767 research outputs found
Space processes for extended low-G testing
Results of an investigation of verifying the capabilities of space processes in ground based experiments at low-g periods are presented. Limited time experiments were conducted with the processes. A valid representation of the complete process cycle was achieved at low-g periods ranging from 40 to 390 seconds. A minimum equipment inventory, is defined. A modular equipment design, adopted to assure low cost and high program flexibility, is presented as well as procedures and data established for the synthesis and definition of dedicated and mixed rocket payloads
Physicochemical materials research: Phase relations in the Al-Ir-Os system in the range up to 70 at.% Al
For alloys in the range Os-OsAl2-IrAl2.7-Ir, as-cast and annealed at 1400 °C (Os-OsAl-IrAl-Ir) and 1250°C (OsAl-OsAl2-IrAl2.7-IrAl), phase equilibria are studied by powder x-ray diffraction (PXRD), differential thermal analysis (DTA), scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDX). Between isostructural aluminides OsAl and IrAl there exists a continuous solid solution (Os,Ir)Al. Other unary and binary phases form terminal solid solutions: (Os), (Ir), (OsAl2), and (IrAl2.7
Sedimentological and petrographic characteristics of volcanic ashes and siliceous claystones (altered ashes) from sites 1173, 1174, and 1177, leg 190
A remark on the trace-map for the Silver mean sequence
In this work we study the Silver mean sequence based on substitution rules by
means of a transfer-matrix approach. Using transfer-matrix method we find a
recurrence relation for the traces of general transfer-matrices which
characterizes electronic properties of the quasicrystal in question. We also
find an invariant of the trace-map.Comment: 5 pages, minor improvements in style and presentation of calculation
The local atomic quasicrystal structure of the icosahedral Mg25Y11Zn64 alloy
A local and medium range atomic structure model for the face centred
icosahedral (fci) Mg25Y11Zn64 alloy has been established in a sphere of r = 27
A. The model was refined by least squares techniques using the atomic pair
distribution (PDF) function obtained from synchrotron powder diffraction. Three
hierarchies of the atomic arrangement can be found: (i) five types of local
coordination polyhedra for the single atoms, four of which are of Frank-Kasper
type. In turn, they (ii) form a three-shell (Bergman) cluster containing 104
atoms, which is condensed sharing its outer shell with its neighbouring
clusters and (iii) a cluster connecting scheme corresponding to a
three-dimensional tiling leaving space for few glue atoms. Inside adjacent
clusters, Y8-cubes are tilted with respect to each other and thus allow for
overall icosahedral symmetry. It is shown that the title compound is
essentially isomorphic to its holmium analogue. Therefore fci-Mg-Y-Zn can be
seen as the representative structure type for the other rare earth analogues
fci-Mg-Zn-RE (RE = Dy, Er, Ho, Tb) reported in the literature.Comment: 12 pages, 8 figures, 2 table
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Initial Metabolic Profiles Are Associated with 7-Day Survival among Infants Born at 22-25 Weeks of Gestation.
OBJECTIVE:To evaluate the association between early metabolic profiles combined with infant characteristics and survival past 7 days of age in infants born at 22-25 weeks of gestation. STUDY DESIGN:This nested case-control consisted of 465 singleton live births in California from 2005 to 2011 at 22-25 weeks of gestation. All infants had newborn metabolic screening data available. Data included linked birth certificate and mother and infant hospital discharge records. Mortality was derived from linked death certificates and death discharge information. Each death within 7 days was matched to 4 surviving controls by gestational age and birth weight z score category, leaving 93 cases and 372 controls. The association between explanatory variables and 7-day survival was modeled via stepwise logistic regression. Infant characteristics, 42 metabolites, and 12 metabolite ratios were considered for model inclusion. Model performance was assessed via area under the curve. RESULTS:The final model included 1 characteristic and 11 metabolites. The model demonstrated a strong association between metabolic patterns and infant survival (area under the curve [AUC] 0.885, 95% CI 0.851-0.920). Furthermore, a model with just the selected metabolites performed better (AUC 0.879, 95% CI 0.841-0.916) than a model with multiple clinical characteristics (AUC 0.685, 95% CI 0.627-0.742). CONCLUSIONS:Use of metabolomics significantly strengthens the association with 7-day survival in infants born extremely premature. Physicians may be able to use metabolic profiles at birth to refine mortality risks and inform postnatal counseling for infants born at <26 weeks of gestation
Hypercontractivity, Sum-of-Squares Proofs, and their Applications
We study the computational complexity of approximating the 2->q norm of
linear operators (defined as ||A||_{2->q} = sup_v ||Av||_q/||v||_2), as well as
connections between this question and issues arising in quantum information
theory and the study of Khot's Unique Games Conjecture (UGC). We show the
following:
1. For any constant even integer q>=4, a graph is a "small-set expander"
if and only if the projector into the span of the top eigenvectors of G's
adjacency matrix has bounded 2->q norm. As a corollary, a good approximation to
the 2->q norm will refute the Small-Set Expansion Conjecture--a close variant
of the UGC. We also show that such a good approximation can be obtained in
exp(n^(2/q)) time, thus obtaining a different proof of the known subexponential
algorithm for Small Set Expansion.
2. Constant rounds of the "Sum of Squares" semidefinite programing hierarchy
certify an upper bound on the 2->4 norm of the projector to low-degree
polynomials over the Boolean cube, as well certify the unsatisfiability of the
"noisy cube" and "short code" based instances of Unique Games considered by
prior works. This improves on the previous upper bound of exp(poly log n)
rounds (for the "short code"), as well as separates the "Sum of
Squares"/"Lasserre" hierarchy from weaker hierarchies that were known to
require omega(1) rounds.
3. We show reductions between computing the 2->4 norm and computing the
injective tensor norm of a tensor, a problem with connections to quantum
information theory. Three corollaries are: (i) the 2->4 norm is NP-hard to
approximate to precision inverse-polynomial in the dimension, (ii) the 2->4
norm does not have a good approximation (in the sense above) unless 3-SAT can
be solved in time exp(sqrt(n) polylog(n)), and (iii) known algorithms for the
quantum separability problem imply a non-trivial additive approximation for the
2->4 norm.Comment: v1: 52 pages. v2: 53 pages, fixed small bugs in proofs of section 6
(on UG integrality gaps) and section 7 (on 2->4 norm of random matrices).
Added comments about real-vs-complex random matrices and about the
k-extendable vs k-extendable & PPT hierarchies. v3: fixed mistakes in random
matrix section. The result now holds only for matrices with random entries
instead of random column
Hybrid photonic-bandgap accelerating cavities
In a recent investigation, we studied two-dimensional point-defected photonic
bandgap cavities composed of dielectric rods arranged according to various
representative periodic and aperiodic lattices, with special emphasis on
possible applications to particle acceleration (along the longitudinal axis).
In this paper, we present a new study aimed at highlighting the possible
advantages of using hybrid structures based on the above dielectric
configurations, but featuring metallic rods in the outermost regions, for the
design of extremely-high quality factor, bandgap-based, accelerating
resonators. In this framework, we consider diverse configurations, with
different (periodic and aperiodic) lattice geometries, sizes, and
dielectric/metal fractions. Moreover, we also explore possible improvements
attainable via the use of superconducting plates to confine the electromagnetic
field in the longitudinal direction. Results from our comparative studies,
based on numerical full-wave simulations backed by experimental validations (at
room and cryogenic temperatures) in the microwave region, identify the
candidate parametric configurations capable of yielding the highest quality
factor.Comment: 13 pages, 5 figures, 3 tables. One figure and one reference added;
minor changes in the tex
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