728 research outputs found
Medial/skeletal linking structures for multi-region configurations
We consider a generic configuration of regions, consisting of a collection of
distinct compact regions in which may be
either smooth regions disjoint from the others or regions which meet on their
piecewise smooth boundaries in a generic way. We introduce a
skeletal linking structure for the collection of regions which simultaneously
captures the regions' individual shapes and geometric properties as well as the
"positional geometry" of the collection. The linking structure extends in a
minimal way the individual "skeletal structures" on each of the regions,
allowing us to significantly extend the mathematical methods introduced for
single regions to the configuration.
We prove for a generic configuration of regions the existence of a special
type of Blum linking structure which builds upon the Blum medial axes of the
individual regions. This requires proving several transversality theorems for
certain associated "multi-distance" and "height-distance" functions for such
configurations. We show that by relaxing the conditions on the Blum linking
structures we obtain the more general class of skeletal linking structures
which still capture the geometric properties.
In addition to yielding geometric invariants which capture the shapes and
geometry of individual regions, the linking structures are used to define
invariants which measure positional properties of the configuration such as:
measures of relative closeness of neighboring regions and relative significance
of the individual regions for the configuration. These invariants, which are
computed by formulas involving "skeletal linking integrals" on the internal
skeletal structures, are then used to construct a "tiered linking graph," which
identifies subconfigurations and provides a hierarchical ordering of the
regions.Comment: 135 pages, 36 figures. Version to appear in Memoirs of the Amer.
Math. So
Topology of Exceptional Orbit Hypersurfaces of Prehomogeneous Spaces
We consider the topology for a class of hypersurfaces with highly nonisolated
singularites which arise as exceptional orbit varieties of a special class of
prehomogeneous vector spaces, which are representations of linear algebraic
groups with open orbits. These hypersurface singularities include both
determinantal hypersurfaces and linear free (and free*) divisors. Although
these hypersurfaces have highly nonisolated singularities, we determine the
topology of their Milnor fibers, complements and links. We do so by using the
action of linear algebraic groups beginning with the complement, instead of
using Morse type arguments on the Milnor fibers. This includes replacing the
local Milnor fiber by a global Milnor fiber which has a complex geometry
resulting from a transitive action of an appropriate algebraic group, yielding
a compact model submanifold for the homotopy type of the Milnor fiber. The
topology includes the (co)homology (in characteristic 0, and 2 torsion in one
family) and homotopy groups, and we deduce the triviality of the monodromy
transformations on rational (or complex) cohomology. The cohomology of the
Milnor fibers and complements are isomorphic as algebras to exterior algebras
or for one family, modules over exterior algebras; and cohomology of the link
is, as a vector space, a truncated and shifted exterior algebra, for which the
cohomology product structure is essentially trivial. We also deduce from Bott's
periodicity theorem, the homotopy groups of the Milnor fibers for determinantal
hypersurfaces in the stable range as the stable homotopy groups of the
associated infinite dimensional symmetric spaces. Applying a Theorem of Oka we
obtain a class of formal linear combinations of exceptional orbit hypersurfaces
which have Milnor fibers which are homotopy equivalent to joins of the compact
model submanifolds.Comment: to appear in the Journal of Topolog
Solvable Groups, Free Divisors and Nonisolated Matrix Singularities II: Vanishing Topology
In this paper we use the results from the first part to compute the vanishing
topology for matrix singularities based on certain spaces of matrices. We place
the variety of singular matrices in a geometric configuration of free divisors
which are the "exceptional orbit varieties" for repesentations of solvable
groups. Because there are towers of representations for towers of solvable
groups, the free divisors actually form a tower of free divisors , and we
give an inductive procedure for computing the vanishing topology of the matrix
singularities. The inductive procedure we use is an extension of that
introduced by L\^{e}-Greuel for computing the Milnor number of an ICIS. Instead
of linear subspaces, we use free divisors arising from the geometric
configuration and which correspond to subgroups of the solvable groups.
Here the vanishing topology involves a singular version of the Milnor fiber;
however, it still has the good connectivity properties and is homotopy
equivalent to a bouquet of spheres, whose number is called the singular Milnor
number. We give formulas for this singular Milnor number in terms of singular
Milnor numbers of various free divisors on smooth subspaces, which can be
computed as lengths of determinantal modules. In addition to being applied to
symmetric, general and skew-symmetric matrix singularities, the results are
also applied to Cohen--Macaulay singularities defined as 2 x 3 matrix
singularities. We compute the Milnor number of isolated Cohen--Macaulay surface
singularities of this type in and the difference of Betti
numbers of Milnor fibers for isolated Cohen--Macaulay 3--fold singularities of
this type in .Comment: 53 pages. To appear in Geometry & Topology. Changes in response to
helpful referee: replace the erroneous Corollary 6.2 with a new version,
specify that we consider 2x3 Cohen-Macaulay singularities, calculate more
entries of Table 5, improve wording, format for publicatio
On the freeness of equisingular deformations of plane curve singularities
We consider surface singularities in arising as the total space of an equisingular deformation of an isolated curve singularity of the form f(x,y)+zg(x,y) with f and g weighted homogeneous. We give a criterion that such a surface is a free divisor in the sense of Saito. We deduce that the Hessian deformation defines a free divisor for nonsimple weighted homogeneous singularities, and that the failure of this property “almost” characterizes the simple singularities. The criterion also yields distinct deformations of the same curve singularity, exactly one of which is free, showing that freeness is not a topological property
Topological triviality of versal unfoldings of complete intersections
We obtain algebraic and geometric conditions for the topological triviality of versal unfoldings of weighted homogeneous complete intersections along subspaces corresponding to deformations of maximal weight. These results are applied: to infinite families of surface singularities in which begin with the exceptional unimodular singularities, to the intersection of pairs of generic quadrics, and to certain curve singularities
Swept regions and surfaces: Modeling and volumetric properties
We consider “swept regions” and “swept hypersurfaces” in (and especially ) which are a disjoint union of subspaces or obtained from a varying family of affine subspaces . We concentrate on the case where and are obtained from a skeletal structure . This generalizes the Blum medial axis of a region , which consists of the centers of interior spheres tangent to the boundary at two or more points, with denoting the vectors from the centers of the spheres to the points of tangency. We extend methods developed for skeletal structures so that they can be deduced from the properties of the individual intersections or and a relative shape operator , which we introduce to capture changes relative to the varying family .
We use these results to deduce modeling properties of the global in terms of the individual , and determine volumetric properties of regions expressed as global integrals of functions on in terms of iterated integrals over the skeletal structure of which is then integrated over the parameter space
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