75,931 research outputs found
Ten Conferences WORDS: Open Problems and Conjectures
In connection to the development of the field of Combinatorics on Words, we
present a list of open problems and conjectures that were stated during the ten
last meetings WORDS. We wish to continually update the present document by
adding informations concerning advances in problems solving
Radial Mixing in Galactic Disks: The Effects of Disk Structure and Satellite Bombardment
We use a suite of numerical simulations to investigate the mechanisms and
effects of radial migration of stars in disk galaxies like the Milky Way (MW).
An isolated, collisionless stellar disk with a MW-like scale-height shows only
the radial "blurring" expected from epicyclic orbits. Reducing the disk
thickness or adding gas to the disk substantially increases the level of radial
migration, induced by interaction with transient spiral arms and/or a central
bar. We also examine collisionless disks subjected to gravitational
perturbations from a cosmologically motivated satellite accretion history. In
the perturbed disk that best reproduces the observed properties of the MW, 20%
of stars that end up in the solar annulus 7 kpc < R < 9 kpc started at R < 6
kpc, and 7% started at R > 10 kpc. This level of migration would add
considerable dispersion to the age-metallicity relation of solar neighborhood
stars. In the isolated disk models, the probability of migration traces the
disk's radial mass profile, but in perturbed disks migration occurs
preferentially at large radii, where the disk is more weakly bound. The orbital
dynamics of migrating particles are also different in isolated and perturbed
disks: satellite perturbations drive particles to lower angular momentum for a
given change in radius. Thus, satellite perturbations appear to be a distinct
mechanism for inducing radial migration, which can operate in concert with
migration induced by bars and spiral structure. We investigate correlations
between changes in radius and changes in orbital circularity or vertical
energy, identifying signatures that might be used to test models and
distinguish radial migration mechanisms in chemo-dynamical surveys of the MW
disk.Comment: Full resolution paper available at
http://www.astronomy.ohio-state.edu/~bird/bkw11_fullres.pdf . 13 pages, 12
figures; emulate MNRAS format. Accepted for publication in MNRA
Graph Isomorphism for unit square graphs
In the past decades for more and more graph classes the Graph Isomorphism
Problem was shown to be solvable in polynomial time. An interesting family of
graph classes arises from intersection graphs of geometric objects. In this
work we show that the Graph Isomorphism Problem for unit square graphs,
intersection graphs of axis-parallel unit squares in the plane, can be solved
in polynomial time. Since the recognition problem for this class of graphs is
NP-hard we can not rely on standard techniques for geometric graphs based on
constructing a canonical realization. Instead, we develop new techniques which
combine structural insights into the class of unit square graphs with
understanding of the automorphism group of such graphs. For the latter we
introduce a generalization of bounded degree graphs which is used to capture
the main structure of unit square graphs. Using group theoretic algorithms we
obtain sufficient information to solve the isomorphism problem for unit square
graphs.Comment: 31 pages, 6 figure
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