The Anderson-Badawi conjecture for commutative algebras over infinite fields

In [1], Anderson and Badawi conjecture that every n-absorbing ideal of a commutative ring is strongly n-absorbing. In this article we prove their conjecture in certain cases (in particular this is the case for commutative algebras over an infinite field). We also show that an affirmative answer to another conjecture in [1] implies the Anderson-Badawi Conjecture.     

Problems in algebra inspired by universal algebraic geometry

Let Θ be a variety of algebras. In every variety Θ and every algebra H from Θ one can consider algebraic geometry in Θ over H. We also consider a special categorical invariant K _Θ of this geometry. The classical algebraic geometry deals with the variety Θ = Com-P of all associative and commutative algebras over the ground field of constants P. An algebra H in this setting is an extension of the ground field P. Geometry in groups is related to the varieties Grp and Grp-G, where G is a group of constants. The case Grp-F, where F is a free group, is related to Tarski’s problems devoted to logic of a free group. The described general insight on algebraic geometry in different varieties of algebras inspires some new problems in algebra and algebraic geometry. The problems of such kind determine, to a great extent, the content of universal algebraic geometry. For example, a general and natural problem is: When do algebras H ₁ and H ₂ have the same geometry? Or more specifically, what are the conditions on algebras from a given variety Θ that provide the coincidence of their algebraic geometries? We consider two variants of coincidence: 1) K _Θ(H ₁) and K _Θ(H ₂) are isomorphic; 2) these categories are equivalent. This problem is closely connected with the following general algebraic problem. Let Θ⁰ be the category of all algebras W = W(X) free in Θ, where X is finite. Consider the groups of automorphisms Aunt(Θ⁰) for different varieties Θ and also the groups of autoequivalences of Θ⁰. The problem is to describe these groups for different Θ.     

Bounding the number of bases of a matroid

Letb(M) andc(M), respectively, be the number of bases and circuits of a matroidM. For any given minor closed class? of matroids, the following two questions, are investigated in this paper. (1) When is there a polynomial functionp(x) such thatb(M)≤p(c(m)|E(M)|) for every matroidM in?? (2) When is there a polynomial functionp(x) such thatb(M)≤p(|E(M)|) for every matroidM in?? Let us denote byM _Mn the direct sum ofn copies ofU _1,2. We prove that the answer to the first question is affirmative if and only if someM _Mn is not in?. Furthermore, if all the members of? are representable over a fixed finite field, then we prove that the answer to the second question is affirmative if and only if, also, someM _Mn is not in?.     

Edge-bipancyclicity of a hypercube with faulty vertices and edges

A bipartite graph G=(V,E) is said to be bipancyclic if it contains a cycle of every even length from 4 to |V|. Furthermore, a bipancyclic G is said to be edge-bipancyclic if every edge of G lies on a cycle of every even length. Let F_v (respectively, F_e) be the set of faulty vertices (respectively, faulty edges) in an n-dimensional hypercube Q_n. In this paper, we show that every edge of Q_n-F_v-F_e lies on a cycle of every even length from 4 to 2ⁿ-2|F_v| even if |F_v|+|F_e|?n-2, where n?3. Since Q_n is bipartite of equal-size partite sets and is regular of vertex-degree n, both the number of faults tolerated and the length of a longest fault-free cycle obtained are worst-case optimal.     

On n-clone extensions of algebras

We consider algebras of a given type with a set F of fundamental operation symbols and without nullary operations. In this paper we generalize notions and results of [12]. An identity is called clone compatible if and are the same variable or the sets of fundamental operation symbols in and are nonempty and identical. In connection with these identities we define in section 1 a construction called an n-clone extension of an algebra for where n is an integer and we study its properties. For a variety V we denote by V ^c the variety defined by all clone compatible identities from Id (V). We also define a variety V ^c,n called the n-clone extension of V. These two varieties are strictly connected. In section 2 under some assumptions we give representations of algebras from V ^c,n and V ^c using n-clone extensions of algebras from V. We also find equational bases of these varieties. In section 3 we apply these results to some important varieties. In section 4 we find minimal generics of V ^c when V is the variety of distributive lattices or the variety of Boolean algebras. Received November 27, 1996; accepted in final form March 19, 1998.     

Graded tensor products

We study the polynomial identities of regular algebras, introduced in [A. Regev, T. Seeman, Z₂-graded tensor products of P.I. algebras, J. Algebra 291 (2005) 274-296]. For example, a finite-dimensional algebra is regular if it has a basis whose multiplication table satisfies some commutation relations. The matrix algebra M_n(F) over the field F is regular, which is closely related to M_n(F) being Z_n-graded. We study the polynomial identities of various types of tensor products of such algebras. In particular, using the theory of Hopf algebras, we prove a far reaching extension of the A⊗B theorem for Z₂-graded PI algebras.     

VERTEX-FAULT-TOLERANT CYCLES EMBEDDING ON ENHANCED HYPERCUBE NETWORKS

In this paper, we study the enhanced hypercube, an attractive variant of the hypercube and obtained by adding some complementary edges from a hypercube, and focus on cycles embedding on the enhanced hypercube with faulty vertices. Let Fu be the set of faulty vertices in the n-dimensional enhanced hypercube Qn,k （n ≥ 3, 1 ≤ k 〈≤n - 1）. When IFvl = 2, we showed that Qn,k - Fv contains a fault-free cycle of every even length from 4 to 2n - 4 where n （n ≥ 3） and k have the same parity; and contains a fault-free cycle of every even length from 4 to 2n - 4, simultaneously, contains a cycle of every odd length from n-k ＋ 2 to 2^n-3 where n （≥ 3） and k have the different parity. Furthermore, when ｜Fv｜ = fv ≤ n - 2, we prove that there exists the longest fault-free cycle, which is of even length 2^n - 2fv whether n （n ≥ 3） and k have the same parity or not; and there exists the longest fault-free cycle, which is of odd length 2^n - 2fv ＋ 1 in Qn,k - Fv where n （≥ 3） and k have the different parity.     

The elementary equivalence of free algebras in cantor varieties

We study properties of free algebras in the Cantor varieties C_m,n. A free algebra of rank r in C_m,n is denoted F_{C m,n}(r). We argue that the following hold: (1) any two C_m,n-free algebras F_{C m,n}(r) and F_{C m,n}(s) of ranks r and s, where r and s are arbitrary (finite or infinite) cardinals, r≥m, and s≥m, are elementary equivalent; (2) any two C_m,n-free algebras F_{C m,n}(r) and F_{C m,n}(s) of ranks r and s, where r and s are arbitrary (finite or infinite) cardinals, are universally equivalent, that is, share one ∀-theory; (3) an elementary theory Th(F_{C m,n}(r)) for an arbitrary C_m,n-free algebra of (finite or infinite) rank r, treated in a signature Ω, is decidable; (4) an elementary theory Th(K) for an arbitrary nonempty class of free algebras in C_m,n, treated in a signature Ω, is decidable. Translated fromAlgebra i Logika, Vol. 38, No. 2, pp. 228–248, March–April, 1999.     

Three theorems on common splitting fields of central simple algebras

LetA ₁, …A _n be central simple algebras over a fieldF. Suppose that we possess information on the Schur indexes of some tensor products of (some tensor powers of) the algebras. What can be said (in general) about possible degrees of finite field extensions ofF splitting the algebras? In Part I, we prove a positive result of that kind. In Part II, we prove a negative result. In Part III, we develop a general approach.     

Clones satisfying the term condition

The term condition considered here is the property of an operation ? that holds iff ? and all of its variants obtained by permuting the variables satisfy (for all x,y,u₁,…v₁,…)$\text{?(x,u}{}_1\text{,…) = ?(x,v}{}_1\text{…)??(y,u}{}_1\text{,…) = ?(y,v)}{}_1\text{,…)}$. Clones consisting entirely of operations satisfying this term condition are called TC clones; algebras whose clone of term operations is a TC clone are called TC algebras; varieties such that every algebra in the variety is a TC algebra are called TC varieties. The paper is a systematic study of these notions, giving primary attention to operations and algebras on finite base sets, and to varieties generated by finite algebras. It is proved, among other results, that the number of n-ary TC operations on a k-element set is logarithmically asymptotic to k^(k?1)n when n increases without bound and k is held fixed; that there exist only countably many TC clones on any finite set; that the maximal TC clones on a finite set are finite in number (for each set). Some necessary conditions for an algebra to generate a TC variety are given, also some sufficient conditions.     

An algorithm for constructing a variety of arbitrary finite dimension

The dimension of a variety V of algebras is the greatest length of a basis (i.e., of an independent generating set) for an SC-theory SC(V), consisting of strong Mal'tsev conditions satisfied in V. The variety V is assumed infinite-dimensional if the lengths of the bases in SC(V) are not bounded. A simple algorithm is found for constructing a variety of any finite dimension r≥1. Using the sieve of Eratosthenes, r distinct primes p₁, p₂,…, p_r are written and their product n=p₁p₂…p_r computed. The variety G_n of algebras (A, f) with one n-ary operation satisfying the identity f(x₁, x₂,…,x_n)=f(x₂,…,x_n, x₁) has, then, dimension r. Translated fromAlgebra i Logika, Vol. 37, No. 2, pp. 167–180, March–April, 1998.     

Poisson algebras of polynomial growth

Consider the sequence c _n (V) of codimensions of a variety V of Poisson algebras. We show that the growth of every variety V of Poisson algebras over an arbitrary field is either bounded by a polynomial or at least exponential. Furthermore, if the growth of V is polynomial then there is a polynomial R(x) with rational coefficients such that c _n (V) = R(n) for all sufficiently large n. We present lower and upper bounds for the polynomials R(x) of an arbitrary fixed degree. We also show that the varieties of Poisson algebras of polynomial growth are finitely based in characteristic zero.     

Non-embeddable simple relation algebras

This paper presents solutions or partial solutions for several problems in the theory of relation algebras. In a simple relation algebra an element x satisfying the condition (a) must be an atom of . It follows that x must also be an atom in every simple extension of . Andréka, Jónsson and Németi [1, Problem 4] (see [12, Problem P5]) asked whether the converse holds: if x is an atom in every simple extension of a simple relation algebra, must it satisfy (a)? We show that the answer is “no”.? The only known examples of simple relation algebras without simple proper extensions are the algebras of all binary relations on a finite set. Jónsson proposed finding all finite simple relation algebras without simple proper extensions [12, Problem P6]. We show how to construct many new examples of finite simple relation algebras that have no simple proper extensions, thus providing a partial answer for this second problem. These algebras are also integral and nonrepresentable.? Andréka, Jónsson, Németi [1, Problem 2] (see [12, Problem P7]) asked whether there is a countable simple relation algebra that cannot be embedded in a one-generated relation algebra. The answer is “yes”. Givant [3, Problem 9] asked whether there is some k such that every finitely generated simple relation algebra can be embedded in a k-generated simple relation algebra. The answer is “no”. Received November 27, 1996; accepted in final form July 3, 1997.     

Self-similar fractals: An algorithmic point of view

This paper studies the self-similar fractals with overlaps from an algorithmic point of view.A decidable problem is a question such that there is an algorithm to answer"yes"or"no"to the question for every possible input.For a classical class of self-similar sets{E b.d}b,d where E b.d=Sn i=1(E b,d/d+b i)with b=(b1,...,b n)∈Qn and d∈N∩[n,∞),we prove that the following problems on the class are decidable:To test if the Hausdorff dimension of a given self-similar set is equal to its similarity dimension,and to test if a given self-similar set satisfies the open set condition(or the strong separation condition).In fact,based on graph algorithm,there are polynomial time algorithms for the above decidable problem.     

Integral closure of invariant ideals, toroidal resolution, and equivariant vector bundles

We establish a connection between equivariant integrally closed ideal sheaves on a G-fibration Y over a G-spherical variety X with an affine fiber V and equivariant vector bundles on the universal toroidal resolution of X. As an application, we reduce the study of invariant integrally closed ideals of V×X to that of some smaller variety in the case of X=M_n,m. Moreover, we present an affirmative answer to a problem raised by Michel Brion [Comment. Math. Helv. 66 (1991) 237-262] for two special infinite series.     

Essential variables in hypersubstitutions

For a variety V of algebras of type , we consider the set M _i (V) of all hypersubstitutions such that the variable x _i is essential in the term (f) with respect to the variety V. We will give a complete answer to the question for which varieties V of type = (n) the set M _i (v) of hypersubstitutions forms a monoid. This is important since to every monoid of hypersubstitutions there corresponds a complete sublattice of the lattice of all varieties of algebras of the given type. For varieties of semigroups we get the monoid of all leftmost and all rightmost hypersubstitutions. Received December 2, 1998; accepted in final form December 18, 2000.     

Dimensions of Cantor and post varieties

A dimension of a finitely based variety V of algebras is the greatest length of a basis (that is, an independent generating set) for the SC-theory SC(V) with the strong Mal'tsev conditions satisfied in V. A dimension is said to be infinite if the lengths of bases in SC(V) are unbounded. We prove that the dimension of a Cantor variety C_m,n in the general form, i.e., with n>m≥1, is infinite. The algorithm of constructing a basis of any given length in SC(C_m,n) is presented. By contrast, any Post variety P_n generated by a primal algebra of order n>1 is shown to have a finite dimension not exceeding the number of maximal subalgebras in the iterative Post algebra over the set {0,1,…,n−1}. Specifically, the dimension of the variety of Boolean algebras is at most four. Translated fromAlgebra i Logika, Vol. 35, No. 3, pp. 359–369, May–June, 1996.     

Algebras Defined from Ordered Sets and the Varieties they Generate

We investigate ways of representing ordered sets as algebras and how the order relation is reflected in the algebraic properties of the variety (equational class) generated by these algebras. In particular we consider two different but related methods for constructing an algebra with one binary operation from an arbitrary ordered set with a top element. The two varieties generated by all these algebras are shown to be well-behaved in that they are locally finite, finitely based, and have an equationally definable order relation. We exhibit a bijection between the subdirectly irreducible algebras in each variety and the class of all ordered sets with top element. We determine the structure and cardinality of the free algebra on n-free generators and provide sharp bounds on the number of n-generated algebras in each variety. These enumeration results involve the number of quasi-orders on an n-element set.     

Identities in algebras with involution

One of the main features of the theory of polynomial identities is the existence (for anyn) of a division algebra of degreen, formed by adjoining quotients of central elements of the algebra of genericn×n matrices; this division algebra is extremely interesting and has been used by Amitsur (forn divisible by either 8 or the square of an odd prime) as an example of a non-crossed product central division algebra. The main object of this paper is to obtain, in a parallel method, division algebras with involution of the first kind, knowledge of which would answer some long-standing questions in the theory of division algebras with involution. One such question is, “Is every division algebra with involution of the first kind a tensor product of quaternion division algebras?” In the process, a theory of (polynomial) identities in algebras with involution is developed with emphasis on prime PI-algebras with involution.