897 research outputs found
Nonnegatively curved Euclidean submanifolds in codimension two
We provide a classification of compact Euclidean submanifolds
with nonnegative sectional curvature, for . The classification is in terms of the induced metric (including the
diffeomorphism classification of the manifold), and we study the structure of
the immersions as well. In particular, we provide the first known example of a
nonorientable quotient
with nonnegative curvature. For the 3-dimensional case, we show that either the
universal cover is isometric to , or is
diffeomorphic to a lens space, and the complement of the (nonempty) set of flat
points is isometric to a twisted cylinder
. As a consequence we conclude that, if
the set of flat points is not too big, there exists a unique flat totally
geodesic surface in whose complement is the union of one or two twisted
cylinders over disks.Comment: Accepted for publication in Commentarii Mathematici Helvetic
Topological obstructions to fatness
Alan Weinstein showed that certain characteristic numbers of any Riemannian
submersion with totally geodesic fibers and positive vertizontal curvatures are
nonzero. In this paper we explicitly compute these invariants in terms of Chern
and Pontrjagin numbers of the bundle. This allows us to show that many bundles
do not admit such metrics.Comment: 32 pages. To appear in Geom. Topo
New string vacua from twistor spaces
We find a new family of AdS_4 vacua in IIA string theory. The internal space
is topologically either the complex projective space CP^3 or the "flag
manifold" SU(3)/(U(1)xU(1)), but the metric is in general neither Einstein nor
Kaehler. All known moduli are stabilized by fluxes, without using quantum
effects or orientifold planes. The analysis is completely ten--dimensional and
does not rely on assumptions about Kaluza--Klein reduction.Comment: 19 pages. v3: published version, further minor correction
Superconductivity without Fe or Ni in the phosphides BaIr2P2 and BaRh2P2
Heat capacity, resistivity, and magnetic susceptibility measurements confirm
bulk superconductivity in single crystals of BaIrP (T=2.1K) and
BaRhP (T = 1.0 K). These compounds form in the ThCrSi (122)
structure so they are isostructural to both the Ni and Fe pnictides but not
isoelectronic to either of them. This illustrates the importance of structure
for the occurrence of superconductivity in the 122 pnictides. Additionally, a
comparison between these and other ternary phosphide superconductors suggests
that the lack of interlayer bonding favors superconductivity. These
stoichiometric and ambient pressure superconductors offer an ideal playground
to investigate the role of structure for the mechanism of superconductivity in
the absence of magnetism.Comment: Published in Phys Rev B: Rapid Communication
Crystal Structure of 2-(2,6-diisopropylphenyl)-N,Ndiethyl- 3,3-dimethyl-2-azaspiro[4.5]decan-1- amine: A Diethylamine Adduct of a Cyclic(alkyl)- (amino)carbene (CAAC)
The structure of the title compound, C27H46N2, at 93 K has monoclinic (P21/n) symmetry. The title compound was prepared by treatment of 2-(2,6-diisopropylphenyl)-3,3-dimethyl-2-azaspiro[4.5]dec-1-en-2-ium hydrogen dichloride with two equivalents of lithium diethylamide. Characterization of the title compound by single-crystal X-ray diffraction and 1H and 13C NMR spectroscopy is presented. Formation of the diethylamine adduct of the cyclic(alkyl)(amino)carbene (CAAC) was unexpected, as deprotonation using lithium diisopropylamide results in free CAAC formation
Crystal Structure of 2-(2,6-diisopropylphenyl)-N,N-diethyl-3,3-dimethyl-2-azaspiro[4.5]decan-1-amine: A Diethylamine Adduct of a Cyclic(alkyl)(amino)carbene (CAAC)
The structure of the title compound, C27H46N2, at 93 K has monoclinic (P21/n) symmetry. The title compound was prepared by treatment of 2-(2,6-diisopropylphenyl)-3,3-dimethyl-2-azaspiro[4.5]dec-1-en-2-ium hydrogen dichloride with two equivalents of lithium diethylamide. Characterization of the title compound by single-crystal X-ray diffraction and 1H and 13C NMR spectroscopy is presented. Formation of the diethylamine adduct of the cyclic(alkyl)(amino)carbene (CAAC) was unexpected, as deprotonation using lithium diisopropylamide results in free CAAC formation
Reduzierung des Zuckerkonsums für eine bessere Mundgesundheit – Welche Strategien sind Erfolg versprechend? [Reducing sugar consumption to improve oral health-which strategies are effective?]
Health promotion strategies can make a significant contribution to the prevention of non-communicable diseases. The most important non-communicable diseases affecting the mouth and teeth are dental caries and periodontal disease. Sugar is a causal factor in the development of dental caries, obesity and obesity-related diseases. Sugar is therefore recognised as an important commercial determinant of health and a public health priority. However, existing strategies to reduce sugar consumption often focus on individual behaviour change while ignoring the role of the wider social and commercial environment.In this article we describe the current recommendations on sugar intake made by the World Health Organization, existing data on sugar intake in Germany as well as the social and commercial factors that influence sugar consumption. Important public health principles will be outlined, followed by a discussion of appropriate sugar reduction strategies. Examples for upstream and downstream approaches to prevention are presented and opportunities for dental health professionals to influence policy and practice highlighted
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