arXiv subjects
Publications and source records attributed to Gabriel Picavet.
Alan and al. defined and studied quasi-maximal ideals. We add a comprehensive characterization of these ideals, introducing submaximal ideals. The conductor of a finite minimal extension $R\subset S$ is quasi-maximal in $S$. This allows us to give a new characterization of these extensions. We also examine the links between quasi-maximal ideals and Badawi 2-absorbing ideals.
We are working in the category of commutative unital rings and denote by $\mathrm U(R)$ the group of units of a nonzero ring $R$. An extension of rings $R\subseteq S$, satisfying $\mathrm U(R)=R \cap\mathrm U(S)$ is usually called local. This paper is devoted to the study of ring extensions such that $\mathrm U(R)=\mathrm U(S)$, that we call strongly local. P. M. Cohn in a paper, entitled Rings with zero divisors, introduced some strongly local extensions. We generalized under the name Cohn's rings his definition and give a comprehensive study of these extensions. As a consequence, we give a constructive proof of his main result. Now Lequain and Doering studied strongly local extensions, where $S$ is semilocal, so that $S/\mathrm J(S)$, where $\mathrm J(S)$ is the Jacobson radical of $S$, is Von Neumann regular. These rings are usually called $J$-regular. We establish many results on $J$-regular rings in order to get substantial results on strongly local extensions when $S$ is $J$-regular. The Picard group of a $J$-regular ring is trivial, allowing to evaluate the group $\mathrm U(S)/\mathrm U(R)$ when $R$ is $J$-regular. We then are able to give a complete characterization of the Doering-Lequain context. A Section is devoted to examples. In particular, when $R$ is a field, the strongly local and weakly strongly inert properties are equivalent.
When $E$ is an $R$-module over a commutative unital ring $R$, the Zariski closure of its support is of the form $\mathrm V(\mathcal O(E))$ where $\mathcal O(E)$ is a unique radical ideal. We give an explicit form of $\mathcal O(E)$ and study its behavior under various operations of algebra. Applications are given, in particular for ring extensions of commutative unital rings whose supports are closed. We provide some applications to crucial and critical ideals of ring extensions.
We are dealing with extensions of commutative rings $R\subseteq S$ whose chains of the poset $[R,S]$ of their subextensions are finite ({\em i.e.} $R\subseteq S$ has the FCP property) and such that $[R,S]$ is a distributive lattice, that we call distributive FCP extensions. Note that the lattice $[R,S]$ of a distributive FCP extension is finite. This paper is the continuation of our earlier papers where we studied catenarian and Boolean extensions. Actually, for an FCP extension, the following implications hold: Boolean $\Rightarrow$ distributive $\Rightarrow$ catenarian. A comprehensive characterization of distributive FCP extensions actually remains a challenge, essentially because the same problem for field extensions is not completely solved. Nevertheless, we are able to exhibit a lot of positive results for some classes of extensions. A main result is that an FCP extension $R\subseteq S$ is distributive if and only if $R\subseteq\overline R$ is distributive, where $\overline R$ is the integral closure of $R$ in $S$. A special attention is paid to distributive field extensions.
The paper intends to apply the properties of Prüfer extensions, investigated in the Knebusch-Zhang book, to ring extensions $R\subseteq S$. The integral closure $\overline R$ of $R$ in $S$ is shown to be the intersection of all $T\in [R,S]$, such that $T\subseteq S$ is Prüfer. We are then able to establish an avoidance lemma for integrally closed subextensions. Rings of sections of the affine scheme defined by $R$ provide results on $S$-regular ideals. Some results on pullbacks characterizations of Prüfer extensions are given. We introduce locally strong divisors, examining the properties of strong divisors of a local ring and their links with Prüfer extensions. The locally strong divisors allow us to give characterizations of QR-extensions. We apply our results to Nagata extensions of rings. We also look at the Prüfer hull of a Nagata extension. We define quasi-Prüferian rings that may differ from quasi-Prüfer integral domains. We then derive some results on minimal and FCP extensions. Finally, we study the set of all primitive elements in an extension.
The paper deals with ring extensions $R\subseteq S$ and the poset $[R,S]$ of their subextensions, with a special look at FCP extensions (extensions such that $[R,S]$ is Artinian and Noetherian). When the extension has FCP, we show that there exists a co-integral closure, that is a least element $\underline R$ in $[R,S]$ such that $\underline R \subseteq S$ is integral. Replacing the integral property by the integrally closed property, we are able to prove a similar result for an FCP extension. The radicial closure of $R$ in $S$ is well known. We are able to exhibit a suitable separable closure of $R$ in $S$ in case the extension has FCP, and then results are similar to those of field theory. The FCP property being always guaranteed, we discuss when an extension has a co-subintegral or a co-infra-integral closure. Our theory is made easier by using anodal extensions. These (co)-closures exist for example when the extension is catenarian, an interesting special case for the study of distributive extensions to appear in a forthcoming paper.
The paper deals with ring extensions $R\subseteq S$ and their lattices $[R,S]$ of subextensions and is mainly devoted to FCP extensions (extensions whose lattices are Artinian and Noetherian). The object of the paper is the introduction and the study of elements of the lattices that split in some sense ring extensions. The reason why is that this splitting was used in earlier paper without their common nature being recognized. There are some favorable cases allowing to build splitters, mainly when we are dealing with $\mathcal B$-extensions, for example integral extensions. Integral closures and Prufer hulls of extensions play a dual role. The paper gives many combinatorics results with the explicit computation of the Prufer hull of an FCP extension. We show that a split extension cannot be pinched, except trivially.
We consider ring extensions whose set of all subextensions is stable under the formation of sums, the so-called Delta extensions and exhibit new examples of these extensions.
If $R\subseteq S$ is a ring extension of commutative unital rings, the poset $[R,S]$ of $R$-subalgebras of $S$ is called catenarian if it verifies the Jordan-Hölder property. This property has already been studied by Dobbs and Shapiro for finite extensions of fields. We investigate this property for arbitrary ring extensions, showing that many type of extensions are catenarian. We reduce the characterization of catenarian extensions to the case of field extensions, an unsolved question at that time.
We examine the Loewy series associated to the lattice of subalgebras of an extension of commutative rings.
We characterize extensions of commutative rings $R \subseteq S$ whose sets of subextensions $[R,S]$ are finite ({\it i.e.} $R\subseteq S$ has the FIP property) and are Boolean lattices, that we call Boolean FIP extensions. Some characterizations involve ``factorial" properties of the poset $[R,S]$. A non trivial result is that each subextension of a Boolean FIP extension is simple (i.e. $R \subseteq S$ is a simple pair).
We characterize extensions of commutative rings $R\subset S$ such that $R\subset T$ is minimal for each $R$-subalgebra $T$ of $S$ with $T\neq R,S$. This property is equivalent to $R\subset S$ has length 2. Such extensions are either pointwise minimal or simple. We are able to compute the number of subextensions of $R\subset S$. Besides commutative algebra considerations, our main result is a consequence of the recently introduced by van Hoeij et al. concept of principal subfields of a finite separable field extension. As a corollary of this paper, we get that simple extensions of length 2 have FIP.
We characterize pointwise minimal extensions of rings introduced by P.-J. Cahen, D. E. Dobbs and T. G. Lucas.
We introduce quasi-Prufer extensions of rings in order to relativize the notion of quasi-Prufer domains and to take into account some contexts recently introduced in the literature. We also introduce almost-Prufer ring extensions. Characterizations and properties are given. In particular, we examine the relationship with the finite fibers property.
We study etale extensions of rings that have FIP.
We show that lengths of ring extensions are preserved under the formation of Nagata extensions as well as Dobbs-Mullins invariant. We exhibit a new condition for the FIP property be preserved under the formation of Nagata extensions by using arithmetic extensions of rings.
We characterize some types of FIP and FCP ring extensions $R \subset S$, where $S$ is not an integral domain and $R$ may not be an integral domain. In this paper S is mostly a product of rings related to R and also the idealization of an R-module.