective resolution and Ext _S ⁿ (M _S , S) = 0 for all n > 0. In this case, (S′,R,M′) = (S, R, Hom _S (M, S)).     

A note on the existence of singular integrals

Abstract. In this note the existence of a singular integral operator T acting on Lipo(R“) spacesis studied. Suppose