Positive ternary quadratic forms with finitely many exceptions

An integral quadratic form is said to be almost regular if globally represents all but finitely many integers that are represented by the genus of . In this paper, we study and characterize all almost regular positive definite ternary quadratic forms.

     

On positive definite hermitian forms

Kneser's method of constructing adjacent lattices will be used to determine class numbers of unimodular positive definite hermitian lattices of rank 2 and 3 over rings of integers in some imaginary quadratic fields. The same method will also be applied in order to construct indecomposable unimodular positive definite hermitian lattices of rank 2 and 3 over almost all orders in imaginary quadratic fields, even non-maximal ones. All exceptional cases will be determined explicitly. In part supported by NSF grant DMS 8805262. I would like to thank Prof. Martin Kneser for the support and the many helpful hints I received during the work on my Diplom thesis on which this paper is based, and also Prof. T.-Y. Lam for providing me with the grant without which I would not have been able to devote part of my time to writing this paper.     

On indecomposable definite hermitian forms

In this paper, for any given natural numbersn anda, we can construct explicitly positive definite indecomposable integral Hermitian forms of rankn over with discriminanta, with the following ten exceptions:n=2,a=1, 2, 4, 10;n=3,a=1, 2, 5;n=4,a=1;n=5,a=1; andn=7,a=1. In the exceptional cases there are no forms with the desired properties. The method given here can be applied to solving the problem of constructing indecomposable positive definite HermitianR _m -lattices of any given rankn and discriminanta, whereR _m is the ring of algebraic integers in an imaginary quadratic field with class number unity.     

On indecomposable definite unimodular hermitian forms

For any natural numbersm andn≥17 we can construct explicitly indecomposable definite unimodular normal Hermitian lattices of rankn over the ring of algebraic integersR _m in an imaginary quadratic field . It is proved that for anyn (in casem=11, there is one exceptionn=3) there exist indecomposable definite unimodular normal HermitianR ₁₅(R ₁₁)-lattices of rankn, and we exhibit representatives for each class. In the exceptional case there are no lattices with the desired properties. The method given in this paper can solve completely the problem of constructing indecomposable definite unimodular normal HermitianR _m -lattices of any rankn for eachm. Dedicated to the memory of Prof. Lee Hwa-Chung.     

Equivalence systems with finitely many relations

We consider a setX with a finite totally ordered setE of equivalence relations onX. We describe the automorphism group of this system, that is, the group of all those permutations ofX that leave each relation inE invariant.     

Groups with finitely many linear orders

It is proved that there exist orderable groups having exactly 6 and 14 distinct linear orders. For any natural number k, we construct examples of orderable groups on which 2(4k+3) linear orders are defined. Supported by RFFR grant No. 96-01-00088. Translated fromAlgebra i Logika, Vol. 38, No. 2, pp. 176–200, March–April, 1999.     

Representations of integers by certain positive definite binary quadratic forms

We prove part of a conjecture of Borwein and Choi concerning an estimate on the square of the number of solutions to n=x ²+Ny ² for a squarefree integer N.      

Linear associative algebras with finitely many idempotents

Let A be a linear (i.e., finite-dimensional) associative algebra with unity defined over K, an algebraically closed field. Then A with respect to its multiplication is an algebraic monoid over k, denoted by A_M, and with respect to the the bracket forms a Lie algebra over K, denoted by A_L. The following theorem is established A_M is nilpotent as an algebraic monoid (equivalentlyA_L is so as a Lie algebra) if and only if the set of idempotents of A is finite if and only if all irreducible closed submonoids of codimension 1 are nilpotent.     

Kernels of digraphs with finitely many ends

According to Richardson’s theorem, every digraph $G$ without directed odd cycles that is either (a) locally finite or (b) rayless has a kernel (an independent subset $K$ with an incoming edge from every vertex in $G?K$). We generalize this theorem showing that a digraph without directed odd cycles has a kernel when (a) for each vertex, there is a finite set separating it from all rays, or (b) each ray contains at most finitely many vertices dominating it (having an infinite fan to the ray) and the digraph has finitely many ends. The restriction to finitely many ends in (b) can be weakened, admitting infinitely many ends with a specific structure, but the possibility of dropping it remains a conjecture.     

Quadratic descent of hermitian forms

Quadratic descent of hermitian and skew hermitian forms over division algebras with involution of the first kind in arbitrary characteristic is investigated and a criterion, in terms of systems of quadratic forms, is obtained. A refined result is also obtained for hermitian (resp. skew hermitian) forms over a quaternion algebra with symplectic (resp. orthogonal) involution.