Cartan determinants for gentle algebras

The determinant of the Cartan matrix of a finite dimensional algebra is an invariant of the derived category and can be very helpful for derived equivalence classifications. In this paper we determine the determinants of the Cartan matrices for all gentle algebras. This is a class of algebras of tame representation type which occurs naturally in various places in representation theory. The definition of these algebras is of a purely combinatorial nature, and so are our formulae for the Cartan determinants.Received: 29 October 2004     

A torsion-free abelian group of finite rank exists whose quotient group modulo the square subgroup is not a nil-group

The first example of a finite rank torsion-free abelian group A such that the quotient group of A modulo the square subgroup of A is not a nil-group is indicated (in both cases of associative and general rings). In particular, the answer to the question posed by A.E. Stratton and M.C. Webb in [18], Abelian groups, nil modulo a subgroup, need not have nil quotient group, Publ. Math. Debrecen. 27 (1980), 127–130, is given for finite rank torsion-free groups. A relationship between nontrivial p-pure subgroups of the additive group of p-adic integers and nontrivial ? [p^?1]-submodules of the field of p-adic numbers is investigated. In particular, a bijective correspondence between these structures is proven using only elementary methods.     

Homomorphisms of matrix semigroups over division rings from dimension two to four

Let D be an arbitrary division ring and M_n(D) the multiplicative semigroup of all n×n matrices over D. We study non-degenerate, injective homomorphisms from M₂(D) to M₄(D). In particular, we present a structural result for the case when D is the ring of quaternions.     

Mesh geometries of root orbits of integral quadratic forms

Integral quadratic forms q:Zⁿ→Z, with n≥1, and the sets R_q(d)={v∈Zⁿ;q(v)=d}, with d∈Z, of their integral roots are studied by means of mesh translation quivers defined by Z-bilinear morsifications b_A:Zⁿ×Zⁿ→Z of q, with Z-regular matrices A∈M_n(Z). Mesh geometries of roots of positive definite quadratic forms q:Zⁿ→Z are studied in connection with root mesh quivers of forms associated to Dynkin diagrams A_n,D_n,E₆,E₇,E₈ and the Auslander-Reiten quivers of the derived category D^b(R) of path algebras R=KQ of Dynkin quivers Q. We introduce the concepts of a Z-morsification b_A of a quadratic form q, a weighted Φ_A-mesh of vectors in Zⁿ, and a weighted Φ_A-mesh orbit translation quiver Γ(R_q,Φ_A) of vectors in Zⁿ, where R_q?R_q(1) and Φ_A:Zⁿ→Zⁿ is the Coxeter isomorphism defined by A. The existence of mesh geometries on R_q is discussed. It is shown that, under some assumptions on the morsification b_A:Zⁿ×Zⁿ→Z, the set admit a Φ_A-orbit mesh quiver , where Φ_A:Zⁿ→Zⁿ is the Coxeter isomorphism defined by A. Moreover, splits into three infinite connected components , , and , where are isomorphic to a translation quiver Z⋅Δ, with Δ an extended Dynkin quiver, and has the shape of a sand-glass tube.     

Orbit semigroups and the representation type of quivers

We show that a finite, connected quiver Q without oriented cycles is a Dynkin or Euclidean quiver if and only if all orbit semigroups of representations of Q are saturated.     

Constructing indecomposable representations over quantum group ${\cal U}_q(sl(2))$

In this article, we discuss the infinite dimensional indecomposable Harish-Chandra representations over ${\cal U}_q(sl(2))$. As an application, we answer a question of Ringel on the annihilator ideals of simple modules.     

A note on Cartan matrices for symmetric groups

Using generating functions, a very simple explicit formula for the determinants of the p-Cartan matrices of symmetric groups is given. Our method works also when p is a composite number.Received: 5 September 2001     

Submodule categories of wild representation type

Let Λ be a commutative local uniserial ring with radical factor field k. We consider the category S(Λ) of embeddings of all possible submodules of finitely generated Λ-modules. In case Λ=Z/〈pⁿ〉, where p is a prime, the problem of classifying the objects in S(Λ), up to isomorphism, has been posed by Garrett Birkhoff in 1934. In this paper we assume that Λ has Loewy length at least seven. We show that S(Λ) is controlled k-wild with a single control object I∈S(Λ). It follows that each finite dimensional k-algebra can be realized as a quotient End(X)/End(X)_I of the endomorphism ring of some object X∈S(Λ) modulo the ideal End(X)_I of all maps which factor through a finite direct sum of copies of I.     

Multiplicative preprojective algebras, middle convolution and the Deligne-Simpson problem

We introduce a family of algebras which are multiplicative analogues of preprojective algebras, and their deformations, as introduced by M.P. Holland and the first author. We show that these algebras provide a natural setting for the ‘middle convolution’ operation introduced by N.M. Katz in his book ‘Rigid local systems’, and put in an algebraic setting by M. Dettweiler and S. Reiter, and H. Völklein. We prove a homological formula relating the dimensions of Hom and Ext spaces, study varieties of representations of multiplicative preprojective algebras, and use these results to study simple representations. We apply this work to the Deligne-Simpson problem, obtaining a sufficient (and conjecturally necessary) condition for the existence of an irreducible solution to the equation A₁A₂…A_k=1 with the A_i in prescribed conjugacy classes in GL_n(C).     

The reciprocal of the weighted geometric mean of many positive numbers is a Stieltjes function

In the paper, by the Cauchy integral formula in the theory of complex functions, an integral representation for the reciprocal of the weighted geometric mean of many positive numbers is established. As a result, the reciprocal of the weighted geometric mean of many positive numbers is verified to be a Stieltjes function and, consequently, a (logarithmically) completely monotonic function. Finally, as applications of the integral representation, in the form of remarks, several integral formulas for a kind of improper integrals are derived, an alternative proof of the famous inequality between the weighted arithmetic and geometric means is supplied, and two explicit formulas for the large Schröder numbers are discovered.     

Factorization of the Non-Stationary Schrödinger Operator

We consider a factorization of the non-stationary Schr?dinger operator based on the parabolic Dirac operator introduced by Cerejeiras, K?hler and Sommen. Based on the fundamental solution for the parabolic Dirac operators, we shall construct appropriated Teodurescu and Cauchy-Bitsadze operators. Afterwards we will describe how to solve the nonlinear Schr?dinger equation using Banach fixed point theorem.     

The automorphism-order in finite groups

The notion of automorphism-order is introduced as a generalization of elemental order in finite groups. Some theorems involving orders of elements are then generalized. Divisibility properties involving this concept are considered. Necessary and sufficient conditions for an abelian group to be represented by number-theoretic functions involving divisibility properties are given. Explicit formulas of these functions are also given.     

A matrix problem over a central quadratic skew field extension

Let F⊂G=F(u) be a central quadratic skew field extension (such that the generator u is central in G) and a natural (G,G)-bimodule. We deal with the matrix problem on finding a canonical form for rectangular matrices over W with help of left elementary transformations of their rows and right elementary transformations of columns over G. We solve this problem reducing it in the separable (resp. inseparable) case to the semilinear (resp. pseudolinear) pencil problem.     

Characterizations of Positive Linear Volterra Integro-differential Systems

We first give a criterion for positivity of the solution semigroup of linear Volterra integro-differential systems. Then, we offer some explicit conditions under which the solution of a positive linear Volterra system is exponentially stable or (robustly) lies in L²[0,+∞). The first and last author are supported by the Japan Society for Promotion of Science (JSPS) ID No. P 05049.