46,422 research outputs found
Cycle-Level Products in Equivariant Cohomology of Toric Varieties
In this paper, we define an action of the group of equivariant Cartier
divisors on a toric variety X on the equivariant cycle groups of X, arising
naturally from a choice of complement map on the underlying lattice. If X is
nonsingular, this gives a lifting of the multiplication in equivariant
cohomology to the level of equivariant cycles. As a consequence, one naturally
obtains an equivariant cycle representative of the equivariant Todd class of
any toric variety. These results extend to equivariant cohomology the results
of Thomas and Pommersheim. In the case of a complement map arising from an
inner product, we show that the equivariant cycle Todd class obtained from our
construction is identical to the result of the inductive, combinatorial
construction of Berline-Vergne. In the case of arbitrary complement maps, we
show that our Todd class formula yields the local Euler-Maclarurin formula
introduced in Garoufalidis-Pommersheim.Comment: 15 pages, to be published in Michigan Mathematical Journal; LaTe
BASEL - The base language for an extensible language facility
Basic language for extensible language facilit
VR/Urban: spread.gun - design process and challenges in developing a shared encounter for media façades
Designing novel interaction concepts for urban environments is not only a technical challenge in terms of scale, safety, portability and deployment, but also a challenge of designing for social configurations and spatial settings. To outline what it takes to create a consistent and interactive experience in urban space, we describe the concept and multidisciplinary design process of VR/Urban's media intervention tool called Spread.gun, which was created for the Media Façade Festival 2008 in Berlin. Main design aims were the anticipation of urban space, situational system configuration and embodied interaction. This case study also reflects on the specific technical, organizational and infrastructural challenges encountered when developing media façade installations
The nonrelativistic limit of Dirac-Fock codes: the role of Brillouin configurations
We solve a long standing problem with relativistic calculations done with the
widely used Multi-Configuration Dirac-Fock Method (MCDF). We show, using
Relativistic Many-Body Perturbation Theory (RMBPT), how even for relatively
high-, relaxation or correlation causes the non-relativistic limit of states
of different total angular momentum but identical orbital angular momentum to
have different energies. We show that only large scale calculations that
include all single excitations, even those obeying the Brillouin's theorem have
the correct limit. We reproduce very accurately recent high-precision
measurements in F-like Ar, and turn then into precise test of QED. We obtain
the correct non-relativistic limit not only for fine structure but also for
level energies and show that RMBPT calculations are not immune to this problem.Comment: AUgust 9th, 2004 Second version Nov. 18th, 200
Perturbations of Spatially Closed Bianchi III Spacetimes
Motivated by the recent interest in dynamical properties of topologically
nontrivial spacetimes, we study linear perturbations of spatially closed
Bianchi III vacuum spacetimes, whose spatial topology is the direct product of
a higher genus surface and the circle. We first develop necessary mode
functions, vectors, and tensors, and then perform separations of (perturbation)
variables. The perturbation equations decouple in a way that is similar to but
a generalization of those of the Regge--Wheeler spherically symmetric case. We
further achieve a decoupling of each set of perturbation equations into
gauge-dependent and independent parts, by which we obtain wave equations for
the gauge-invariant variables. We then discuss choices of gauge and stability
properties. Details of the compactification of Bianchi III manifolds and
spacetimes are presented in an appendix. In the other appendices we study
scalar field and electromagnetic equations on the same background to compare
asymptotic properties.Comment: 61 pages, 1 figure, final version with minor corrections, to appear
in Class. Quant. Gravi
On Combining Lensing Shear Information from Multiple Filters
We consider the possible gain in the measurement of lensing shear from
imaging data in multiple filters. Galaxy shapes may differ significantly across
filters, so that the same galaxy offers multiple samples of the shear. On the
other extreme, if galaxy shapes are identical in different filters, one can
combine them to improve the signal-to-noise and thus increase the effective
number density of faint, high redshift galaxies. We use the GOODS dataset to
test these scenarios by calculating the covariance matrix of galaxy
ellipticities in four visual filters (B,V,i,z). We find that galaxy shapes are
highly correlated, and estimate the gain in galaxy number density by combining
their shapes.Comment: 8 pages, no figures, submitted to JCA
On the Structure of Equilibria in Basic Network Formation
We study network connection games where the nodes of a network perform edge
swaps in order to improve their communication costs. For the model proposed by
Alon et al. (2010), in which the selfish cost of a node is the sum of all
shortest path distances to the other nodes, we use the probabilistic method to
provide a new, structural characterization of equilibrium graphs. We show how
to use this characterization in order to prove upper bounds on the diameter of
equilibrium graphs in terms of the size of the largest -vicinity (defined as
the the set of vertices within distance from a vertex), for any
and in terms of the number of edges, thus settling positively a conjecture of
Alon et al. in the cases of graphs of large -vicinity size (including graphs
of large maximum degree) and of graphs which are dense enough.
Next, we present a new swap-based network creation game, in which selfish
costs depend on the immediate neighborhood of each node; in particular, the
profit of a node is defined as the sum of the degrees of its neighbors. We
prove that, in contrast to the previous model, this network creation game
admits an exact potential, and also that any equilibrium graph contains an
induced star. The existence of the potential function is exploited in order to
show that an equilibrium can be reached in expected polynomial time even in the
case where nodes can only acquire limited knowledge concerning non-neighboring
nodes.Comment: 11 pages, 4 figure
Free-induction decay and envelope modulations in a narrowed nuclear spin bath
We evaluate free-induction decay for the transverse components of a localized
electron spin coupled to a bath of nuclear spins via the Fermi contact
hyperfine interaction. Our perturbative treatment is valid for special
(narrowed) bath initial conditions and when the Zeeman energy of the electron
exceeds the total hyperfine coupling constant : . Using one unified
and systematic method, we recover previous results reported at short and long
times using different techniques. We find a new and unexpected modulation of
the free-induction-decay envelope, which is present even for a purely isotropic
hyperfine interaction without spin echoes and for a single nuclear species. We
give sub-leading corrections to the decoherence rate, and show that, in
general, the decoherence rate has a non-monotonic dependence on electron Zeeman
splitting, leading to a pronounced maximum. These results illustrate the
limitations of methods that make use of leading-order effective Hamiltonians
and re-exponentiation of short-time expansions for a strongly-interacting
system with non-Markovian (history-dependent) dynamics.Comment: 13 pages, 9 figure
On the Number of Facets of Three-Dimensional Dirichlet Stereohedra III: Full Cubic Groups
We are interested in the maximum possible number of facets that Dirichlet
stereohedra for three-dimensional crystallographic groups can have. The problem
for non-cubic groups was studied in previous papers by D. Bochis and the second
author (Discrete Comput. Geom. 25:3 (2001), 419-444, and Beitr. Algebra Geom.,
47:1 (2006), 89-120). This paper deals with ''full'' cubic groups, while
''quarter'' cubic groups are left for a subsequent paper. Here, ''full'' and
''quarter'' refers to the recent classification of three-dimensional
crystallographic groups by Conway, Delgado-Friedrichs, Huson and Thurston
(math.MG/9911185, Beitr. Algebra Geom. 42.2 (2001), 475-507).
Our main result in this paper is that Dirichlet stereohedra for any of the 27
full groups cannot have more than 25 facets. We also find stereohedra with 17
facets for one of these groups.Comment: 28 pages, 12 figures. Changes from v1: apart of some editing (mostly
at the end of the introduction) and addition of references, an appendix has
been added, which analyzes the case where the base point does not have
trivial stabilize
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