Wang counterexamples lead to noncrossed products

Two famous counterexamples in algebra and number theory are Wang's counterexample to Grunwald's Theorem and Amitsur's noncrossed product division algebra. In this paper we use Wang's counterexample to construct a noncrossed product division algebra.

In the 30's, Grunwald's Theorem was used in the proof of a major result of class field theory, that all division algebras over number fields are (cyclic) crossed products. It is ironic that now Grunwald-Wang's Theorem is the decisive factor in a noncrossed product construction.

     

A free product of finitely generated nilpotent groups amalgamating a cycle that is not subgroup separable

We exhibit a counterexample to a recent assertion concerning the subgroup separability of groups in the title. The example also serves as a simplification of work of Gitik and Rips.

     

On the crossing number of positive knots and braids and braid index criteria of Jones and Morton-Williams-Franks

We give examples of knots with some unusual properties of the crossing number of positive diagrams or strand number of positive braid representations. In particular, we show that positive braid knots may not have positive minimal (strand number) braid representations, giving a counterpart to results of Franks-Williams and Murasugi. Other examples answer questions of Cromwell on homogeneous and (partially) of Adams on almost alternating knots.

We give a counterexample to, and a corrected version of, a theorem of Jones on the Alexander polynomial of 4-braid knots. We also give an example of a knot on which all previously applied braid index criteria fail to estimate sharply (from below) the braid index. A relation between (generalizations of) such examples and a conjecture of Jones that a minimal braid representation has unique writhe is discussed.

Finally, we give a counterexample to Morton's conjecture relating the genus and degree of the skein polynomial.

     

Artin characters,Hurwitz trees and the lifting problem

We study finite groups of automorphisms of the p-adic open disk. In particular, we generalize results of Green, Matignon and Henrio from cyclic groups of order p to arbitrary finite groups. As an application, we produce a counterexample to a question of Chinburg, Guralnick and Harbater, concerning the local lifting problem for generalized quaternion groups.     

On an example of Aspinwall and Morrison

In this paper, a family of smooth multiply-connected Calabi-Yau threefolds is investigated. The family presents a counterexample to global Torelli as conjectured by Aspinwall and Morrison.

     

A robust nonconforming -element

Finite element methods for some elliptic fourth order singular perturbation problems are discussed. We show that if such problems are discretized by the nonconforming Morley method, in a regime close to second order elliptic equations, then the error deteriorates. In fact, a counterexample is given to show that the Morley method diverges for the reduced second order equation. As an alternative to the Morley element we propose to use a nonconforming -element which is -conforming. We show that the new finite element method converges in the energy norm uniformly in the perturbation parameter.

     

A counterexample to a conjecture on flat bilinear forms

We provide a counterexample to a conjecture on the dimension of the nullity of a flat symmetric bilinear form.

     

A note on de Branges' weights

We discuss the de Branges weight systems and give a counterexample uncovering pitfalls.

     

A counterexample to a lower bound for a class of pseudodifferential operators

We present a counterexample to a possible improvement of a lower bound for a class of pseudodifferential operators with symplectic characteristic manifold.

     

On the intermediate integral for Monge-Ampère equations

Goursat showed that in the presence of an intermediate integral, the problem of solving a second-order Monge-Ampère equation can be reduced to solving a first-order equation, in the sense that the generic solution of the first-order equation will also be a solution of the original equation. An attempt by Hermann to give a rigorous proof of this fact contains an error; we show that there exists an essentially unique counterexample to Hermann's assertion and state and prove a correct theorem.

     

On a theorem of Favard

We obtain sufficient conditions for the existence of almost periodic solutions of almost periodic linear differential equations thereby extending Favard's classical theorem. These results are meant to complement previous results of the authors who have shown by means of a counterexample that the boundedness of all solutions is not, by itself, sufficient to guarantee the existence of an almost periodic solution to a linear almost periodic differential equation.

     

Towards the Eaton-Moretó conjecture on the minimal height of characters

Abstract

In this note we prove that the Eaton-Moretó conjecture holds for all blocks of finite general linear and unitary groups for all primes. Also, we show that no block of a finite quasi-simple group of classical Lie type provides a minimal counterexample to the conjecture, and so for ? > 5 no ?-block of any quasi-simple group can be a minimal counterexample.     

On the lift-off constant for elastically supported plates

In this paper we continue the study begun by Kawohl and Sweers of the precise constant at which the elastic foundation supporting a bending plate can allow lift-off in the case of downward loading. We provide a number of numerical results and a rigorous result on a different counterexample than the one suggested in Kawohl and Sweers (2002). Important open problems are summarized at the conclusion.

     

Fixed points for some non-obviously contractive operators

The use of K-normed spaces gives us the possibility to prove that a fixed point theorem due to B. Luo is equivalent to the Banach Contraction Principle. This confirms the conspiracy among fixed point theorems. Moreover the theorem of Lou is improved and extended to different contexts. A counterexample about the fixed points of the sum of a contraction and an integral operator is given. The usefulness of K-norm is tested on a Volterra integral equation as well.

     

Compact-covering maps and -networks

In this paper, we show a characterization of compact-covering -images of metric spaces and prove a theorem about them. Also we give a set theoretical assumption and under the assumption construct a counterexample which gives a negative answer to some questions.

     

A characterization of finite nonsimple groups by the set of orders of their elements

It is proved that the permutation wreath product H of a simple Suzuki group Sz(2⁷) and a subgroup fo a symmetric group of degree 23, isomorphic to a Frobenius group of order 253, is (up to isomorphism) distinguished among all finite groups by the set of orders of its elements. Since H possesses a minimal normal subgroup N that contains an element of order equal to the exponent of N, this result furnishes a counterexample to one of the conjectures set forth by Shi [1]. In addition, we show that the direct square of a group Sz(2⁷) is also distinguished by the set of orders of its elements. Supported by RFFR grant No. 96-01-01893. Translated fromAlgebra i Logika, Vol. 36, No. 3, pp. 304–322, May–June, 1997.     

A counterexample to Herzog’s Conjecture on the number of involutions

In 1979, Herzog put forward the following conjecture: if two simple groups have the same number of involutions, then they are of the same order. We give a counterexample to this conjecture.     

A note on the left and right translations in L-topological groups

In this note, we show by a counterexample that a proposition on L-homeomorphism property of the left and right translations in L-topological groups given by Bayoumi [On initial and final L-topological groups, Fuzzy Sets and Systems 156 (2005) 43–54] is incorrect, and offer its modified version.     

A counterexample to ``An extension of the Vitali-Hahn-Saks theorem' and a compactness result

We give a counterexample to ``An extension of the Vitali-Hahn-Saks theorem' and from that highlight the sharp frame within which any attempt to change the version of such an extension should be possible. Lastly a sequential compactness criterion for Radon measures absolutely continuous with respect to a prescribed Radon measure defined on a locally compact separable metric space (taking into account the ideas of Hernandez-Lerma and Lasserre) is proved. The results deal with Radon measures but yield obvious corollaries on real (or vector-valued) Radon measures and so on functions with bounded variation on open subsets of .

     

An affine PI Hopf algebra not finite over a normal commutative Hopf subalgebra

In formulating a generalized framework to study certain noncommutative algebras naturally arising in representation theory, K. A. Brown asked if every finitely generated Hopf algebra satisfying a polynomial identity was finite over a normal commutative Hopf subalgebra. In this note we show that Radford's biproduct, applied to the enveloping algebra of the Lie superalgebra , provides a noetherian prime counterexample.