Search results for "geometry."
showing 10 items of 4386 documents
Quotients of the Dwork Pencil
2012
In this paper we investigate the geometry of the Dwork pencil in any dimension. More specifically, we study the automorphism group G of the generic fiber of the pencil over the complex projective line, and the quotients of it by various subgroups of G. In particular, we compute the Hodge numbers of these quotients via orbifold cohomology.
Groups acting freely on Calabi-Yau threefolds embedded in a product of del Pezzo surfaces
2011
In this paper, we investigate quotients of Calabi-Yau manifolds $Y$ embedded in Fano varieties $X$, which are products of two del Pezzo surfaces — with respect to groups $G$ that act freely on $Y$. In particular, we revisit some known examples and we obtain some new Calabi-Yau varieties with small Hodge numbers. The groups $G$ are subgroups of the automorphism groups of $X$, which is described in terms of the automorphism group of the two del Pezzo surfaces.
Flots de Smale en dimension 3: présentations finies de voisinages invariants d'ensembles selles
2002
Abstract Given a vector field X on a compact 3-manifold, and a hyperbolic saddle-like set K of that vector field, we consider all the filtering neighbourhood of K: by such, we mean any submanifold which boundary is tranverse to X, the maximal invariant of which is equal to K and which intersection with every orbit of X is connected. Up to topological equivalence, there is only a finite number of such neighbourhoods. We give a finite combinatorial presentation of the global dynamics on any such neighbourhood. A key step is the construction of a unique model of the germ of X along K; this model is, roughly speaking, the simplest three-dimensional manifold and the simplest Smale flow exhibitin…
Stepwise formation of a pentanuclear Ni4Cu heterometallic complex exhibiting a vertex-sharing defective double-cubane core and diphenoxo- and phenoxo…
2013
Sequential reaction of a N5O3 octadentate tripodal ligand with Ni(2+) and subsequently with Cu(2+) and azide ligand afforded the first example of a heterobridged (phenoxo/μ(1,1)-azido) pentanuclear heterometallic (Ni4Cu) compound, which exhibits a centrosymmetric vertex-sharing defective double-cubane structure. The study of the magnetic properties reveals that the compound shows ferromagnetic interaction interactions, leading to an S = 9/2 spin ground state. Density functional theory calculations on the X-ray structure and model compounds predict ferromagnetic interactions through the magnetic exchange pathways involving each couple of metal ions.
Novel azobenzene precursors for NLO active polyuretanes: Synthesis, quantum chemical and experimental characterization
2009
Abstract For the development of electro optical active polyurethanes six new derivatives of 2-{(2-hydroxyethyl)-[4-(4-nitrophenylazo)phenyl]amino}ethanol and two of 2-{(2-hydroxyethyl)-[4-(5-nitropyridin-2-ylazo)phenyl]amino}ethanol were synthesized by azocoupling reaction. Molecular geometry, hyperpolarizability βFF and ground state dipole moment μg were acquired by RHF ab initio (6–31G∗∗) calculations using HyperChem software package. To characterize NLO performance of synthesized azo compounds second order non linear coefficients d31 and d33 were measured as function of chromophore load in guest–host films (PMMA). Eight synthesized compounds can be grouped in four similar gross formula p…
4-hydroxy-ONN-azoxybenzene
2000
The oxidation of 4-hydroxyazobenzene provided a mixture of two azoxy compounds, which were separated by column chromatography. The isomer with the higher melting point appeared to belong to the α (ONN) series, as determined by X-ray diffraction. The molecule, C12H10N2O2, is almost planar. The benzene rings are twisted by 11.7 (2) (substituted) and 4.1 (1)° (unsubstituted) with respect to the ONN plane. The molecules are connected to one another by strong O—H⋯O hydrogen bonds forming chains extended along [001], which are bound by much weaker C—H⋯O hydrogen bonds forming layers in the bc plane.
Disorder in the crystals of trans-4-fluoroazoxybenzene. Synthesis, spectral properties, crystal structures and DFT calculations
2005
Abstract Two crystals of trans-4-fluoroazoxybenzene were obtained using two different methods. Oxidation of 4-fluoroazobenzene provided crystals of trans-4-fluoroazoxybenzene (I) consisting of two isomers; 51% α (ONN) and 49% β (NNO) isomer. From trans-amino-azoxybenzenes in Schiemann reaction it were obtained crystals of trans-4-fluoro-NNO-azoxybenzenes (II, β isomer) containing 4.7% of the α-isomer according to the HPLC analysis. The crystal structures of I and II were determined by the X-ray diffraction method. In the crystal I two nitrogen atoms of azoxy bridge and fluorine atom are disordered. In the crystal II, there are two independent molecules of trans-4-fluoroazoxybenzene; the mol…
Twotrans-4-aminoazoxybenzenes
2001
Two isomeric trans-4-aminoazoxybenzenes, trans-1-(4-aminophenyl)-2-phenyldiazene 2-oxide (alpha, C(12)H(11)N(3)O) and trans-2-(4-aminophenyl)-1-phenyldiazene 2-oxide (beta, C(12)H(11)N(3)O), have been characterized by X-ray diffraction. The alpha isomer is almost planar, having torsion angles along the C(aryl)-N bonds of only 4.9 (2) and 8.0 (2) degrees. The relatively short C(aryl)-N bond to the non-oxidized site of the azoxy group [1.401 (2) A], together with the significant quinoid deformation of the respective phenyl ring, is evidence of conjugation between the aromatic sextet and the pi-electron system of the azoxy group. The geometry of the beta isomer is different. The non-substitute…
Bruhat–Tits Trees and Modular Groups
2019
In this chapter, we give background information and preliminary results on the main link between the geometry and the algebra used for our arithmetic applications: the (discrete-time) geodesic ow on quotients of Bruhat{Tits trees by arithmetic lattices.
Estimation of Vegetation Structure Parameters From SMAP Radar Intensity Observations
2021
In this article, we present a multipolarimetric estimation approach for two model-based vegetation structure parameters (shape A and orientation distribution ψ of the main canopy elements). The approach is based on a reduced observation set of three incoherent (no phase information) polarimetric backscatter intensities (|S HH | 2 , |S HV | 2 , and |S VV | 2 ) combined with a two-parameter (A P and ψ) discrete scatterer model of vegetation. The objective is to understand whether this confined set of observations contains enough information to estimate the two vegetation structure parameters from the L-band radar signals. In order to disentangle soil and vegetation scattering influences on th…