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121.
Light propagation in uniform arrays of photonic crystal waveguides is studied. We demonstrate that, in stark contrast to the case of conventional waveguide arrays, diffraction can be tailored both in magnitude and sign by varying only the spacing between adjacent waveguides. Diffraction management in ultracompact arrays of straight photonic crystal waveguides is demonstrated by solving Maxwell's equations through the time-domain finite-element method. 相似文献
122.
The one-loop vacuum energy is explicitly computed for a class of perturbative string vacua where supersymmetry is spontaneously broken by a T-duality invariant asymmetric Scherk–Schwarz deformation. The low-lying spectrum is tachyon-free for any value of the compactification radii and thus no Hagedorn-like phase-transition takes place. Indeed, the induced effective potential is free of divergence, and has a global anti-de Sitter minimum where geometric moduli are naturally stabilised. 相似文献
123.
We report a systematic analysis of anomalous refractive effects at interfaces between two photonic crystal waveguide arrays. Discrete negative refraction can be easily predicted from the sign of the coupling coefficient between adjacent waveguides, regardless of handedness of propagation. 相似文献
124.
We consider Maxwell’s equations with periodic coefficients as it is usually done for the modeling of photonic crystals. Using Bloch/Floquet theory, the problem reduces in a standard way to a modification of the Maxwell cavity eigenproblem with periodic boundary conditions. Following [8], a modification of edge finite elements is considered for the approximation of the band gap. The method can be used with meshes of tetrahedrons or parallelepipeds. A rigorous analysis of convergence is presented, together with some preliminary numerical results in 2D, which fully confirm the robustness of the method. The analysis uses well established results on the discrete compactness for edge elements, together with new sharper interpolation estimates. 相似文献
125.
126.
Pierre Bonami Gérard Cornuéjols Sanjeeb Dash Matteo Fischetti Andrea Lodi 《Mathematical Programming》2008,113(2):241-257
Recent experiments by Fischetti and Lodi show that the first Chvátal closure of a pure integer linear program (ILP) often
gives a surprisingly tight approximation of the integer hull. They optimize over the first Chvátal closure by modeling the
Chvátal–Gomory (CG) separation problem as a mixed integer linear program (MILP) which is then solved by a general- purpose
MILP solver. Unfortunately, this approach does not extend immediately to the Gomory mixed integer (GMI) closure of an MILP,
since the GMI separation problem involves the solution of a nonlinear mixed integer program or a parametric MILP. In this
paper we introduce a projected version of the CG cuts, and study their practical effectiveness for MILP problems. The idea
is to project first the linear programming relaxation of the MILP at hand onto the space of the integer variables, and then
to derive Chvátal–Gomory cuts for the projected polyhedron. Though theoretically dominated by GMI cuts, projected CG cuts
have the advantage of producing a separation model very similar to the one introduced by Fischetti and Lodi, which can typically
be solved in a reasonable amount of computing time.
Gérard Cornuéjols was supported in part by NSF grant DMI-0352885, ONR grant N00014-03-1-0188, and ANR grant BLAN 06-1-138894.
Matteo Fischetti was supported in part by the EU projects ADONET (contract n. MRTN-CT-2003-504438) and ARRIVAL (contract n.
FP6-021235-2). Andrea Lodi was supported in part by the EU projects ADONET (contract n. MRTN-CT-2003-504438) and ARRIVAL (contract
n. FP6-021235-2). 相似文献
127.
In this paper, new lower bounds for the asymmetric travelling salesman problem are presented, based on spanning arborescences. The new bounds are combined in an additive procedure whose theoretical performance is compared with that of the Balas and Christofides procedure (1981). Both procedures have been imbedded in a simple branch and bound algorithm and experimentally evaluated on hard test problems. 相似文献
128.
129.
Given the integer polyhedronP
t
:= conv{x ∈ℤ
n
:Ax⩽b}, whereA ∈ℤ
m × n
andb ∈ℤ
m
, aChvátal-Gomory (CG)cut is a valid inequality forP
1 of the type λτAx⩽⌊λτb⌋ for some λ∈ℝ
+
m
such that λτA∈ℤ
n
. In this paper we study {0, 1/2}-CG cuts, arising for λ∈{0, 1/2}
m
. We show that the associated separation problem, {0, 1/2}-SEP, is equivalent to finding a minimum-weight member of a binary
clutter. This implies that {0, 1/2}-SEP is NP-complete in the general case, but polynomially solvable whenA is related to the edge-path incidence matrix of a tree. We show that {0, 1/2}-SEP can be solved in polynomial time for a
convenient relaxation of the systemAx<-b. This leads to an efficient separation algorithm for a subclass of {0, 1/2}-CG cuts, which often contains wide families of
strong inequalities forP
1. Applications to the clique partitioning, asymmetric traveling salesman, plant location, acyclic subgraph and linear ordering
polytopes are briefly discussed. 相似文献
130.
Manzoni L Arosio D Belvisi L Bracci A Colombo M Invernizzi D Scolastico C 《The Journal of organic chemistry》2005,70(10):4124-4132
[reaction: see text] An efficient and operationally simple method for the synthesis of functionalized azaoxobicyclo[X.3.0]alkane amino acids has been devised. The key step is an intramolecular nitrone cycloaddition on 5-allyl- or 5-homoallylproline that was found to be completely regio- and stereoselective. 相似文献