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1. CJM 2018 (vol 70 pp. 961)

Ara, Pere; Claramunt, Joan
 Uniqueness of the von Neumann Continuous Factor For a division ring $D$, denote by $\mathcal M_D$ the $D$-ring obtained as the completion of the direct limit $\varinjlim_n M_{2^n}(D)$ with respect to the metric induced by its unique rank function. We prove that, for any ultramatricial $D$-ring $\mathcal B$ and any non-discrete extremal pseudo-rank function $N$ on $\mathcal B$, there is an isomorphism of $D$-rings $\overline{\mathcal B} \cong \mathcal M_D$, where $\overline{\mathcal B}$ stands for the completion of $\mathcal B$ with respect to the pseudo-metric induced by $N$. This generalizes a result of von Neumann. We also show a corresponding uniqueness result for $*$-algebras over fields $F$ with positive definite involution, where the algebra $\mathcal M_F$ is endowed with its natural involution coming from the $*$-transpose involution on each of the factors $M_{2^n}(F)$. Keywords:rank function, von Neumann regular ring, completion, factor, ultramatricialCategories:16E50, 16D70

2. CJM Online first

Geroldinger, Alfred; Zhong, Qinghai
 Long sets of lengths with maximal elasticity We introduce a new invariant describing the structure of sets of lengths in atomic monoids and domains. For an atomic monoid $H$, let $\Delta_{\rho} (H)$ be the set of all positive integers $d$ which occur as differences of arbitrarily long arithmetical progressions contained in sets of lengths having maximal elasticity $\rho (H)$. We study $\Delta_{\rho} (H)$ for transfer Krull monoids of finite type (including commutative Krull domains with finite class group) with methods from additive combinatorics, and also for a class of weakly Krull domains (including orders in algebraic number fields) for which we use ideal theoretic methods. Keywords:transfer Krull monoid, weakly Krull monoid, set of length, elasticityCategories:13A05, 13F05, 16H10, 16U30, 20M13

3. CJM 2018 (vol 70 pp. 868)

Ivorra, Florian; Yamazaki, Takao
 Nori Motives of Curves with Modulus and Laumon $1$-motives Let $k$ be a number field. We describe the category of Laumon $1$-isomotives over $k$ as the universal category in the sense of Nori associated with a quiver representation built out of smooth proper $k$-curves with two disjoint effective divisors and a notion of $H^1_\mathrm{dR}$ for such "curves with modulus". This result extends and relies on the theorem of J. Ayoub and L. Barbieri-Viale that describes Deligne's category of $1$-isomotives in terms of Nori's Abelian category of motives. Keywords:motive, curve with modulus, quiver representationCategories:19E15, 16G20, 14F42

4. CJM 2017 (vol 70 pp. 773)

Du, Jie; Zhao, Zhonghua
 Multiplication Formulas and Canonical Bases for Quantum Affine $gl_n$ We will give a representation-theoretic proof for the multiplication formula in the Ringel-Hall algebra $\mathfrak{H}_\Delta(n)$ of a cyclic quiver $\Delta(n)$. As a first application, we see immediately the existence of Hall polynomials for cyclic quivers, a fact established by J. Y. Guo and C. M. Ringel, and derive a recursive formula to compute them. We will further use the formula and the construction of a certain monomial base for $\mathfrak{H}_\Delta(n)$ given by Deng, Du, and Xiao together with the double Ringel--Hall algebra realisation of the quantum loop algebra $\mathbf{U}_v(\widehat{\mathfrak{g}\mathfrak{l}}_n)$ given by Deng, Du, and Fu to develop some algorithms and to compute the canonical basis for $\mathbf{U}_v^+(\widehat{\mathfrak{g}\mathfrak{l}}_n)$. As examples, we will show explicitly the part of the canonical basis associated with modules of Lowey length at most $2$ for the quantum group $\mathbf{U}_v(\widehat{\mathfrak{g}\mathfrak{l}}_2)$. Keywords:Ringel-Hall algebra, quantum group, cyclic quiver, monomial basis, canonical basisCategories:16G20, 20G42

5. CJM 2015 (vol 69 pp. 408)

Klep, Igor; Špenko, Špela
 Free Function Theory Through Matrix Invariants This paper concerns free function theory. Free maps are free analogs of analytic functions in several complex variables, and are defined in terms of freely noncommuting variables. A function of $g$ noncommuting variables is a function on $g$-tuples of square matrices of all sizes that respects direct sums and simultaneous conjugation. Examples of such maps include noncommutative polynomials, noncommutative rational functions and convergent noncommutative power series. In sharp contrast to the existing literature in free analysis, this article investigates free maps \emph{with involution} -- free analogs of real analytic functions. To get a grip on these, techniques and tools from invariant theory are developed and applied to free analysis. Here is a sample of the results obtained. A characterization of polynomial free maps via properties of their finite-dimensional slices is presented and then used to establish power series expansions for analytic free maps about scalar and non-scalar points; the latter are series of generalized polynomials for which an invariant-theoretic characterization is given. Furthermore, an inverse and implicit function theorem for free maps with involution is obtained. Finally, with a selection of carefully chosen examples it is shown that free maps with involution do not exhibit strong rigidity properties enjoyed by their involution-free counterparts. Keywords:free algebra, free analysis, invariant theory, polynomial identities, trace identities, concomitants, analytic maps, inverse function theorem, generalized polynomialsCategories:16R30, 32A05, 46L52, 15A24, 47A56, 15A24, 46G20

6. CJM 2015 (vol 67 pp. 1144)

Nystedt, Patrik; Öinert, Johan
 Outer Partial Actions and Partial Skew Group Rings We extend the classicial notion of an outer action $\alpha$ of a group $G$ on a unital ring $A$ to the case when $\alpha$ is a partial action on ideals, all of which have local units. We show that if $\alpha$ is an outer partial action of an abelian group $G$, then its associated partial skew group ring $A \star_\alpha G$ is simple if and only if $A$ is $G$-simple. This result is applied to partial skew group rings associated with two different types of partial dynamical systems. Keywords:outer action, partial action, minimality, topological dynamics, partial skew group ring, simplicityCategories:16W50, 37B05, 37B99, 54H15, 54H20

7. CJM 2014 (vol 67 pp. 28)

 Bounded Derived Categories of Infinite Quivers: Grothendieck Duality, Reflection Functor We study bounded derived categories of the category of representations of infinite quivers over a ring $R$. In case $R$ is a commutative noetherian ring with a dualising complex, we investigate an equivalence similar to Grothendieck duality for these categories, while a notion of dualising complex does not apply to them. The quivers we consider are left, resp. right, rooted quivers that are either noetherian or their opposite are noetherian. We also consider reflection functor and generalize a result of Happel to noetherian rings of finite global dimension, instead of fields. Keywords:derived category, Grothendieck duality, representation of quivers, reflection functorCategories:18E30, 16G20, 18E40, 16D90, 18A40

8. CJM 2014 (vol 66 pp. 902)

Levandovskyy, Viktor; Shepler, Anne V.
 Corrigendum to Example in "Quantum Drinfeld Hecke Algebras" The last example of the article contains an error which we correct. We also indicate some indices in Theorem 11.1 that were accidently transposed. Keywords:quantum/skew polynomial rings, noncommutative Groebner basesCategories:16S36, 16S35, 16S80, 16W20, 16Z05, 16E40

9. CJM 2013 (vol 66 pp. 453)

Vaz, Pedro; Wagner, Emmanuel
 A Remark on BMW algebra, $q$-Schur Algebras and Categorification We prove that the 2-variable BMW algebra embeds into an algebra constructed from the HOMFLY-PT polynomial. We also prove that the $\mathfrak{so}_{2N}$-BMW algebra embeds in the $q$-Schur algebra of type $A$. We use these results to suggest a schema providing categorifications of the $\mathfrak{so}_{2N}$-BMW algebra. Keywords:tangle algebras, BMW algebra, HOMFLY-PT Skein algebra, q-Schur algebra, categorificationCategories:57M27, 81R50, 17B37, 16W99

10. CJM 2013 (vol 66 pp. 874)

Levandovskyy, Viktor; Shepler, Anne V.
 Quantum Drinfeld Hecke Algebras We consider finite groups acting on quantum (or skew) polynomial rings. Deformations of the semidirect product of the quantum polynomial ring with the acting group extend symplectic reflection algebras and graded Hecke algebras to the quantum setting over a field of arbitrary characteristic. We give necessary and sufficient conditions for such algebras to satisfy a PoincarÃ©-Birkhoff-Witt property using the theory of noncommutative GrÃ¶bner bases. We include applications to the case of abelian groups and the case of groups acting on coordinate rings of quantum planes. In addition, we classify graded automorphisms of the coordinate ring of quantum 3-space. In characteristic zero, Hochschild cohomology gives an elegant description of the PBW conditions. Keywords:skew polynomial rings, noncommutative GrÃ¶bner bases, graded Hecke algebras, symplectic reflection algebras, Hochschild cohomologyCategories:16S36, 16S35, 16S80, 16W20, 16Z05, 16E40

11. CJM 2013 (vol 66 pp. 625)

Giambruno, Antonio; Mattina, Daniela La; Zaicev, Mikhail
 Classifying the Minimal Varieties of Polynomial Growth Let $\mathcal{V}$ be a variety of associative algebras generated by an algebra with $1$ over a field of characteristic zero. This paper is devoted to the classification of the varieties $\mathcal{V}$ which are minimal of polynomial growth (i.e., their sequence of codimensions growth like $n^k$ but any proper subvariety grows like $n^t$ with $t\lt k$). These varieties are the building blocks of general varieties of polynomial growth. It turns out that for $k\le 4$ there are only a finite number of varieties of polynomial growth $n^k$, but for each $k \gt 4$, the number of minimal varieties is at least $|F|$, the cardinality of the base field and we give a recipe of how to construct them. Keywords:T-ideal, polynomial identity, codimension, polynomial growth,Categories:16R10, 16P90

12. CJM 2013 (vol 66 pp. 481)

Aguiar, Marcelo; Mahajan, Swapneel
 On the Hadamard Product of Hopf Monoids Combinatorial structures that compose and decompose give rise to Hopf monoids in Joyal's category of species. The Hadamard product of two Hopf monoids is another Hopf monoid. We prove two main results regarding freeness of Hadamard products. The first one states that if one factor is connected and the other is free as a monoid, their Hadamard product is free (and connected). The second provides an explicit basis for the Hadamard product when both factors are free. The first main result is obtained by showing the existence of a one-parameter deformation of the comonoid structure and appealing to a rigidity result of Loday and Ronco that applies when the parameter is set to zero. To obtain the second result, we introduce an operation on species that is intertwined by the free monoid functor with the Hadamard product. As an application of the first result, we deduce that the Boolean transform of the dimension sequence of a connected Hopf monoid is nonnegative. Keywords:species, Hopf monoid, Hadamard product, generating function, Boolean transformCategories:16T30, 18D35, 20B30, 18D10, 20F55

13. CJM 2013 (vol 66 pp. 205)

Iovanov, Miodrag Cristian
 Generalized Frobenius Algebras and Hopf Algebras "Co-Frobenius" coalgebras were introduced as dualizations of Frobenius algebras. We previously showed that they admit left-right symmetric characterizations analogue to those of Frobenius algebras. We consider the more general quasi-co-Frobenius (QcF) coalgebras; the first main result in this paper is that these also admit symmetric characterizations: a coalgebra is QcF if it is weakly isomorphic to its (left, or right) rational dual $Rat(C^*)$, in the sense that certain coproduct or product powers of these objects are isomorphic. Fundamental results of Hopf algebras, such as the equivalent characterizations of Hopf algebras with nonzero integrals as left (or right) co-Frobenius, QcF, semiperfect or with nonzero rational dual, as well as the uniqueness of integrals and a short proof of the bijectivity of the antipode for such Hopf algebras all follow as a consequence of these results. This gives a purely representation theoretic approach to many of the basic fundamental results in the theory of Hopf algebras. Furthermore, we introduce a general concept of Frobenius algebra, which makes sense for infinite dimensional and for topological algebras, and specializes to the classical notion in the finite case. This will be a topological algebra $A$ that is isomorphic to its complete topological dual $A^\vee$. We show that $A$ is a (quasi)Frobenius algebra if and only if $A$ is the dual $C^*$ of a (quasi)co-Frobenius coalgebra $C$. We give many examples of co-Frobenius coalgebras and Hopf algebras connected to category theory, homological algebra and the newer q-homological algebra, topology or graph theory, showing the importance of the concept. Keywords:coalgebra, Hopf algebra, integral, Frobenius, QcF, co-FrobeniusCategories:16T15, 18G35, 16T05, 20N99, 18D10, 05E10

14. CJM 2012 (vol 64 pp. 1222)

Bobiński, Grzegorz
 Normality of Maximal Orbit Closures for Euclidean Quivers Let $\Delta$ be an Euclidean quiver. We prove that the closures of the maximal orbits in the varieties of representations of $\Delta$ are normal and Cohen--Macaulay (even complete intersections). Moreover, we give a generalization of this result for the tame concealed-canonical algebras. Keywords:normal variety, complete intersection, Euclidean quiver, concealed-canonical algebraCategories:16G20, 14L30

15. CJM 2012 (vol 65 pp. 241)

Aguiar, Marcelo; Lauve, Aaron
 Lagrange's Theorem for Hopf Monoids in Species Following Radford's proof of Lagrange's theorem for pointed Hopf algebras, we prove Lagrange's theorem for Hopf monoids in the category of connected species. As a corollary, we obtain necessary conditions for a given subspecies $\mathbf k$ of a Hopf monoid $\mathbf h$ to be a Hopf submonoid: the quotient of any one of the generating series of $\mathbf h$ by the corresponding generating series of $\mathbf k$ must have nonnegative coefficients. Other corollaries include a necessary condition for a sequence of nonnegative integers to be the dimension sequence of a Hopf monoid in the form of certain polynomial inequalities, and of a set-theoretic Hopf monoid in the form of certain linear inequalities. The latter express that the binomial transform of the sequence must be nonnegative. Keywords:Hopf monoids, species, graded Hopf algebras, Lagrange's theorem, generating series, PoincarÃ©-Birkhoff-Witt theorem, Hopf kernel, Lie kernel, primitive element, partition, composition, linear order, cyclic order, derangementCategories:05A15, 05A20, 05E99, 16T05, 16T30, 18D10, 18D35

16. CJM 2010 (vol 63 pp. 3)

Banica, T.; Belinschi, S. T.; Capitaine, M.; Collins, B.
 Free Bessel Laws We introduce and study a remarkable family of real probability measures $\pi_{st}$ that we call free Bessel laws. These are related to the free Poisson law $\pi$ via the formulae $\pi_{s1}=\pi^{\boxtimes s}$ and ${\pi_{1t}=\pi^{\boxplus t}}$. Our study includes definition and basic properties, analytic aspects (supports, atoms, densities), combinatorial aspects (functional transforms, moments, partitions), and a discussion of the relation with random matrices and quantum groups. Keywords:Poisson law, Bessel function, Wishart matrix, quantum groupCategories:46L54, 15A52, 16W30

17. CJM 2009 (vol 61 pp. 904)

Saliola, Franco V.
 The Face Semigroup Algebra of a Hyperplane Arrangement This article presents a study of an algebra spanned by the faces of a hyperplane arrangement. The quiver with relations of the algebra is computed and the algebra is shown to be a Koszul algebra. It is shown that the algebra depends only on the intersection lattice of the hyperplane arrangement. A complete system of primitive orthogonal idempotents for the algebra is constructed and other algebraic structure is determined including: a description of the projective indecomposable modules, the Cartan invariants, projective resolutions of the simple modules, the Hochschild homology and cohomology, and the Koszul dual algebra. A new cohomology construction on posets is introduced, and it is shown that the face semigroup algebra is isomorphic to the cohomology algebra when this construction is applied to the intersection lattice of the hyperplane arrangement. Categories:52C35, 05E25, 16S37

18. CJM 2009 (vol 61 pp. 315)

Enochs, E.; Estrada, S.; Rozas, J. R. Garc\'{\i}a
 Injective Representations of Infinite Quivers. Applications In this article we study injective representations of infinite quivers. We classify the indecomposable injective representations of trees and describe Gorenstein injective and projective representations of barren trees. Categories:16G20, 18A40

19. CJM 2008 (vol 60 pp. 923)

Okoh, F.; Zorzitto, F.
 Endomorphisms of Kronecker Modules Regulated by Quadratic Algebra Extensions of a Function Field The Kronecker modules $\mathbb{V}(m,h,\alpha)$, where $m$ is a positive integer, $h$ is a height function, and $\alpha$ is a $K$-linear functional on the space $K(X)$ of rational functions in one variable $X$ over an algebraically closed field $K$, are models for the family of all torsion-free rank-2 modules that are extensions of finite-dimensional rank-1 modules. Every such module comes with a regulating polynomial $f$ in $K(X)[Y]$. When the endomorphism algebra of $\mathbb{V}(m,h,\alpha)$ is commutative and non-trivial, the regulator $f$ must be quadratic in $Y$. If $f$ has one repeated root in $K(X)$, the endomorphism algebra is the trivial extension $K\ltimes S$ for some vector space $S$. If $f$ has distinct roots in $K(X)$, then the endomorphisms form a structure that we call a bridge. These include the coordinate rings of some curves. Regardless of the number of roots in the regulator, those $\End\mathbb{V}(m,h,\alpha)$ that are domains have zero radical. In addition, each semi-local $\End\mathbb{V}(m,h,\alpha)$ must be either a trivial extension $K\ltimes S$ or the product $K\times K$. Categories:16S50, 15A27

20. CJM 2008 (vol 60 pp. 379)

rgensen, Peter J\o
 Finite Cohen--Macaulay Type and Smooth Non-Commutative Schemes A commutative local Cohen--Macaulay ring $R$ of finite Cohen--Macaulay type is known to be an isolated singularity; that is, $\Spec(R) \setminus \{ \mathfrak {m} \}$ is smooth. This paper proves a non-commutative analogue. Namely, if $A$ is a (non-commutative) graded Artin--Schelter \CM\ algebra which is fully bounded Noetherian and has finite Cohen--Macaulay type, then the non-commutative projective scheme determined by $A$ is smooth. Keywords:Artin--Schelter Cohen--Macaulay algebra, Artin--Schelter Gorenstein algebra, Auslander's theorem on finite Cohen--Macaulay type, Cohen--Macaulay ring, fully bounded Noetherian algebra, isolated singularity, maximal Cohen--Macaulay module, non-commutative Categories:14A22, 16E65, 16W50

21. CJM 2008 (vol 60 pp. 266)

Bergeron, Nantel; Reutenauer, Christophe; Rosas, Mercedes; Zabrocki, Mike
 Invariants and Coinvariants of the Symmetric Group in Noncommuting Variables We introduce a natural Hopf algebra structure on the space of noncommutative symmetric functions. The bases for this algebra are indexed by set partitions. We show that there exists a natural inclusion of the Hopf algebra of noncommutative symmetric functions in this larger space. We also consider this algebra as a subspace of noncommutative polynomials and use it to understand the structure of the spaces of harmonics and coinvariants with respect to this collection of noncommutative polynomials and conclude two analogues of Chevalley's theorem in the noncommutative setting. Categories:16W30, 05A18;, 05E10

22. CJM 2007 (vol 59 pp. 1260)

Deng, Bangming; Du, Jie; Xiao, Jie
 Generic Extensions and Canonical Bases for Cyclic Quivers We use the monomial basis theory developed by Deng and Du to present an elementary algebraic construction of the canonical bases for both the Ringel--Hall algebra of a cyclic quiver and the positive part $\bU^+$ of the quantum affine $\frak{sl}_n$. This construction relies on analysis of quiver representations and the introduction of a new integral PBW-like basis for the Lusztig $\mathbb Z[v,v^{-1}]$-form of~$\bU^+$. Categories:17B37, 16G20

23. CJM 2007 (vol 59 pp. 880)

van, John E.
 Radical Ideals in Valuation Domains An ideal $I$ of a ring $R$ is called a radical ideal if $I={\mathcalR}(R)$ where ${\mathcal R}$ is a radical in the sense of Kurosh--Amitsur. The main theorem of this paper asserts that if $R$ is a valuation domain, then a proper ideal $I$ of $R$ is a radical ideal if and only if $I$ is a distinguished ideal of $R$ (the latter property means that if $J$ and $K$ are ideals of $R$ such that $J\subset I\subset K$ then we cannot have $I/J\cong K/I$ as rings) and that such an ideal is necessarily prime. Examples are exhibited which show that, unlike prime ideals, distinguished ideals are not characterizable in terms of a property of the underlying value group of the valuation domain. Categories:16N80, 13A18

24. CJM 2007 (vol 59 pp. 658)

 Division Algebras of Prime Degree and Maximal Galois $p$-Extensions Let $p$ be an odd prime number, and let $F$ be a field of characteristic not $p$ and not containing the group $\mu_p$ of $p$-th roots of unity. We consider cyclic $p$-algebras over $F$ by descent from $L = F(\mu_p)$. We generalize a theorem of Albert by showing that if $\mu_{p^n} \subseteq L$, then a division algebra $D$ of degree $p^n$ over $F$ is a cyclic algebra if and only if there is $d\in D$ with $d^{p^n}\in F - F^p$. Let $F(p)$ be the maximal $p$-extension of $F$. We show that $F(p)$ has a noncyclic algebra of degree $p$ if and only if a certain eigencomponent of the $p$-torsion of $\Br(F(p)(\mu_p))$ is nontrivial. To get a better understanding of $F(p)$, we consider the valuations on $F(p)$ with residue characteristic not $p$, and determine what residue fields and value groups can occur. Our results support the conjecture that the $p$ torsion in $\Br(F(p))$ is always trivial. Category:16K20
 Endomorphism Rings of Finite Global Dimension For a commutative local ring $R$, consider (noncommutative) $R$-algebras $\Lambda$ of the form $\Lambda = \operatorname{End}_R(M)$ where $M$ is a reflexive $R$-module with nonzero free direct summand. Such algebras $\Lambda$ of finite global dimension can be viewed as potential substitutes for, or analogues of, a resolution of singularities of $\operatorname{Spec} R$. For example, Van den Bergh has shown that a three-dimensional Gorenstein normal $\mathbb{C}$-algebra with isolated terminal singularities has a crepant resolution of singularities if and only if it has such an algebra $\Lambda$ with finite global dimension and which is maximal Cohen--Macaulay over $R$ (a noncommutative crepant resolution of singularities''). We produce algebras $\Lambda=\operatorname{End}_R(M)$ having finite global dimension in two contexts: when $R$ is a reduced one-dimensional complete local ring, or when $R$ is a Cohen--Macaulay local ring of finite Cohen--Macaulay type. If in the latter case $R$ is Gorenstein, then the construction gives a noncommutative crepant resolution of singularities in the sense of Van den Bergh. Keywords:representation dimension, noncommutative crepant resolution, maximal Cohen--Macaulay modulesCategories:16G50, 16G60, 16E99