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Gregor Fels

Publications and source records attributed to Gregor Fels.

7 recordsLinked to original sources
arXiv2011

Nilpotent algebras and affinely homogeneous surfaces

The paper is devoted to the investigation of finite dimensional commutative nilpotent (associative) algebras N over an arbitrary base field of characteristic zero. Due to the lack of a general structure theory for algebras of this type (as opposed to the semi-simple case) we associate various objects to every N which encode the algebra structure. Our main results are in the subclass of algebras having 1-dimensional annihilator, that is, are maximal ideals of Gorenstein algebras of finite vector dimension > 1. Associated structural objects are then, for instance, a class of mutually affinely equivalent algebraic hypersurfaces S in N, and a class of so-called nil-polynomials p, whose degree is the nil-index of N. Then N can be reconstructed from S and even from the quadratic plus cubic part of p. If the algebra N is graded the hypersurface S is affinely homogeneous. The paper closes with an example of an N of dimension 23 and nil-index 5, for which S is not affinely homogeneous.

math.AC↗
arXiv2009

Local tube realizations of CR-manifolds and maximal abelian subalgebras

For every CR-manifold germ (M,a) local tube realizations are characterized by certain abelian subalgebras of the real Lie algebra hol(M,a) of all germs of (real-analytic) infinitesimal transformations. For instance, if M is holomorphically non-degenerate, every such subalgebra is maximal abelian and the classification of all local tube realizations for (M,a) reduces to a purely algebraic problem.

math.CV↗
arXiv2006

Locally homogeneous finitely nondegenerate CR-manifolds

Germs of locally homogeneous CR manifolds M can be characterized in terms of certain algebraic data, e.g., by CR-algebras. We give an explicit formula which relates the Levi form of such an M and its higher order analogues to the Lie brackets in certain finite dimensional Lie algebras. As an application we give a simple characterization of geometric properties of M such as minimality, k-nondegeneracy, holomorphic degeneracy etc. in purely algebraic terms. We present an example of a homogeneous 3-nondegenerate CR-manifold. We also determine a universal upper bound for the order k of k-nondegenerate orbits of real forms in flag manifolds.

math.CV↗
arXiv2006

Homogeneous Levi degenerate CR-manifolds in dimension 5

We investigate CR-manifolds which are tubes M:= F x iV over general bases F in a real vector space V and characterize the k-nondegeneracy of M in terms of the real affine geometry of F. We give a method for an explicit computation of the Lie algebra hol(M,a) of all local infinitesimal CR-transformations at a and use these local invariants to establish the CR-inequivalence of certain families of CR-manifolds. In dimension 5 we present, apart from the well known tube over the future light cone, new examples of 2-nondegenerate homogeneous CR-manifolds that are mutually locally CR-inequivalent.

math.CV↗
arXiv2005

CR-manifolds of dimension 5: A Lie algebra approach

We study real-analytic Levi degenerate hypersurfaces M in complex manifolds of dimension 3, for which the CR-automorphism group Aut(M) is a real Lie group acting transitively on M. We provide large classes of examples for such M, compute the corresponding groups Aut(M) and determine the maximal subsets of M that cannot be separated by global continuous CR-functions. It turns out that all our examples, although partly arising in different contexts, are locally CR-equivalent to the tube over the future light cone in 3-dimensional space-time.

math.DS↗
arXiv2002

Characterization of cycle domains via Kobayashi hyperbolicity

A real form $G$ of a complex semisimple Lie group $G^C$ has only finitely many orbits in any given $G^C$-flag manifold $Z=G^C/Q$. The complex geometry of these orbits is of interest, e.g., for the associated representation theory. The open orbits $D$ generally possess only the constant holomorphic functions, and the relevant associated geometric objects are certain positive-dimensional compact complex submanifolds of $D$ which, with very few well-understood exceptions, are parameterized by the Wolf cycle domains $Ω_W(D)$ in $G^C/K^C$, where $K$ is a maximal compact subgroup of $G$. Thus, for the various domains $D$ in the various ambient spaces $Z$, it is possible to compare the cycle spaces $Ω_W(D)$. The main result here is that, with the few exceptions mentioned above, for a fixed real form $G$ all of the cycle spaces $Ω_W(D)$ are the same. They are equal to a universal domain $Ω_{AG}$ which is natural from the the point of view of group actions and which, in essence, can be explicitly computed. The essential technical result is that if $\hat Ω$ is a $G$-invariant Stein domain which contains $Ω_{AG}$ and which is Kobayashi hyperbolic, then $\hat Ω=Ω_{AG}$. The equality of the cycle domains follows from the fact that every $Ω_W(D)$ is itself Stein, is hyperbolic, and contains $Ω_{AG}$.

math.AG↗