1,107 research outputs found
Lin-Kernighan Heuristic Adaptations for the Generalized Traveling Salesman Problem
The Lin-Kernighan heuristic is known to be one of the most successful
heuristics for the Traveling Salesman Problem (TSP). It has also proven its
efficiency in application to some other problems. In this paper we discuss
possible adaptations of TSP heuristics for the Generalized Traveling Salesman
Problem (GTSP) and focus on the case of the Lin-Kernighan algorithm. At first,
we provide an easy-to-understand description of the original Lin-Kernighan
heuristic. Then we propose several adaptations, both trivial and complicated.
Finally, we conduct a fair competition between all the variations of the
Lin-Kernighan adaptation and some other GTSP heuristics. It appears that our
adaptation of the Lin-Kernighan algorithm for the GTSP reproduces the success
of the original heuristic. Different variations of our adaptation outperform
all other heuristics in a wide range of trade-offs between solution quality and
running time, making Lin-Kernighan the state-of-the-art GTSP local search.Comment: 25 page
Branching on multi-aggregated variables
open5siopenGamrath, Gerald; Melchiori, Anna; Berthold, Timo; Gleixner, Ambros M.; Salvagnin, DomenicoGamrath, Gerald; Melchiori, Anna; Berthold, Timo; Gleixner, Ambros M.; Salvagnin, Domenic
Theoretical study of scattering in graphene ribbons in the presence of structural and atomistic edge roughness
We investigate the diffusive electron-transport properties of charge-doped
graphene ribbons and nanoribbons with imperfect edges. We consider different
regimes of edge scattering, ranging from wide graphene ribbons with (partially)
diffusive edge scattering to ribbons with large width variations and
nanoribbons with atomistic edge roughness. For the latter, we introduce an
approach based on pseudopotentials, allowing for an atomistic treatment of the
band structure and the scattering potential, on the self-consistent solution of
the Boltzmann transport equation within the relaxation-time approximation and
taking into account the edge-roughness properties and statistics. The resulting
resistivity depends strongly on the ribbon orientation, with zigzag (armchair)
ribbons showing the smallest (largest) resistivity and intermediate ribbon
orientations exhibiting intermediate resistivity values. The results also show
clear resistivity peaks, corresponding to peaks in the density of states due to
the confinement-induced subband quantization, except for armchair-edge ribbons
that show a very strong width dependence because of their claromatic behavior.
Furthermore, we identify a strong interplay between the relative position of
the two valleys of graphene along the transport direction, the correlation
profile of the atomistic edge roughness, and the chiral valley modes, leading
to a peculiar strongly suppressed resistivity regime, most pronounced for the
zigzag orientation.Comment: 13 pages, 7 figure
The role of electron-electron scattering in spin transport
We investigate spin transport in quasi 2DEG formed by III-V semiconductor
heterojunctions using the Monte Carlo method. The results obtained with and
without electron-electron scattering are compared and appreciable difference
between the two is found. The electron-electron scattering leads to suppression
of Dyakonov-Perel mechanism (DP) and enhancement of Elliott-Yafet mechanism
(EY). Finally, spin transport in InSb and GaAs heterostructures is investigated
considering both DP and EY mechanisms. While DP mechanism dominates spin
decoherence in GaAs, EY mechanism is found to dominate in high mobility InSb.
Our simulations predict a lower spin relaxation/decoherence rate in wide gap
semiconductors which is desirable for spin transport.Comment: to appear in Journal of Applied Physic
The new Dutch timetable: The OR revolution
In December 2006, Netherlands Railways introduced a completely new timetable. Its objective was to facilitate the growth of passenger and freight transport on a highly utilized railway network, and improve the robustness of the timetable resulting in less train delays in the operation. Further adjusting the existing timetable constructed in 1970 was not option anymore, because further growth would then require significant investments in the rail infrastructure. Constructing a railway timetable from scratch for about 5,500 daily trains was a complex problem. To support this process, we generated several timetables using sophisticated operations research techniques, and finally selected and implemented one of these timetables. Furthermore, because rolling-stock and crew costs are principal components of the cost of a passenger railway operator, we used innovative operations research tools to devise efficient schedules for these two resources. The new resource schedules and the increased number of passengers resulted in an additional annual profit of 40 million euros (105 million) annually in the coming years. However, the benefits of the new timetable for the Dutch society as a whole are much greater: more trains are transporting more passengers on the same railway infrastructure, and these trains are arriving and departing on schedule more than they ever have in the past. In addition, the rail transport system will be able to handle future transportation demand growth and thus allow cities to remain accessible. Therefore, people can switch from car transport to rail transport, which will reduce the emission of greenhouse gases.
Characterisation of the secondary-neutron production in particle therapy treatments with the MONDO tracking detector
Particle Therapy (PT) is a non-invasive technique that exploits charged light ions for the irradiation of tumours that cannot be effectively treated with surgery or conventional radiotherapy. While the largest dose fraction is released to the tumour volume by the primary beam, a non-negligible amount of additional dose is due to the beam fragmentation that occurs along the path towards the target volume. In particular, the produced neutrons are particularly dangerous as they can release their energy far away from the treated area, increasing the risk of developing a radiogenic secondary malignant neoplasm after undergoing a treatment. A precise measurement of the neutron flux, energy spectrum and angular distributions is eagerly needed in order to improve the treatment planning system software, so as to predict the normal tissue toxicity in the target region and the risk of late complications in the whole body. The MONDO (MOnitor for Neutron Dose in hadrOntherapy) project is dedicated to the characterisation of the secondary ultra-fast neutrons ([20-400] MeV energy range) produced in PT. The neutron tracking system exploits the reconstruction of the recoil protons produced in two consecutive (n, p) elastic scattering interactions to measure simultaneously the neutron incoming direction and energy. The tracker active media is a matrix of thin squared scintillating fibers arranged in orthogonally oriented layers that are read out by a sensor (SBAM) based on SPAD (Single-Photon Avalanche Diode) detectors developed in collaboration with the Fondazione Bruno Kessler (FBK)
A Discrete State Transition Algorithm for Generalized Traveling Salesman Problem
Generalized traveling salesman problem (GTSP) is an extension of classical
traveling salesman problem (TSP), which is a combinatorial optimization problem
and an NP-hard problem. In this paper, an efficient discrete state transition
algorithm (DSTA) for GTSP is proposed, where a new local search operator named
\textit{K-circle}, directed by neighborhood information in space, has been
introduced to DSTA to shrink search space and strengthen search ability. A
novel robust update mechanism, restore in probability and risk in probability
(Double R-Probability), is used in our work to escape from local minima. The
proposed algorithm is tested on a set of GTSP instances. Compared with other
heuristics, experimental results have demonstrated the effectiveness and strong
adaptability of DSTA and also show that DSTA has better search ability than its
competitors.Comment: 8 pages, 1 figur
Spin diffusion/transport in -type GaAs quantum wells
The spin diffusion/transport in -type (001) GaAs quantum well at high
temperatures ( K) is studied by setting up and numerically solving the
kinetic spin Bloch equations together with the Poisson equation
self-consistently. All the scattering, especially the electron-electron Coulomb
scattering, is explicitly included and solved in the theory. This enables us to
study the system far away from the equilibrium, such as the hot-electron effect
induced by the external electric field parallel to the quantum well. We find
that the spin polarization/coherence oscillates along the transport direction
even when there is no external magnetic field. We show that when the scattering
is strong enough, electron spins with different momentums oscillate in the same
phase which leads to equal transversal spin injection length and ensemble
transversal injection length. It is also shown that the intrinsic scattering is
already strong enough for such a phenomena. The oscillation period is almost
independent on the external electric field which is in agreement with the
latest experiment in bulk system at very low temperature [Europhys. Lett. {\bf
75}, 597 (2006)]. The spin relaxation/dephasing along the diffusion/transport
can be well understood by the inhomogeneous broadening, which is caused by the
momentum-dependent diffusion and the spin-orbit coupling, and the scattering.
The scattering, temperature, quantum well width and external magnetic/electric
field dependence of the spin diffusion is studied in detail.Comment: 12 pages, 6 figures, to be published in J Appl. Phy
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