1,804 research outputs found
Computing Volume Bounds of Inclusions by EIT Measurements
The size estimates approach for Electrical Impedance Tomography (EIT) allows
for estimating the size (area or volume) of an unknown inclusion in an
electrical conductor by means of one pair of boundary measurements of voltage
and current. In this paper we show by numerical simulations how to obtain such
bounds for practical application of the method. The computations are carried
out both in a 2D and a 3D setting.Comment: 20 pages with figure
Globally diffeomorphic Ï -harmonic mappings
Given a two-dimensional mapping U whose components solve a divergence structure elliptic equation,we give necessary and sufficient conditions on the boundary so that U is a global diffeomorphism
Seismic Station Installations and Their Impact on the Recorded Signals and Derived Quantities
The role of local geology in controlling ground motion has long been acknowledged.
Consequently, increasing attention is paid to the assessment of the geophysical properties
of the soils at the seismic stations, which impact the station recordings and a series
of related quantities, particularly those referring to seismic hazard estimates. Not the
same level of attention is commonly dedicated to the seismic station installation, to the
point that it is generally believed that housings and shelters containing seismic instruments
are of no interest, because they can only affect frequencies well above the engineering
range of interest. Using examples from seismometric and accelerometric
stations, we describe the (1) housing, (2) foundation, and (3) pillar effects on the seismic
records. We propose a simple working scheme to identify the existence of potential
installation-related issues and to assess the frequency fidelity range of response of
a seismic station to ground motion. Our scheme is developed mostly on ambient noise
recordings and, thus, surface waves. The hope is that, besides the parameters that start
to be routinely introduced in the seismic archives (VS30, soil classes, etc.), the assessment
of the maximum reliable frequency, under which no soilâstructure interaction is
expected, also becomes a mandatory information. In our experience, for some installation
sites, the maximum reliable frequency can even be less than a very few hert
Sewing Constraints and Non-Orientable Open Strings
We extend to non-orientable surfaces previous work on sewing constraints in
Conformal Field Theory. A new constraint, related to the real projective plane,
is described and is used to illustrate the correspondence with a previous
construction of open-string spectra.Comment: phyzzx, 11 pages and 4 figures, ROM2F-93/3
Characterization of ellipses as uniformly dense sets with respect to a family of convex bodies
Let K \subset R^N be a convex body containing the origin. A measurable set G
\subset R^N with positive Lebesgue measure is said to be uniformly K-dense if,
for any fixed r > 0, the measure of G \cap (x + rK) is constant when x varies
on the boundary of G (here, x + rK denotes a translation of a dilation of K).
We first prove that G must always be strictly convex and at least C1,1-regular;
also, if K is centrally symmetric, K must be strictly convex, C1,1-regular and
such that K = G - G up to homotheties; this implies in turn that G must be
C2,1- regular. Then for N = 2, we prove that G is uniformly K-dense if and only
if K and G are homothetic to the same ellipse. This result was already proven
by Amar, Berrone and Gianni in [3]. However, our proof removes their regularity
assumptions on K and G and, more importantly, it is susceptible to be
generalized to higher dimension since, by the use of Minkowski's inequality and
an affine inequality, avoids the delicate computations of the higher-order
terms in the Taylor expansion near r = 0 for the measure of G\cap(x+rK) (needed
in [3])
Diffeomorphic approximation of Sobolev homeomorphisms
Every homeomorphism h : X -> Y between planar open sets that belongs to the
Sobolev class W^{1,p}(X,Y), 1<p<\infty, can be approximated in the Sobolev norm
by diffeomorphisms.Comment: 21 pages, 5 figure
Global stability for an inverse problem in soil-structure interaction
We consider the inverse problem of determining the Winkler
subgrade reaction coefficient of a slab foundation modelled as a
thin elastic plate clamped at the boundary. The plate is loaded by
a concentrated force and its transversal deflection is measured at
the interior points. We prove a global Holder stability
estimate under (mild) regularity assumptions on the unknown
coefficient
Path integral evaluation of Dbrane amplitudes
We extend Polchinski's evaluation of the measure for the one-loop closed
string path integral to open string tree amplitudes with boundaries and
crosscaps embedded in Dbranes. We explain how the nonabelian limit of
near-coincident Dbranes emerges in the path integral formalism. We give a
careful path integral derivation of the cylinder amplitude including the
modulus dependence of the volume of the conformal Killing group.Comment: Extended version replacing hep-th/9903184, includes discussion of
nonabelian limit, Latex, 10 page
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