11,763 research outputs found
Finsler bordifications of symmetric and certain locally symmetric spaces
We give a geometric interpretation of the maximal Satake compactification of
symmetric spaces of noncompact type, showing that it arises by
attaching the horofunction boundary for a suitable -invariant Finsler metric
on . As an application, we establish the existence of natural
bordifications, as orbifolds-with-corners, of locally symmetric spaces
for arbitrary discrete subgroups . These bordifications
result from attaching -quotients of suitable domains of proper
discontinuity at infinity. We further prove that such bordifications are
compactifications in the case of Anosov subgroups. We show, conversely, that
Anosov subgroups are characterized by the existence of such compactifications
among uniformly regular subgroups. Along the way, we give a positive answer, in
the torsion free case, to a question of Ha\"issinsky and Tukia on convergence
groups regarding the cocompactness of their actions on the domains of
discontinuity.Comment: 88 page
Planet Formation by Concurrent Collapse
After reviewing the difficulties faced by the conventional theory of planet
formation (based upon the aggregation of microscopic dust particles), we
describe an alternative hypothesis. We propose that planets form by
gravitational collapse at the same time as the star about which they orbit.
This 'concurrent collapse' hypothesis avoids theoretical difficulties
associated with the conventional model and suggests satisfying explanations for
various poorly understood phenomena. We introduce new explanations for FU
Orionis outbursts seen in young stars, the discovery of exoplanets with
eccentric orbits and the existence of small rocky objects such as chondrules in
the solar system.Comment: 16 pages, no figures, extended and refined version of earlier
submissio
The Shape of Unlabeled Rooted Random Trees
We consider the number of nodes in the levels of unlabelled rooted random
trees and show that the stochastic process given by the properly scaled level
sizes weakly converges to the local time of a standard Brownian excursion.
Furthermore we compute the average and the distribution of the height of such
trees. These results extend existing results for conditioned Galton-Watson
trees and forests to the case of unlabelled rooted trees and show that they
behave in this respect essentially like a conditioned Galton-Watson process.Comment: 34 pages, 1 figur
Discrete isometry groups of symmetric spaces
This survey is based on a series of lectures that we gave at MSRI in Spring
2015 and on a series of papers, mostly written jointly with Joan Porti. Our
goal here is to:
1. Describe a class of discrete subgroups of higher rank
semisimple Lie groups, which exhibit some "rank 1 behavior".
2. Give different characterizations of the subclass of Anosov subgroups,
which generalize convex-cocompact subgroups of rank 1 Lie groups, in terms of
various equivalent dynamical and geometric properties (such as asymptotically
embedded, RCA, Morse, URU).
3. Discuss the topological dynamics of discrete subgroups on flag
manifolds associated to and Finsler compactifications of associated
symmetric spaces . Find domains of proper discontinuity and use them to
construct natural bordifications and compactifications of the locally symmetric
spaces .Comment: 77 page
An equivariant Quillen theorem
A classical theorem due to Quillen (1969) identifies the unitary bordism ring
with the Lazard ring, which classifies the universal one-dimensional
commutative formal group law. We prove an equivariant generalization of this
result by identifying the homotopy theoretic -equivariant unitary
bordism ring, introduced by tom Dieck (1970), with the
-equivariant Lazard ring, introduced by Cole-Greenlees-Kriz
(2000). Our proof combines a computation of the homotopy theoretic
-equivariant unitary bordism ring due to Strickland (2001) with a
detailed investigation of the -equivariant Lazard ring.Comment: 19 pages; v3: Minor changes. Accepted for publication in Adv. Mat
Lexical typology through similarity semantics: Toward a semantic map of motion verbs
This paper discusses a multidimensional probabilistic semantic map of lexical motion verb stems based on data collected from parallel texts (viz. translations of the Gospel according to Mark) for 100 languages from all continents. The crosslinguistic diversity of lexical semantics in motion verbs is illustrated in detail for the domain of `go', `come', and `arrive' type contexts. It is argued that the theoretical bases underlying probabilistic semantic maps from exemplar data are the isomorphism hypothesis (given any two meanings and their corresponding forms in any particular language, more similar meanings are more likely to be expressed by the same form in any language), similarity semantics (similarity is more basic than identity), and exemplar semantics (exemplar meaning is more fundamental than abstract concepts)
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