arXiv subjects

George M. Bergman

Publications and source records attributed to George M. Bergman.

At least 37 records · Page 2Linked to original sources

arXiv2017

Strong inner inverses in endomorphism rings of vector spaces

For $V$ a vector space over a field, or more generally, over a division ring, it is well-known that every $x\in\mathrm{End}(V)$ has an inner inverse , i.e., an element $y\in\mathrm{End}(V)$ satisfying $xyx=x.$ We show here that a large class of such $x$ have inner inverses $y$ that satisfy with $x$ an infinite family of additional monoid relations, making the monoid generated by $x$ and $y$ what is known as an inverse monoid (definition recalled). We obtain consequences of these relations, and related results. P. Nielsen and J. Šter, in a paper to appear, show that a much larger class of elements $x$ of rings $R,$ including all elements of von Neumann regular rings, have inner inverses satisfying arbitrarily large finite subsets of the abovementioned set of relations. But we show by example that the endomorphism ring of any infinite-dimensional vector space contains elements having no inner inverse that simultaneously satisfies all those relations. A tangential result proved is a condition on an endomap $x$ of a set $S$ that is necessary and sufficient for $x$ to belong to an inverse submonoid of the monoid of all endomaps of $S.$

math.RA↗

arXiv2016

Simplicial complexes with lattice structures

If $L$ is a finite lattice, we show that there is a natural topological lattice structure on the geometric realization of its order complex $Δ(L)$ (definition recalled). Lattice-theoretically, the resulting object is a subdirect product of copies of $L$. We note properties of this construction and of some variants thereof, and pose several questions. For $M_3$ the $5$-element nondistributive modular lattice, $Δ(M_3)$ is modular, but its underlying topological space does not admit a structure of distributive lattice, answering a question of Walter Taylor. We also describe a construction of "stitching together" a family of lattices along a common chain, and note how $Δ(M_3)$ can be obtained as a case of this construction.

math.RA↗

arXiv2016

Tensor products of faithful modules

If $k$ is a field, $A$ and $B$ $k$-algebras, $M$ a faithful left $A$-module, and $N$ a faithful left $B$-module, we recall the proof that the left $A\otimes_k B$-module $M\otimes_k N$ is again faithful. If $k$ is a general commutative ring, we note some conditions on $A,$ $B,$ $M$ and $N$ that do, and others that do not, imply the same conclusion. Finally, we note a version of the main result that does not involve any algebra structures on $A$ and $B.$

math.RA↗

arXiv2015

On Vaughan Pratt's crossword problem

Vaughan Pratt has introduced objects consisting of pairs $(A,W)$ where $A$ is a set and $W$ a set of subsets of $A,$ such that (i) $W$ contains $\emptyset$ and $A,$ (ii) if $C$ is a subset of $A\times A$ such that for every $a\in A,$ both $\{b\mid (a,b)\in C\}$ and $\{b\mid (b,a)\in C\}$ are members of $W$ (a "crossword" with all "rows" and "columns" in $W),$ then $\{b\mid (b,b)\in C\}$ (the "diagonal word") also belongs to $W,$ and (iii) for all distinct $a,b\in A,$ the set $W$ has an element which contains $a$ but not $b.$ He has asked whether for every $A,$ the only such $W$ is the set of all subsets of $A.$ We answer that question in the negative. We also obtain several positive results, in particular, a positive answer to the above question if $W$ is closed under complementation. We obtain partial results on whether there can exist counterexamples to Pratt's question with $W$ countable.

math.CO↗

arXiv2015

Adjoining a universal inner inverse to a ring element

Let $R$ be an associative unital algebra over a field $k,$ let $p$ be an element of $R,$ and let $R'=R\langle q\mid pqp= p\rangle.$ We obtain normal forms for elements of $R',$ and for elements of $R'$-modules arising by extension of scalars from $R$-modules. The details depend on where in the chain $pR\cap Rp \subseteq pR\cup Rp \subseteq pR + Rp \subseteq R$ the unit $1$ of $R$ first appears. This investigation is motivated by a hoped-for application to the study of the possible forms of the monoid of isomorphism classes of finitely generated projective modules over a von Neumann regular ring; but that goal remains distant. We end with a normal form result for the algebra obtained by tying together a $k$-algebra $R$ given with a nonzero element $p$ satisfying $1\notin pR+Rp$ and a $k$-algebra $S$ given with a nonzero $q$ satisfying $1\notin qS+Sq,$ via the pair of relations $p=pqp,$ $q=qpq.$

math.RA↗

arXiv2014

Homomorphisms on infinite direct products of groups, rings and monoids

We study properties of a group, abelian group, ring, or monoid $B$ which (a) guarantee that every homomorphism from an infinite direct product $\prod_I A_i$ of objects of the same sort onto $B$ factors through the direct product of finitely many ultraproducts of the $A_i$ (possibly after composition with the natural map $B\to B/Z(B)$ or some variant), and/or (b) guarantee that when a map does so factor (and the index set has reasonable cardinality), the ultrafilters involved must be principal. A number of open questions, and topics for further investigation, are noted.

math.GR↗

arXiv2014

Minimal faithful modules over Artinian rings

Let R be a left Artinian ring, and M a faithful left R-module which is minimal, in the sense that no proper submodule or proper homomorphic image of M is faithful. If R is local, and socle(R) is central in R, we show that length(M/J(R)M)+length(socle(M))$\leq$ length(socle(R))+1, strengthening a result of T. Gulliksen. Most of the rest of the paper is devoted to the situation where the Artinian ring R is not necessarily local, and does not necessarily have central socle. In the case where R is a finite-dimensional algebra over an algebraically closed field, we get an inequality similar to the above, with the length of socle(R) interpreted as its length as a bimodule, and with the final summand +1 replaced by the Euler characteristic of a bipartite graph determined by the structure of socle(R). More generally, that inequality holds if, rather than assuming k algebraically closed, we assume that R/J(R) is a direct product of full matrix algebras over k, and exclude the case where k has small finite cardinality. Examples show that the restriction on the cardinality of k is needed; we do not know whether versions of our result hold with other hypotheses significantly weakened. The situation for faithful modules with only one minimality property, i.e., having no faithful proper submodules or having no faithful proper homomorphic images, is more straightforward: The length of M/J(R)M in the former case, and of socle(M) in the latter, is $\leq$ length(socle(R)) (again meaning length as a bimodule). The final section, essentially independent of the rest of the note, obtains these bounds, and shows that every faithful module over a left Artinian ring has a faithful submodule with the former minimality condition and a faithful factor module with the latter. The proofs involve some nice general results on decompositions of modules.

math.RT↗

arXiv2014

On monoids, 2-firs, and semifirs

Several authors have studied the question of when the monoid ring DM of a monoid M over a ring D is a right and/or left fir (free ideal ring), a semifir, or a 2-fir (definitions recalled in section 1). It is known that for M nontrivial, a necessary condition for any of these properties to hold is that D be a division ring. Under that assumption, necessary and sufficient conditions on M are known for DM to be a right or left fir, and various conditions on M have been proved necessary or sufficient for DM to be a 2-fir or semifir. A sufficient condition for DM to be a semifir is that M be a direct limit of monoids which are free products of free monoids and free groups. W.Dicks has conjectured that this is also necessary. However F.Cedó has given an example of a monoid M which is not such a direct limit, but satisfies the known necessary conditions for DM to be a semifir. It is an open question whether for this M, the rings DM are semifirs. We note some reformulations of the known necessary conditions for DM to be a 2-fir or a semifir, motivate Cedó's construction and a variant, and recover Cedó's results for both constructions. Any homomorphism from a monoid M into \Z induces a \Z-grading on DM, and we show that for the two monoids in question, the rings DM are "homogeneous semifirs" with respect to all such nontrivial \Z-gradings; i.e., have (roughly) the property that every finitely generated homogeneous one-sided ideal is free. If M is a monoid such that DM is an n-fir, and N a "well-behaved" submonoid of M, we obtain results on DN. Using these, we show that for M a monoid such that DM is a 2-fir, mutual commutativity is an equivalence relation on nonidentity elements of M, and each equivalence class, together with the identity element, is a directed union of infinite cyclic groups or infinite cyclic monoids. Several open questions are noted.

math.RA↗

arXiv2013

On group topologies determined by families of sets

Let $G$ be an abelian group, and $F$ a downward directed family of subsets of $G$. The finest topology $\mathcal{T}$ on $G$ under which $F$ converges to $0$ has been described by I.Protasov and E.Zelenyuk. In particular, their description yields a criterion for $\mathcal{T}$ to be Hausdorff. They then show that if $F$ is the filter of cofinite subsets of a countable subset $X\subseteq G$, there is a simpler criterion: $\mathcal{T}$ is Hausdorff if and only if for every $g\in G-\{0\}$ and positive integer $n$, there is an $S\in F$ such that $g$ does not lie in the n-fold sum $n(S\cup\{0\}\cup-S)$. In this note, their proof is adapted to a larger class of families $F$. In particular, if $X$ is any infinite subset of $G$, $κ$ any regular infinite cardinal $\leq\mathrm{card}(X)$, and $F$ the set of complements in $X$ of subsets of cardinality $<κ$, then the above criterion holds. We then give some negative examples, including a countable downward directed set $F$ of subsets of $\mathbb{Z}$ not of the above sort which satisfies the "$g\notin n(S\cup\{0\}\cup-S)$" condition, but does not induce a Hausdorff topology. We end with a version of our main result for noncommutative $G$.

math.GR↗

arXiv2013

Commuting matrices, and modules over Artinian local rings

Gerstenhaber showed in 1961 that any commuting pair of n x n matrices over a field k generates a k-algebra A of k-dimension \leq n. A well-known example shows that the corresponding statement for 4 matrices is false. The question for 3 matrices is open. Gerstenhaber's result can be looked at as a statement about the relation between the length of a 2-generator finite-dimensional commutative k-algebra A and the lengths of faithful A-modules. Wadsworth generalized this result to a larger class of commutative rings than those generated by two elements over a field. We recover his generalization, using a slightly improved argument. We then explore some examples, raise further questions, and make a bit of progress toward answering some of them. An appendix gives some results on generation and subdirect decompositions of modules over not necessarily commutative Artinian rings, generalizing a special case used in the paper. What I originally thought of as my main result turned out to have been anticipated by Wadsworth, so I probably won't submit this for publication unless I find further strong results to add. However, others may find interesting the observations, partial results, and questions noted, and perhaps make some progress on them.

math.AC↗

arXiv2013

Families of ultrafilters, and homomorphisms on infinite direct product algebras

Criteria are obtained for a filter F of subsets of a set I to be an intersection of finitely many ultrafilters, respectively, finitely many κ-complete ultrafilters for a given uncountable cardinal κ. From these, general results are deduced concerning homomorphisms on infinite direct product groups, which yield quick proofs of some results in the literature: the Łoś-Eda theorem (characterizing homomorphisms from a not-necessarily-countable direct product of modules to a slender module), and some results of N. Nahlus and the author on homomorphisms on infinite direct products of not-necessarily-associative k-algebras. The same tools allow other results of Nahlus and the author to be nontrivially strengthened, and yield an analog to one of their results, with nonabelian groups taking the place of k-algebras. We briefly examine the question of how the common technique used in applying the general results of this note to k-algebras on the one hand, and to nonabelian groups on the other, might be extended to more general varieties of algebras in the sense of universal algebra. In a final section, the Erdős-Kaplansky Theorem on dimensions of vector spaces D^I (D a division ring) is extended to reduced products D^I/F, and an application is noted.

math.LO↗

arXiv2013

Thoughts on Eggert's Conjecture

Eggert's Conjecture says that if R is a finite-dimensional nilpotent commutative algebra over a perfect field F of characteristic p, and R^{(p)} is the image of the p-th power map on R, then dim_F R \geq p dim_F R^{(p)}. Whether this very elementary statement is true is not known. We examine heuristic evidence for this conjecture, versions of the conjecture that are not limited to positive characteristic and/or to commutative R, consequences the conjecture would have for semigroups, and examples that give equality in the conjectured inequality. We pose several related questions, and briefly survey the literature on the subject.

math.AC↗

arXiv2012

On diagram-chasing in double complexes

Diagram-chasing arguments frequently lead to "magical" relations between distant points of diagrams: exactness implications, connecting morphisms, etc.. These long connections are usually composites of short "unmagical" connections, but the latter, and the objects they join, are not visible in the proofs. I try to remedy this situation. Given a double complex in an abelian category, we consider, for each object A of the complex, the familiar horizontal and vertical homology objects at A, and two other objects, which we name the "donor" A_{\box} and and the "receptor" ^{\box}A at A. For each arrow of the double complex, we prove the exactness of a 6-term sequence of these objects (the "Salamander Lemma"). Standard results such as the 3x3-Lemma, the Snake Lemma, and the long exact sequence of homology associated with a short exact sequence of complexes, are obtained as easy applications of this lemma. We then obtain some generalizations of the last of the above examples, getting various exact diagrams from double complexes with all but a few rows and columns exact. The total homology of a double complex is also examined in terms of the constructions we have introduced. We end with a brief look at the world of triple complexes, and two exercises.

math.CT↗

arXiv2012

Linear maps on k^I, and homomorphic images of infinite direct product algebras

Let k be an infinite field, I an infinite set, V a k-vector-space, and g:k^I\to V a k-linear map. It is shown that if dim_k(V) is not too large (under various hypotheses on card(k) and card(I), if it is finite, respectively countable, respectively < card(k)), then ker(g) must contain elements (u_i)_{i\in I} with all but finitely many components u_i nonzero. These results are used to prove that any homomorphism from a direct product \prod_I A_i of not-necessarily-associative algebras A_i onto an algebra B, where dim_k(B) is not too large (in the same senses) must factor through the projection of \prod_I A_i onto the product of finitely many of the A_i, modulo a map into the subalgebra \{b\in B | bB=Bb=\{0\}\}\subseteq B. Detailed consequences are noted in the case where the A_i are Lie algebras.

math.RA↗

arXiv2011

Every module is an inverse limit of injective modules

It is shown that any left module A over a ring R can be written as the intersection of a downward directed system of injective submodules of an injective module; equivalently, as an inverse limit of one-to-one homomorphisms of injectives. If R is left Noetherian, A can also be written as the inverse limit of a system of surjective homomorphisms of injectives. Some questions are raised.

math.RA↗

arXiv2011

Bilinear maps on Artinian modules

It is shown that if a bilinear map f: A x B --> C of modules over a commutative ring k is nondegenerate (i.e., if no nonzero element of A annihilates all of B, and vice versa), and A and B are Artinian, then A and B are of finite length. Some consequences are noted. Counterexamples are given to some attempts to generalize the above statement to balanced bilinear maps of bimodules over noncommutative rings, while the question is raised whether other such generalizations are true.

math.RA↗

arXiv2011

More Abelian groups with free duals

In answer to a question of A. Blass, J. Irwin and G. Schlitt, a subgroup G of the additive group Z^ω is constructed whose dual, Hom(G,Z), is free abelian of rank 2^{\aleph_0}. The question of whether Z^ω has subgroups whose duals are free of still larger rank is discussed, and some further classes of subgroups of Z^ω are noted.

math.GR↗

arXiv2011

An inner automorphism is only an inner automorphism, but an inner endomorphism can be something strange

The inner automorphisms of a group G can be characterized within the category of groups without reference to group elements: they are precisely those automorphisms of G that can be extended, in a functorial manner, to all groups H given with homomorphisms G --> H. Unlike the group of inner automorphisms of G itself, the group of such extended systems of automorphisms is always isomorphic to G. A similar characterization holds for inner automorphisms of an associative algebra R over a field K; here the group of functorial systems of automorphisms is isomorphic to the group of units of R modulo units of K. If one substitutes "endomorphism" for "automorphism" in these considerations, then in the group case, the only additional example is the trivial endomorphism; but in the K-algebra case, a construction unfamiliar to ring theorists, but known to functional analysts, also arises. Systems of endomorphisms with the same functoriality property are examined in some other categories; other uses of the phrase "inner endomorphism" in the literature, some overlapping the one introduced here, are noted; the concept of an inner {\em derivation} of an associative or Lie algebra is looked at from the same point of view, and the dual concept of a "co-inner" endomorphism is briefly examined. Several questions are posed.

math.RA↗