Searcharxiv⌕ Search

arXiv subjects

Martin Lorenz

Publications and source records attributed to Martin Lorenz.

29 records · Page 2Linked to original sources
arXiv2005

Orders of Finite Groups of Matrices

We present a new proof of a theorem of Schur's determining the least common multiple of the orders of all finite groups of complex $n \times n$-matrices whose elements have traces in the field of rational numbers. The basic method of proof goes back to Minkowski and proceeds by reduction to the case of finite fields. For the most part, we work over an arbitrary number field rather than the rationals. The first half of the article is expository and is intended to be accessible to graduate students and advanced undergraduates. It gives a self-contained treatment, following Schur, over the field of rational numbers.

math.GR↗
arXiv2004

On the Cohen-Macaulay property of multiplicative invariants

We investigate the Cohen-Macaulay property for rings of invariants under multiplicative actions of a finite group $G$. By definition, these are $G$-actions on Laurent polynomial algebras that stabilize the multiplicative group consisting of all monomials in the variables. For the most part, we concentrate on the case where the base ring is the ring of rational integers. Our main result states that if $G$ acts non-trivially and the invariant algebra is Cohen-Macaulay then the abelianized isotropy groups $G_m/[G_m,G_m]$ of all monomials m are generated by bireflections and at least one $G_m/[G_m,G_m]$ is non-trivial. As an application, we prove the multiplicative version of Kemper's 3-copies conjecture.

math.AC↗
arXiv2003

Fields of definition for division algebras

Let $A$ be a finite-dimensional division algebra containing a base field $k$ in its center $F$. We say that $A$ is defined over a subfield $F_0$ of $F$ if $A = A_0\otimes_{F_0} F$ for some $F_0$-subalgebra $A_0$ of $A$. We show that: (1) In many cases $A$ can be defined over a rational extension of $k$. (2) If $A$ has odd degree $n \ge 5$, then $A$ is defined over a field $F_0$ of transcendence degree at most $(n-1)(n-2)/2$ over $k$. (3) If $A$ is a $Z/m \times Z/2$-crossed product for some $m \ge 2$ (and in particular, if $A$ is any algebra of degree 4) then $A$ is Brauer equivalent to a tensor product of two symbol algebras. Consequently, $M_m(A)$ can be defined over a field $F_0$ of transcendence degree at most 4 over $k$. (4) If $A$ has degree 4 then the trace form of $A$ can be defined over a field $F_0$ of transcendence degree at most 4. (In (1), (3), and (4) we assume that the center of $A$ contains certain roots of unity.)

math.RA↗
arXiv2001

On Euler classes of abelian-by-finite groups

Let $G$ be a finitely generated abelian-by-finite group and $k$ a field of characteristic $p\ge 0$. The Euler class $[k_G]$ of $G$ over $k$ is the class of the trivial $kG$-module in the Grothendieck group $G_0(kG)$. We show that $[k_G]$ has finite order if and only if every $p$-regular element of $G$ has infinite centralizer in $G$. We also give a lower bound for the order of the Euler class in terms of suitable finite subgroups of $G$. This lower bound is derived from a more general result on finite-dimensional representations of smash products of Hopf algebras.

math.RA↗
arXiv2000

On Cohen-Macaulay rings of invariants

We investigate the transfer of the Cohen-Macaulay property from a commutative ring to a subring of invariants under the action of a finite group. Our point of view is ring theoretic and not a priori tailored to a particular type of group action. As an illustration, we briefly discuss the special case of multiplicative actions, that is, actions on group algebras $k[\bbZ^n]$ via an action on $\bbZ^n$.

math.AC↗
arXiv2000

Lattices and Parameter Reduction in Division Algebras

Let k be an algebraically closed field of characteristic 0 and let D be a division algebra whose center F contains k. We shall say that D can be reduced to r parameters if D = D_0 tensor_{F_0} F, where D_0 is a division algebra, the center F_0 of D_0 contains k and trdeg(F_0/k) = r. We show that every division algebra of odd degree n >= 5 can be reduced to at most (n-1)(n-2)/2 parameters. Moreover, every crossed product division algebra of degree n >= 4 can be reduced to at most (log_2(n) - 1)n + 1 parameters. Our proofs of these results rely on lattice-theoretic techniques.

math.RA↗
arXiv1999

On certain lattices associated with generic division algebras

Let S_n denote the symmetric group on n letters. We consider the S_n-root lattice A_{n-1} = {(z1,...,zn) in Z^n | z1+...+zn = 0}, where S_n acts on Z^n by permuting the coordinates, and its tensor, symmetric, and exterior squares. For odd values of n, we show that the tensor square is equivalent, in the sense of Colliot-Thelene and Sansuc, to the exterior square. Consequently, the rationality problem for generic division algebras, for odd values of n, amounts to proving stable rationality of the multiplicative S_n-invariant field of the exterior square of A_{n-1}. Furthermore, confirming a conjecture of Le Bruyn, we show that n=2 and n=3 are the only cases where the tensor square of A_{n-1} is equivalent to a permutation S_n-lattice. In the course of the proof of this result, we construct subgroups H of S_n, for all n that are not prime, so that the algebra of multiplicative H-invariants of A_{n-1} has a non-trivial Picard group.

math.RA↗
arXiv1999

Multiplicative Invariants and Semigroup Algebras

Let G be a finite group acting by automorphism on a lattice A, and hence on the group algebra S=k[A]. The algebra of G-invariants in S is called an algebra of multiplicative invariants. We investigate when algebras of multiplicative invariants are semigroup algebras. In particular, we present an explicit version of a result of Farkas stating that multiplicative invariants of finite reflection groups are indeed semigroup algebras. On the other hand, multiplicative invariants arising from fixed point free actions are shown to never be semigroup algebras. In particular, this holds whenever G has odd prime order.

math.AC↗
arXiv1998

Polycyclic-by-finite group algebras are catenary

We show that group algebras kG of polycyclic-by-finite groups G, where k is a field, are catenary: Given prime ideals P and P' of kG, with P contained in P', all saturated chains of primes between P and P' have the same length.

math.RA↗
arXiv1998

K_0 of invariant rings and nonabelian H^1

We give a description of the kernel of the induction map K_0(R)->K_0(S), where S is a commutative ring and R is the ring of invariants of the action of a finite group G on S. The description is in terms of H^1(G,GL(S)).

math.RA↗