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Discrete Temporal Models of Social Networks
We propose a family of statistical models for social network evolution over
time, which represents an extension of Exponential Random Graph Models (ERGMs).
Many of the methods for ERGMs are readily adapted for these models, including
maximum likelihood estimation algorithms. We discuss models of this type and
their properties, and give examples, as well as a demonstration of their use
for hypothesis testing and classification. We believe our temporal ERG models
represent a useful new framework for modeling time-evolving social networks,
and rewiring networks from other domains such as gene regulation circuitry, and
communication networks
Generic singularities of nilpotent orbit closures
According to a well-known theorem of Brieskorn and Slodowy, the intersection
of the nilpotent cone of a simple Lie algebra with a transverse slice to the
subregular nilpotent orbit is a simple surface singularity. At the opposite
extremity of the nilpotent cone, the closure of the minimal nilpotent orbit is
also an isolated symplectic singularity, called a minimal singularity. For
classical Lie algebras, Kraft and Procesi showed that these two types of
singularities suffice to describe all generic singularities of nilpotent orbit
closures: specifically, any such singularity is either a simple surface
singularity, a minimal singularity, or a union of two simple surface
singularities of type . In the present paper, we complete the picture
by determining the generic singularities of all nilpotent orbit closures in
exceptional Lie algebras (up to normalization in a few cases). We summarize the
results in some graphs at the end of the paper.
In most cases, we also obtain simple surface singularities or minimal
singularities, though often with more complicated branching than occurs in the
classical types. There are, however, six singularities which do not occur in
the classical types. Three of these are unibranch non-normal singularities: an
-variety whose normalization is , an
-variety whose normalization is , and a
two-dimensional variety whose normalization is the simple surface singularity
. In addition, there are three 4-dimensional isolated singularities each
appearing once. We also study an intrinsic symmetry action on the
singularities, in analogy with Slodowy's work for the regular nilpotent orbit.Comment: 56 pages (5 figures). Minor corrections. Accepted in Advances in Mat
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