George M. Bergman - abstracts of preprints, and of reprints since 2006

Completeness results for metrized rings and lattices

The Boolean ring  B  of measurable subsets of the unit interval, modulo sets of measure zero, has proper primary ideals that are closed under the natural metric (e.g.,  {0}),  but has no prime ideals closed under that metric; hence closed primary ideals are not, in general, intersections of closed prime ideals.  Moreover,  B  is known to be complete in its metric.  Together, these facts answer a question posed by J. Gleason.  From this example, rings of arbitrary characteristic with the corresponding properties are obtained.

The result that  B  is complete is generalized to show that if  L  is a lattice given with a metric  d  satisfying identically either the relation  d(xyxz)  ≤  d(yz)  or the relation  d(xyxz)  ≤  d(yz),  and if in  L  if every increasing Cauchy sequence converges and every decreasing Cauchy sequence converges, then every Cauchy sequence converges; i.e.,  L  is complete as a metric space.

We end with an example showing that if the above inequalities are replaced by the weaker conditions  d(xxy)  ≤  d(xy),  respectively  d(xxy)  ≤  d(xy),  then this completeness statement can fail.

16 pp., last revised 28 Sept., 2018.  texdvipdfpsarXiv:1808.04455Back to publications-list

Submonoids of groups, and group-representability of restricted relation algebras

Algebra Universalis 79 (2018) ?-? (15 pp)

M. Kuczma asked in 1980 whether for every positive integer  n,  there exists a subsemigroup  M  of a group  G,  such that  G  is equal to the n-fold product  M M−1 M M−1 ... M(−1)n-1,  but not to any proper initial subproduct of this product.  We answer his question affirmatively, and prove a more general result on representing a certain sort of relation algebra by a family of subsets of a group. We also sketch several variants of the latter result.

DOI.  Preprint version:  texdvipdfpsarXiv:1702.06088Back to publications-list

Some results relevant to embeddability of rings (especially group algebras) in division rings

P. M. Cohn showed in 1971 that given a ring  R,  to describe, up to isomorphism, a division ring  D  generated by a homomorphic image of  R  is equivalent to specifying the set of square matrices over  R  which map to singular matrices over  D,  and he determined the conditions that such a set of matrices must satisfy.  The present author later developed another version of this data, in terms of closure operators on free R-modules.

In this note, we examine these constructions (mainly the latter) further, and show how such data can be obtained from an R-module  M  satisfying certain conditions.

Our motivation is the longstanding question of whether, for  G  a right-orderable group and  k  a field, the group algebra  kG  is always embeddable in a division ring.  Our hope is that the right kG-module  M = k((G))  might induce a closure operator of the required sort.  We review a partial result in this direction due to N. I. Dubrovin, and note a plausible generalization thereof which would give the desired embedding.

22 pp., last modified 26 Oct., 2016.  texdvipdfpsBack to publications-list

Strong inner inverses in endomorphism rings of vector spaces

Publ. Mat. 62 (2018) 253-284.

For  V  a vector space over a field, or more generally, over a division ring, it is well-known that every  x ∈ End(V)  has an inner inverse, i.e., an element y ∈ End(V)  satisfying  xyx=x.  We show here that a large class of such  x  have inner inverses  y  that satisfy an infinite family of additional monoid relations, making the monoid generated by  x  and  y  an inverse monoid (definition recalled).  We obtain consequences of these relations, and related results.

P. Nielsen and J. Šter (TAMS 370 (2018) 1759-1782) show that a much larger class of elements  x  of rings  R,  including all elements of von Neumann regular rings, have inner inverses satisfying any finite subset of the abovementioned system of relations.  But we show by example that the endomorphism ring of an infinite-dimensional vector space contains elements having no inner inverse that simultaneously satisfies all those relations.

A tangential result proved gives a necessary and sufficient condition on an endomap  x  of a set  S,  for x to have a strong inner inverse in the monoid of all endomaps of  S

DOI.  Preprint version: texdvipdfpsarXiv:1611.00972Back to publications-list

Simplicial complexes with lattice structures

Algebr. Geom. Topol. 17 (2017) 439-486.

A standard construction associates to every finite partially ordered set  P  a finite simplicial complex  Δ(P),  called the "order complex" of  P.  Simplicial complexes come in two versions, the "abstract" complex (a set of points called "vertices", with distinguished subsets called "simplices") and its "geometric realization" (a topological space in which the 2-element simplices are replaced by line-segments, the 3-element simplices by triangles, etc.); we shall  understand  Δ(P)  to denote the geometric realization.  I give a variant way of describing this object, which leads to a natural partial ordering of  Δ(P),  such that as a poset,  Δ(P)  is a subdirect product of copies of  P,  and for  L  a lattice, the poset  Δ(L)  is likewise a lattice.

The lattice  Δ(M3)  answers W. Taylor's question "Is there a simplicial complex which admits a continuous structure of lattice, but not a continuous structure of distributive lattice?"  Properties of the construction  Δ(L)  suggest some further questions, such as whether in a topological lattice whose underlying space is a finite simplicial complex, "almost every" point must have a neighborhood that is a distributive sublattice.

Variants of the construction  Δ(L)  are sketched, and some of their properties noted.

I also describe a construction of "stitching" together a family of lattices along a common chain, and note that  Δ(M3)  can be obtained by stitching together three copies of  Δ(C),  where  C  is a 3-element chain.

DOIMR3604382.  Preprint version: texdvipdfpsarXiv:1602.00034Back to publications-list

Some embedding results for associative algebras

(This is an extensively revised version of a preprint that was posted here under the title Rings with few generators from Feb. 2012 through Jan. 2013.

Suppose we wish to embed an (associative) k-algebra  A  in a k-algebra  R  generated in some specified way; e.g., by two elements, or by copies of given k-algebras  A1A2A3.  Mal'cev, Shirshov, and Bokut' et al., have obtained sufficient conditions for such embeddings to exist.  We prove here some further results on this theme.

In particular, we merge the ideas of existing constructions based on two generating elements, and on three generating subalgebras, to get a construction using two generating subalgebras.

We pose some questions on how far these results can be strengthened.

10 pp., last revised 5 March, 2013.  texdvipdfpsBack to publications-list

On Vaughan Pratt's crossword problem (with Pace P. Nielsen)

J. Lond. Math. Soc. (2) 93 (2016) 825-845.

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  ∅  and  A
(ii)  if  C  is a subset of  A × A  such that for every  a ∈ A,  both  {b | (a,b) ∈ C}  and  {b | (b,a)∈ C}  are members of  W  (a "crossword" with all "rows" and "columns" in  W),  then  {a | (a,a)∈ C}  (the "diagonal word") also belongs to  W,  and
(iii) for all  a, b ∈ A,  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 the problem of whether Pratt's question has a positive answer if  W  is required to be countable.

MR3509966DOI  Preprint version:  texdvipdfpsarXiv:1504.07310Back to publications-list

Adjoining a universal inner inverse to a ring element,

J. Algebra, 449 (2016) 355-399

Let  R  be an algebra over a field  k,  let  p  be an element of  R,  and let  R' = R< q | pqp = p>.  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 ∩ Rp  ⊆ pR ∪ RppR + RpR 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∉pR+Rp  and a k-algebra  S  given with a nonzero element  q  satisfying  1∉qS+Sq  via the pair of relations  p = pqpq = qpq

MR3448178DOI.  Preprint version: texdvipdfpsarXiv:1505.02312Back to publications-list

On group topologies determined by families of sets

Matematychni Studii, 43 (2015) 115-128

Let  G  be an abelian group, and  F  a downward directed family of subsets of  G.  The finest topology  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  T  to be Hausdorff.  They then show that if  F  is the filter of cofinite subsets of a countable subset  X ⊆ G  (the Fréchet filter on  X),  then there is a simpler criterion:  T  is Hausdorff if and only if for every  g ∈ G−{0}  and positive integer  n,  there is an  S ∈ F  such that  g  does not lie in the n-fold sum  n (S ∪ {0} ∪ −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  ≤ 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  Z  not of the above sort which satisfies the "g ∉ n (S ∪ {0} ∪ −S)" condition, but does not induce a Hausdorff topology.

We end with a version of our main result for noncommutative  G

(The printed version has some errors -- in particular, an added "the" after the first word of the title, and some spelling errors in the added Russian translation of the abstract -- but these have been corrected in the online version.)

MR3444020DOI.  Preprint version: texdvipdfpsarXiv:1311.2648Back to publications-list

Minimal faithful modules over Artinian rings

Publicacions Matemàtiques, 59 (2015) 271-300.

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))  ≤  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 preceding, 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 bimodule 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 are true with the hypotheses on  R/J(R)  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  ≤ length(socle(R))  (where this again means length as a bimodule).  We end with a section, essentially independent of the rest of the note, which 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.

MR3374608DOI.  Preprint version: texdvipdfpsarXiv:1310.5365Back to publications-list

Homomorphisms on infinite direct products of groups, rings and monoids

Pacific Journal of Mathematics, 274 (2015) 451-495.

We study properties of a group, abelian group, ring, or monoid  B  which (a) guarantee that every homomorphism from an infinite direct product  ΠI Ai  of objects of the same sort onto  B  factors through the direct product of finitely many ultraproducts of the  Ai  (possibly after composition with the natural map  B → 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 in question must be principal.

We note a number of open questions, and topics for further investigation.

MR3332912DOI.  Preprint version:  texdvipdfpsarXiv:1406.1932Back to publications-list

Paul Moritz Cohn. 8 January 1924 -- 20 April 2006 (with Trevor Stuart)

Biographical Memoirs of Fellows of the Royal Society 60 (2014) 127-150.

Sections 1-4, by Trevor Stuart, sketch Cohn's personal and professional life: his childhood in Hamburg, his move to England on the Kindertransport at age 15, his years directing Ph.D. students, and his service to the London Mathematical Society.  Of the sections I wrote on his research and publications, §5 tells the non-mathematician what noncommutative rings are, §§6-11 sketch his research for the specialist, and §12 surveys his publications.

The memoir is supplemented by two online files, a 209-item bibliography of his works, and an English translation of an account he wrote of his early childhood.

Families of ultrafilters, and homomorphisms on infinite direct product algebras

J. Symb. Log., 79 (2014) 223–239.

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 N. Nahlus and the author on that topic 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  DI  (D a division ring) is extended to reduced products  DI/F  and used to generalize to division rings another result of N. Nahlus and the author.

MR3226022DOI.  Preprint version:  texdvipspdfarXiv:1301.6383Back to publications-list

On monoids, 2-firs, and semifirs

Semigroup Forum, 89 (2014) 293--335.

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.  Warren 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 all 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 here some reformulations of the known necessary conditions for a monoid ring  DM  to be a 2-fir or a semifir, motivate Cedó's construction and a variant of that construction, 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 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  N  of  M,  we prove some properties of  DN.  Using these, we show that if  M  is any monoid having a monoid ring  DM  which is a 2-fir, then 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.

MR3258484DOI.  Preprint version:  texdvipdfpsarXiv:1309.0564Back to publications-list

Thoughts on Eggert's Conjecture

Contemporary Mathematics, 609 (2014), Ring Theory and Its Applications, proceedings of a conference honoring T.Y.Lam, pp.1-17.

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  dimF R ≥  p dimF 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 finite abelian semigroups, and examples that give equality in the conjectured inequality.

We pose several related questions, and briefly survey the literature on the subject.

MR3204348DOI.  Preprint version:  texdvipdfarXiv:1206.0326Back to publications-list

Homomorphic images of pro-nilpotent algebras

Illinois J. Math., 55 (2011) 719–748.  (Actually appeared 2013.)

It is shown that any finite-dimensional homomorphic image of an inverse limit of nilpotent not-necessarily-associative algebras over a field is nilpotent.  More generally, this is true of algebras over a general commutative ring  k,  with "finite-dimensional" replaced by "of finite length as a  k-module".

These results are obtained by considering the multiplication algebra  M(A)  of an algebra  A  (the associative algebra of  k-linear maps  A → A  generated by left and right multiplications by elements of  A),  and its behavior with respect to nilpotence, inverse limits, and homomorphic images.

As a corollary, it is shown that a finite-dimensional homomorphic image of an inverse limit of finite-dimensional solvable Lie algebras over a field of characteristic 0 is solvable.

Examples are given showing that infinite-dimensional homomorphic images of inverse limits of nilpotent algebras can have properties far from those of nilpotent algebras; in particular, properties that imply that they are not residually nilpotent.

Several open questions and directions for further investigation are noted.

(Erratum:  In the second sentence of the Introduction, which recalls the definitions of right, left, and 2-sided ideals using a "respectively" construction, the conditions for "right" and "left" are reversed.)

Continuity of homomorphisms on pro-nilpotent algebras

Illinois J. Math., 55 (2011) 749–748.  (Actually appeared 2013.)

Let  V  be a variety of not necessarily associative algebras, and  A  an inverse limit of nilpotent algebras  Ai ∈ V,  such that some finitely generated subalgebra  S ⊆ A  is dense in  A  under the inverse limit of the discrete topologies on the  Ai .

A sufficient condition on  V  is obtained for all algebra homomorphisms from  A  to finite-dimensional algebras  B  to be continuous; in other words, for the kernels of all such homomorphisms to be open ideals.  This condition is satisfied, in particular, if  V  is the variety of associative, Lie, or Jordan algebras.

Examples are given showing the need for our hypotheses, and some questions are raised.

Bilinear maps on Artinian modules

Journal of Algebra and its Applications, 11 (2012) No.5, 1250090, 10 pp.

It is shown that if a bilinear map  f : A × 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 if  A  and  B  are Artinian, then  A  and  B  are of finite length.

Some immediate consequences are noted.  Counterexamples are given to certain generalizations of this statement to balanced bilinear maps of bimodules over noncommutative rings, while the question is raised whether other such generalizations may be true.

Every module is an inverse limit of injectives

Proc. A.M.S., 141 (2013) 1177-1183.

It is shown that any left module  A  over a ring  R  can be written as the intersection (and hence the inverse limit) of a downward directed system of injective submodules of an injective R-module.  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, and an example is noted.

(The MR review points out that, calling a module  E  finitely injective if every homomorphism from a finitely generated module  A  into  E  extends to all overmodules of  A,  we can, in the second result above, delete "Noetherian" if we weaken "injective" to "finitely injective".)

Isotone maps on lattices  (with George Grätzer)

Algebra Universalis, 68 (2012) 17-37.

Let  ( Li | i ∈ I )  be a family of lattices in a nontrivial lattice variety  V,  and let  φi : Li → M,  for  i ∈ I,  be isotone maps (not assumed to be lattice homomorphisms) to a common lattice  M  (not assumed to lie in  V).  We show that the maps  φi  can be extended to an isotone map  φ: FreeVLi ) → M,  where  FreeVLi )  is the free product of the  Li  in  V.  This was known for  L = V,  the variety of all lattices.

The above free product  L  can be viewed as the free lattice in  V  on the partial lattice  P  formed by the disjoint union of the  Li.  The analog of the above result does not, however, hold for free lattices  L  on arbitrary partial lattices  P.  We show that the only codomain lattices  M  for which that more general statement holds are the complete lattices.  On the other hand, we prove the analog of our main result for a class of partial lattices  P  that are not-quite-disjoint unions of lattices.

We also obtain some results similar to our main one, but with the relationship  lattices : orders  replaced either by  semilattices : orders  or by  lattices : semilattices

Some open questions are noted.

More abelian groups with free duals

Portugaliae Mathematica, 69 (2012) 69-84.

In answer to a question of A. Blass, J. Irwin and G. Schlitt, a subgroup  G  of the additive group  Zω  is constructed whose dual,  Hom(G, Z ),  is free abelian of rank  20.  The question of whether  Zω  has subgroups whose duals are free of still higher rank is discussed, and some further classes of subgroups of  Zω  are noted.

Linear maps on  kI,  and homomorphic images of infinite direct product algebras  (with Nazih Nahlus)

J. Algebra, 356 (2012) 257-274.

Let  k  be an infinite field, and  g : kI → V  a k-linear map, where  I  is an infinite set and  V  a k-vector-space.  It is shown that if  dimk(V)  is not too large (where depending on the hypotheses on  card(k)  and  card(I),  this can mean that  dimk(V)  is finite, respectively  < card(k),  respectively less than continuum), then  ker(g)  must contain an element  (ui)iI  with all but finitely many components  ui  nonzero.

These results are used to prove that any homomorphism from a direct product  ∏I Ai  of not-necessarily-associative algebras  Ai onto an algebra  B,  where  dimk(B)  is "not too large" (in the same senses) must factor through the projection of  ∏I Ai  onto the product of finitely many of the  Ai,  modulo a map into the subalgebra  {b ∈ B | bB = Bb = {0}} ⊆ B

Detailed consequences are noted in the case where the  Ai  are Lie algebras.

A partial generalization of the above results is proved with the field  k  replaced by a commutative valuation ring.

It is shown in a subsequent note  that cardinality assumptions of this note can be weakened.

Homomorphisms on infinite direct product algebras, especially Lie algebras  (with Nazih Nahlus)

J. Algebra, 333 (2011) 67-104.

We study surjective homomorphisms  f : ∏I Ai  → B  of not-necessarily-associative algebras over a commutative ring  k,  for  I  a generally infinite set; especially when  k  is a field and  B  is countable-dimensional over  k.

Our results have the following consequences when  k  is an infinite field, the algebras are Lie algebras, and  B  is finite-dimensional:

If all the Lie algebras  Ai  are solvable, then so is  B

If all the Lie algebras  Ai  are nilpotent, then so is  B

If  k  is not of characteristic 2 or 3, and all the Lie algebras  Ai  are finite-dimensional and are direct products of simple algebras, then, (i) so is  B,  (ii)  f  splits, and (iii) under a weak cardinality bound on  If  is continuous in the pro-discrete topology.  A key fact used in getting (i)-(iii) is that over any such field, every finite-dimensional simple Lie algebra  L  can be written  L = [x1L] + [x2L].  for some  x1, x2 ∈ L , which we prove from a recent result of J. M. Bois.

The general technique of the paper involves studying conditions under which a homomorphism on  ∏I Ai  must factor through the direct product of finitely many ultraproducts of the  Ai

Several open questions are noted.

On diagram-chasing in double complexes

Theory and Applications of Categories 26 (2012) 60-96.

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 here 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  and the "receptor"  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 3×3-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 that lemma.

We then obtain some further generalizations of the long exact sequence of homology, 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 a couple of exercises.

MR2909639.  Published version:  dvi.  pdf.  ps.  TeX source file of preprint (requires this style-class file).  arXiv:1108.0958Back to publications-list

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

Publicacions Matemàtiques56 (2012) 91-126.

The inner automorphisms of a group  G  can be characterized in terms of 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 .  (Precise statement in section 1.)  The group of such extended systems of automorphisms, unlike the group of inner 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 looks at the above functorial extendibility property for endomorphisms, rather than just automorphisms, 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 above functoriality property are examined in some other categories; other uses of the phrase "inner endomorphism" in the literature, some of which overlap the one introduced here, are noted; the concept of an inner derivation of an associative algebra or Lie algebra is looked at from the same point of view, and a dual concept of "co-inner" endomorphism is briefly examined.  Several questions are posed.

MR2918185DOI.  Preprint:  texdvipdfpsarXivBack to publications-list

On common divisors of multinomial coefficients

Bull. Australian Math. Soc.83 (2011) 138-157.

Erdős and Szekeres showed in 1978 that for any four positive integers satisfying  m1+m2 = n1+n2,  the two binomial coefficients  (m1+m2)! / m1m2!  and  (n1+n2)! / n1n2!  have a common divisor  >1.  The analogous statement for families of  k   k-nomial coefficients  (k > 1)  was conjectured in 1997 by David Wasserman.

Erdős and Szekeres remark that if  m1, m2, n1, n2  as above are all  > 1,  there is probably a lower bound on the common divisor in question which goes to infinity as a function of  m1+m2.  Such a bound is obtained.

Criteria are developed for narrowing the class of possible counterexamples to Wasserman's conjecture.  On the other hand, several plausible generalizations of that conjecture are shown to be false.

(Early preprints of this note contained many digressions, which I moved into 8 pages of "Addenda"; I don't plan on publishing the Addenda.)

MR2765421DOI  preprint:  TeXPDFarXivBack to publications-list

On coproducts in varieties, quasivarieties and prevarieties

If the free algebra  F  on one generator in a variety  V  of algebras (in the sense of universal algebra) has a subalgebra free on two generators, must it also have a subalgebra free on three generators?  In general, no; but yes if  F  generates the variety  V

Generalizing the argument, it is shown that if we are given an algebra and subalgebras,  A0 ⊇ ... ⊇ An,  in a prevariety (SP-closed class of algebras)  P  such that  An  generates  P,  and also subalgebras  Bi ⊆ Ai-1  (0 < i ≤ n)  such that for each  i > 0  the subalgebra of  Ai-1  generated by  Ai  and  Bi  is their coproduct in  P,  then the subalgebra of  A  generated by  B1, ..., Bn  is the coproduct in  P  of these algebras.

Some further results on coproducts are noted:

If  P  satisfies the amalgamation property, then one has the stronger "transitivity" statement, that if  A  has a finite family of subalgebras  (Bi)i ∈ I  such that the subalgebra of  A  generated by the  Bi  is their coproduct, and each  Bi  has a finite family of subalgebras  (Cij)j∈ Ji  with the same property, then the subalgebra of  A  generated by all the  Cij  is their coproduct.

For  P  a residually small prevariety or an arbitrary quasivariety, relationships are proved between the least number of algebras needed to generate  P  as a prevariety or quasivariety, and behavior of the coproduct operation in  P

It is shown by example that for  G  a subgroup of the group  S = Sym(Ω)  of all permutations of an infinite set  Ω,  the group  S  need not have a subgroup isomorphic over  G  to the coproduct with amalgamation  S \coprodS.  But under weak additional hypotheses, that question remains open.

For the reader who is not an expert in universal algebra, a "Prologue" introducing the concepts, and a short glossary at the end, are provided.

MR2011e:08014.  DOI.  preprint:  texdvipdfpsarXivBack to publications-list

On lattices and their ideal lattices, and posets and their ideal posets

Tbilisi Math. J.  1 (2008) 89-103.

For  P  a poset or lattice, let  Id(P)  denote the poset, respectively, lattice, of upward directed downsets in  P,  including the empty set, and let  id(P) = Id(P)- {∅}.  This note obtains various results to the effect that  Id(P)  is always, and  id(P)  often, "essentially larger" than  P.  In the first vein, we find that a poset  P  admits no < -respecting map (and so in particular, no one-to-one isotone map) from  Id(P)  into  P,  and, going the other way, that an upper semilattice  S  admits no semilattice homomorphism from any subsemilattice of itself onto  Id(S).

The slightly smaller object  id(P)  is known to be isomorphic to  P  if and only if  P  has ascending chain condition.  This result is strengthened to say that the only posets  P0  such that for every natural number  n  there exists a poset  Pn  with  idn(Pn) ≅ P0  are those having ascending chain condition.  On the other hand, a wide class of cases is noted here where  id(P)  is embeddable in  P

Counterexamples are given to many variants of the results proved.

MR2010k:06001.  Preprint:   texdvipdfpsBack to publications-list

Colimits of representable algebra-valued functors

Theory and Applications of Categories, 20 (2008) 334-404.

If  C  and  D  are varieties of algebras in the sense of general algebra, then by a representable functor  C → D  we understand a functor which, when composed with the forgetful functor  D → Set,  gives a representable functor in the classical sense; Freyd showed that these functors are determined by D-coalgebra objects of  C.  Let  Rep(CD)  denote the category of all such functors, a full subcategory of  Cat(CD),  opposite to the category of D-coalgebras in  C.  It is proved that  Rep(CD)  has small colimits, and in certain situations, explicit constructions for the representing coalgebras are obtained.

In particular,  Rep(CD)  always has an initial object.  This is shown to be "trivial" unless  C  and  D  either both have no zeroary operations, or both have more than one derived zeroary operation.  In those two cases, the functors in question may have surprisingly opulent structures.

It is also shown that every set-valued representable functor on  C  admits a universal morphism to a D-valued representable functor.

Several examples are worked out in detail, and areas for further investigation noted.

MR 2009c:18012.  Preprint:   texdvipdfpsBack to publications-list

Mapping radii of metric spaces

Pacific Journal of Mathematics, 236 (2008) 223--261.

It is known that every closed curve of length  ≤ 4 in  Rn   (n>0)  can be surrounded by a sphere of radius  1,  and that this is the best bound.  Letting  S  denote the circle of circumference  4,  with the arc-length metric, we here express this fact by saying that the mapping radius of  S  in  Rn  is  1.

Tools are developed for estimating the mapping radius of a metric space  X  in a metric space  Y.  In particular, it is shown that for  X  a bounded metric space, the supremum of the mapping radii of  X  in all convex subsets of normed vector spaces is equal to the infimum of the  sup  norms of all convex linear combinations of the functions  d(x,-): X → R  (x X).

Several explicit mapping radii are calculated, and open questions noted.

MR 2009g:54064.  Article.   DOI.  preprint:  texdvipdfps Back to publications-list

Some results on embeddings of algebras, after de Bruijn and McKenzie

Indagationes Mathematicae, 18 (2007) 349-403

In 1957, N. G. de Bruijn showed that the symmetric group  Sym(Ω)  on an infinite set  Ω  contains a free subgroup on  2card(Ω)  generators, and proved a more general statement, a sample consequence of which is that for any group  A  of cardinality  ≤ card(Ω),  the group  Sym(Ω)  contains a coproduct of  2card(Ω)  copies of  A,  not only in the variety of all groups, but in any variety of groups to which  A  belongs.  His key lemma is here generalized to an arbitrary variety of algebras  V,  and formulated as a statement about functors  Set → V.  From this one easily obtains analogs of the results stated above with "group" and  Sym(Ω)  replaced by "monoid" and the monoid  Self(Ω)  of endomaps of  Ω,  by "associative K-algebra" and the K-algebra  EndK(V)  of endomorphisms of a K-vector-space  V  with basis  Ω,  and by "lattice" and the lattice  Equiv(Ω)  of equivalence relations on  Ω.  It is also shown, extending another result from de Bruijn's 1957 paper, that each of  Sym(Ω),  Self(Ω)  and  EndK(V)  contains a coproduct of  2card(Ω)  copies of itself.

That paper also gave an example of a group of cardinality  2card(Ω)  that was not embeddable in  Sym(Ω),  and R. McKenzie subsequently established a large class of such examples.  Those results are shown here to be instances of a general property of the lattice of solution sets in  Sym(Ω)  of sets of equations with constants in  Sym(Ω).  Again, similar results - this time of varying strengths - are obtained for  Self(Ω),  EndK(V)  and  Equiv(Ω),  and also for the monoid  Rel(Ω),  of binary relations on  Ω.

Many open questions and areas for further investigation are noted.  A couple of these have since been solved:  see Update

MR 2008m:08016.  DOI.  preprint:  texdvipdfpsBack to publications-list

Two statements about infinite products that are not quite true

in Groups, Rings & Algebras (Proceedings of a conference in honor of Donald S. Passman), ed. W. Chin, J. Osterburg, and D. Quinn.  Contemporary Mathematics, 420 (2006) 35-58.

This one is hard to summarize concisely; below are some high points.  In statements (i) and (ii) below,  Rω  and  ⊕ω R  are the direct product, respectively, the direct sum, of countably many copies of the ring  R,  as a left  R-module.

(i) There exist nontrivial rings  R  for which one has surjective module  homomorphisms  ⊕ω R → Rω ;   but in such cases, the module  Rω  must in fact be finitely generated.
(ii) There exist nontrivial rings  R  for which one has surjective module  homomorphisms  Rω → ⊕ω R ;   but in such cases,  R  must have DCC on finitely generated right ideals.
(iii) The full permutation group  S  on an infinite set  Ω  has the property that the card(Ω)-fold direct product of copies of  S  is generated over its diagonal subgroup by a single element.
(iv) Whenever an algebra  S  in the sense of universal algebra has the property that the countable direct product  Sω  is finitely generated over its diagonal subalgebra (or even when the corresponding property holds with a nonprincipal countable ultrapower in place of this direct product),  S  has some of the other strange properties known to hold for infinite symmetric groups.  (The consequences are slightly weaker in the ultrapower case than in the direct product case.)

MR 2007k:16008.  DOI.  preprint:  texdvipdfpsBack to publications-list

Closed subgroups of the infinite symmetric group  (with Saharon Shelah)

Algebra Universalis, 55 (2006) 137-173.

Let  S = Sym(Ω)  be the group of all permutations of a countably infinite set  Ω,  and for subgroups  G1G2  ≤  S  let us write  G1 ~ G2  if there exists a finite set  U ⊆ S  such that  <G1 ∪ U> = <G2 ∪ U>.  It is shown that the subgroups closed in the function topology on  S  lie in precisely four equivalence classes under this relation.  Which of these classes a closed subgroup  G  belongs to depends on which of the following statements about pointwise stabilizer subgroups  G(Γ)  of finite subsets  Γ ⊆ Ω  holds:

(i) For every finite set  Γ,  the subgroup  G(Γ)  has at least one infinite orbit in  Ω.
(ii) There exist finite sets  Γ  such that all orbits of  G(Γ)  are finite, but none for which the cardinalities of such orbits have a common finite bound.
(iii) There exist finite sets  Γ  such that the cardinalities of the orbits of  G(Γ)  have a common finite bound, but none such that  G(Γ) = {1}.
(iv) There exist finite sets  Γ  such that  G(Γ) = {1}.

Some related results and topics for further investigation are noted.

MR 2008a:20005DOI.  preprint:  texdvipsBack to publications-list

Generating infinite symmetric groups

Bull. London Math. Soc., 38 (2006) 429-440.

Let  S = Sym(Ω)  be the group of all permutations of an infinite set  Ω.  Extending an argument of Macpherson and Neumann, it is shown that if  U  is a generating set for  S  as a group, respectively as a monoid, then there exists a positive integer  n  such that every element of  S  may be written as a group word, respectively a monoid word, of length  ≤ n  in the elements of  U

Some related questions and recent results by others are noted, and a brief proof is given of a result of Ore's on commutators that is used in the proof of the above result.

MR 2007e:20004.  DOI.  galley proofs:  pdf.  preprint:  texpsdvi.  Back to publications-list

Can one factor the classical adjoint of a generic matrix?

Transformation Groups, 11 (2006) 7-15.

Let  k  be an integral domain,  n  a positive integer,  X  a generic  n×n  matrix over  k  (i.e., the matrix  (xij)  over a polynomial ring  k[xij]  in  n2  indeterminates  xij),  and  adj(X)  its classical adjoint.  For char k = 0  it is shown that if  n  is odd,  adj(X)  is not the product of two noninvertible  n×n  matrices over  k[xij],  while for  n  even, only one special sort of factorization occurs.  Whether the corresponding results hold in positive characteristic is not known.

The operation  adj  on matrices arises from the (n − 1)-st exterior power functor on modules; the analogous factorization question for matrix constructions arising from other functors is raised, as are several other questions.

MR 2006k:15037DOI.  preprint:  texdvipdfps.  galley proofs: pdfBack to publications-list