Locally Closed Semirings and Iteration Semirings

Locally closed semirings, iteration semirings and Conway semirings play an important role in the algebraic theory of semirings and theoretical computer science. Z. ésik and W. Kuich showed that a locally closed commutative semiring is an iteration semiring (is also a Conway semiring). By study of polynomial semirings and matrix semirings, we obtain new expressions of certain polynomials and show that all matrix semirings over a locally closed semiring are also locally closed, and so a locally closed semiring (which need not be commutative) is an iteration semiring.     

Approximation of singular series and automata

A constant of the form , where the product ranges over all sufficiently large primes p and h is rational, is an example of a singular series. We show that this type of singular series can be expanded in the form , where ζ denotes the zeta-function and e _k is an integer and use this to numerically approximate them. Gerhard Niklasch in an appendix describes how to obtain more than 1000 decimal accuracy. In some cases the coefficients $e_k$ turn out to be related to conjugacy classes of primitive words in cyclic languages. Received: 12 April 1999 / Revised version: 23 October 1999     

On Polynomials Over Rickart Rings and Their Generalizations I

This paper continues the investigation of polynomials and formal power series over a ring with various annihilator conditions which were originally used by Rickart and Kaplansky to abstract the algebraic properties of von Neumann algebras. Results of Armendariz on polynomial rings over a PP ring are extended to analogous annihilator conditions in nearrings of polynomials and nearrings of formal power series. These results are somewhat striking since, in contrast to the polynomial ring case, the nearring of polynomials or formal power series has substitution for its “multiplication” operation. These investigations provide an alternative viewpoint in illustrating the structure of polynomials and formal power series. Extensions of Rickart rings to formal power series rings are also discussed. The author was partially supported by the National Science Council, Taiwan under the grant number NSC 93-2115-M-143-001.     

Growth Series and Random Walks on Some Hyperbolic Graphs

 Consider the tessellation of the hyperbolic plane by m-gons, ℓ per vertex. In its 1-skeleton, we compute the growth series of vertices, geodesics, tuples of geodesics with common extremities. We also introduce and enumerate holly trees, a family of proper loops in these graphs. We then apply Grigorchuk’s result relating cogrowth and random walks to obtain lower estimates on the spectral radius of the Markov operator associated with a symmetric random walk on these graphs. Received 19 September 2001; in revised form 23 December 2001     

<Emphasis Type="Italic">P</Emphasis>≠<Emphasis Type="Italic">NP</Emphasis> for Infinite Time Turing Machines

We state different versions of the P=?NP problem for infinite time Turing machines. It is observed that PNP collapses to the fact that there are analytic sets which are not Borel.The author thanks J. D. Hamkins, R. Schipperus, and P. Welch for comments on earlier versions of this paper. He thanks Sanders Seliverstov for pointing out an inaccuracy in the first version of the proof of Theorem 2.10.Received April 19, 2002; in revised form June 10, 2002 Published online May 16, 2003     

Annihilator Conditions on Polynomials II

In this paper, we continue the study of various annihilator conditions which were used by Rickart and Kaplansky to abstract the algebraic properties of von Neumann algebras. In our main results, we extend results of Armendariz on the Baer and p.p. conditions in a polynomial ring to certain analogous annihilator conditions in a nearring of polynomials. These results are somewhat surprising since, in contrast to the polynomial ring case, the nearring of polynomials has substitution for its multiplication operation. Moreover they indicate connections between the ring and nearring structures on polynomials. Examples are provided to illustrate and delimit our results.This revised version was published online in October 2004 with a corrected Received date.The second author was partially supported by the National Science Council of the Republic of China, Taiwan under grant number NSC 90-2115-M-143-001.     

On some equations in prime rings

The main purpose of this paper is to prove the following result. Let R be a prime ring of characteristic different from two and let T : R → R be an additive mapping satisfying the relation T(x ³) = T(x)x ² − xT(x)x + x ² T(x) for all x ∈ R. In this case T is of the form 4T(x) = qx + xq, where q is some fixed element from the symmetric Martindale ring of quotients. This result makes it possible to solve some functional equations in prime rings with involution which are related to bicircular projections.     

Recursive events in random sequences

Let ω be a Kolmogorov–Chaitin random sequence with ω_{1: n} denoting the first n digits of ω. Let P be a recursive predicate defined on all finite binary strings such that the Lebesgue measure of the set {ω|∃nP(ω_{1: n} )} is a computable real α. Roughly, P holds with computable probability for a random infinite sequence. Then there is an algorithm which on input indices for any such P and α finds an n such that P holds within the first n digits of ω or not in ω at all. We apply the result to the halting probability Ω and show that various generalizations of the result fail. Received: 1 December 1998 / Published online: 3 October 2001     

The Expected Volume of a Tetrahedron whose Vertices are Chosen at Random in the Interior of a Cube

We calculate E[V ₄(C)], the expected volume of a tetrahedron whose vertices are chosen randomly (i.e. independently and uniformly) in the interior of C, a cube of unit volume. We find

Nonlinear Free Boundary Problems with Singular Source Terms

We prove the existence of solutions to nonlinear free boundary problem with singularities at given points.     

A Local Limit Theorem on Certain p-AdicGroups and Buildings

 We present a local limit theorem for a measure on the special linear group with entries in a local field. (Received 20 March 2000; in revised form 8 September 2000)     

Cut elimination for a calculus with context-dependent rules

Context-dependent rules are an obstacle to cut elimination. Turning to a generalised sequent style formulation using deep inferences is helpful, and for the calculus presented here it is essential. Cut elimination is shown for a substructural, multiplicative, pure propositional calculus. Moreover we consider the extra problems induced by non-logical axioms and extend the results to additive connectives and quantifiers. Received: 11 April 1998 / Published online: 25 January 2001     

Cubic Fields with Index 2

 Let d be a squarefree integer with allowed. If (mod 8) it is shown that there do not exist any cubic fields with index 2 whose splitting field contains . If (mod 8) it is shown that there exist infinitely many cubic fields with index 2 and minimal index 2 whose splitting field contains . (Received 23 May 2001)     

Extremely noncommutative elements in rings

We study rings and K-algebras in which all elements or all noncentral elements have smallest possible centralizer. Our principal result asserts that a ring R must be either finite or commutative if each noncentral element a has centralizer equal to the subring generated by a. Supported by the Natural Sciences and Engineering Research Council of Canada, Grant 3961. Authors’ addresses: Howard E. Bell, Department of Mathematics, Brock University, St. Catharines, Ontario, Canada L2S 3A1; Abraham A. Klein, Sackler Faculty of Exact Sciences, School of Mathematical Sciences, Tel Aviv University, Tel Aviv 69978, Israel     

Almost Kähler-Einstein Structures on 8-Dimensional Walker Manifolds

We study almost K?hler-Einstein structures on 8-dimensional Walker manifolds, i.e., pseudo-Riemannian 8-manifolds admitting a field of parallel null 4-planes, whence the metric is of neutral signature. We construct on explicit almost K?hler-Einstein Walker metrics which are not K?hler. An appropriate restriction induces examples of such metrics on the 8-torus, thereby producing a counterexample to Goldberg’s conjecture in the case of neutral signature. S. Haze passed away on 15 March 2006.     

Class-Preserving Coleman Automorphisms of Finite Groups

 Let G be a finite group whose Sylow 2-subgroups are either cyclic, dihedral, or generalized quaternion. It is shown that a class-preserving automorphism of G of order a power of 2 whose restriction to any Sylow subgroup of G equals the restriction of some inner automorphism of G is necessarily an inner automorphism. Interest in such automorphisms arose from the study of the isomorphism problem for integral group rings, see [6, 7, 13, 14]. Received 30 September 2001; in revised form 10 December 2001     

A Remark about Peck’s Method in Positive Characteristic

 In this short paper, we study Peck’s method in positive characteristic. Curiously enough, the simultaneous approximations this method provides are better than in the real field. (Received 11 January 2001; in revised form 26 March 2001)