553 research outputs found
Note on the Complexity of the Mixed-Integer Hull of a Polyhedron
We study the complexity of computing the mixed-integer hull
of a polyhedron .
Given an inequality description, with one integer variable, the mixed-integer
hull can have exponentially many vertices and facets in . For fixed,
we give an algorithm to find the mixed integer hull in polynomial time. Given
and fixed, we compute a vertex description of
the mixed-integer hull in polynomial time and give bounds on the number of
vertices of the mixed integer hull
Equivariant Perturbation in Gomory and Johnson's Infinite Group Problem. VII. Inverse semigroup theory, closures, decomposition of perturbations
In this self-contained paper, we present a theory of the piecewise linear
minimal valid functions for the 1-row Gomory-Johnson infinite group problem.
The non-extreme minimal valid functions are those that admit effective
perturbations. We give a precise description of the space of these
perturbations as a direct sum of certain finite- and infinite-dimensional
subspaces. The infinite-dimensional subspaces have partial symmetries; to
describe them, we develop a theory of inverse semigroups of partial bijections,
interacting with the functional equations satisfied by the perturbations. Our
paper provides the foundation for grid-free algorithms for the Gomory-Johnson
model, in particular for testing extremality of piecewise linear functions
whose breakpoints are rational numbers with huge denominators.Comment: 67 pages, 21 figures; v2: changes to sections 10.2-10.3, improved
figures; v3: additional figures and minor updates, add reference to IPCO
abstract. CC-BY-S
The Triangle Closure is a Polyhedron
Recently, cutting planes derived from maximal lattice-free convex sets have
been studied intensively by the integer programming community. An important
question in this research area has been to decide whether the closures
associated with certain families of lattice-free sets are polyhedra. For a long
time, the only result known was the celebrated theorem of Cook, Kannan and
Schrijver who showed that the split closure is a polyhedron. Although some
fairly general results were obtained by Andersen, Louveaux and Weismantel [ An
analysis of mixed integer linear sets based on lattice point free convex sets,
Math. Oper. Res. 35 (2010), 233--256] and Averkov [On finitely generated
closures in the theory of cutting planes, Discrete Optimization 9 (2012), no.
4, 209--215], some basic questions have remained unresolved. For example,
maximal lattice-free triangles are the natural family to study beyond the
family of splits and it has been a standing open problem to decide whether the
triangle closure is a polyhedron. In this paper, we show that when the number
of integer variables the triangle closure is indeed a polyhedron and its
number of facets can be bounded by a polynomial in the size of the input data.
The techniques of this proof are also used to give a refinement of necessary
conditions for valid inequalities being facet-defining due to Cornu\'ejols and
Margot [On the facets of mixed integer programs with two integer variables and
two constraints, Mathematical Programming 120 (2009), 429--456] and obtain
polynomial complexity results about the mixed integer hull.Comment: 39 pages; made self-contained by merging material from
arXiv:1107.5068v
Minimizing Cubic and Homogeneous Polynomials over Integers in the Plane
We complete the complexity classification by degree of minimizing a
polynomial over the integer points in a polyhedron in . Previous
work shows that optimizing a quadratic polynomial over the integer points in a
polyhedral region in can be done in polynomial time, while
optimizing a quartic polynomial in the same type of region is NP-hard. We close
the gap by showing that this problem can be solved in polynomial time for cubic
polynomials.
Furthermore, we show that the problem of minimizing a homogeneous polynomial
of any fixed degree over the integer points in a bounded polyhedron in
is solvable in polynomial time. We show that this holds for
polynomials that can be translated into homogeneous polynomials, even when the
translation vector is unknown. We demonstrate that such problems in the
unbounded case can have smallest optimal solutions of exponential size in the
size of the input, thus requiring a compact representation of solutions for a
general polynomial time algorithm for the unbounded case
Finite element analysis solution applications to photoreceptor modules
One of the primary components in a Xerox copier is the print engine. The center of this engine is comprised of a photoreceptor, which is a roller/belt module mounted to a frame. The belt revolves around the module acquiring and transposing toner to sheets of paper as they come into contact with the module. The initial design of these modules can often lead to registration and print quality problems later in the assembly and application phases of design. The current analysis procedure includes lengthy commercial FEA packages that require high designer investment. For this reason, many new ideas are never given the opportunity to develop. The implementation of a low investment analysis step which is designed to reveal problems with a design\u27s general formulation could save the corporation both time and money. The means of statically approximating designs before they are modeled in commercial FEA packages could allow for more module configurations to be analyzed and considered. This low investment means of approximation has been developed here. A user friendly Excel spreadsheet based generic photoreceptor module analyzer is derived, explained, and correlated in the ensuing analysis. Although approximate, the ability to compare designs and choose the best one for the application makes this analysis successful. The generic modeling capability is automated such that user interaction is minimal and navigation is relatively simple. Also included in this thesis is a step by step instruction set for inputting module parameters and running the program. A Nastran FEA model was constructed and correlated to this solver, which was shown to retain the correct order of magnitude (micron level) and overall deformation shape. Future adjustments and other software capabilities are also discussed
Geology, Mantle Tomography, and Inclination Corrected Paleogeographic Trajectories Support Westward Subduction During Cretaceous Orogenesis in the North American Cordillera
Geological evidence, including the presence of two passive margin platforms, juxtaposed and mismatched deformation between North America and more outboard terranes, as well as the lack of rift deposits, suggest that North America was the lower plate during both the Sevier and Laramide events and that subduction dipped westward beneath the Cordilleran Ribbon Continent (Rubia). Terranes within the composite ribbon continent, now present in the Canadian Cordillera, collided with western North America during the 125â105 Ma Sevier event and were transported northward during the ~80â58 Ma Laramide event, which affected the Cordillera from South America to Alaska. New high-resolution mantle tomography beneath North America reveals a huge slab wall that extends vertically for over 1000 km, marks the site of long-lived subduction, and provides independent verification of the westward-dipping subduction model. Other workers analyzed paleogeographic trajectories and concluded that the initial collision took place in Canada at about 160 Ma â a time and place for which there is no deformational thickening on the North American platform â and later farther west where subduction was not likely westward, but eastward. However, by utilizing a meridionally corrected North American paleogeographic trajectory, coupled with the geologically most reasonable location for the initial deformation, the position of western North America with respect to the relict superslab parsimoniously accounts for the timing and extents of both the Sevier and Laramide events. SOMMAIRELes indications gĂ©ologiques, en particulier la prĂ©sence de deux marges de plateforme passives, de dĂ©formations adjacentes non-conformes entre lâAmĂ©rique du Nord et les terranes extĂ©rieurs, ainsi que lâabsence de gisements de rift, permet de croire que lâAmĂ©rique du Nord Ă©tait la plaque sous-jacente durant les Ă©vĂ©nements de Sevier et de Laramide et que la subduction plongeait vers lâouest sous le continent rubanĂ© de la CordillĂšres (Rubia). Les terranes du continent rubanĂ© composite, maintenant au sein de la CordillĂšre canadienne, sont entrĂ©s en collision avec lâouest de lâAmĂ©rique du Nord durant lâĂ©vĂ©nement Sevier (125-105 Ma), et ont Ă©tĂ© transportĂ©s vers le nord durant lâĂ©vĂ©nement Laramide (~80â58 Ma), laquelle a affectĂ© la CordillĂšre, de lâAmĂ©rique du Sud Ă lâAlaska. Une nouvelle tomographie haute rĂ©solution du manteau sous lâAmĂ©rique du Nord montre la prĂ©sence dâun gigantesque mur de plaques vertical qui sâĂ©tend sur 1 000 km, marque le site dâune subduction de longue haleine, et offre une validation indĂ©pendante du modĂšle dâune subduction Ă pendage vers lâouest. Dâautres chercheurs ont analysĂ© les trajectoires palĂ©ogĂ©ographiques et conclu que la collision initiale sâest produite au Canada vers 160 Ma â un moment et un endroit sans Ă©paississement par dĂ©formation sur la plateforme dâAmĂ©rique du Nord â et plus tard plus Ă lâouest, lĂ oĂč la subduction nâĂ©tait vraisemblablement pas vers lâouest, mais vers lâest. Cela dit, en considĂ©rant une trajectoire palĂ©ogĂ©ographique de lâAmĂ©rique du Nord corrigĂ©e longitudinalement, avec la position gĂ©ologique la plus probable de la dĂ©formation initiale, la position de la portion ouest de lâAmĂ©rique du Nord par rapport aux restes de la super-plaque explique alors facilement la chronologie et lâĂ©tendue des Ă©pisodes Sevier et Laramide
Continuous Equality Knapsack with Probit-Style Objectives
We study continuous, equality knapsack problems with uniform separable,
non-convex objective functions that are continuous, strictly increasing,
antisymmetric about a point, and have concave and convex regions. For example,
this model captures a simple allocation problem with the goal of optimizing an
expected value where the objective is a sum of cumulative distribution
functions of identically distributed normal distributions (i.e., a sum of
inverse probit functions). We prove structural results of this model under
general assumptions and provide two algorithms for efficient optimization: (1)
running in linear time and (2) running in a constant number of operations given
preprocessing of the objective function
Arc and Slab-Failure Magmatism in Cordilleran Batholiths I â The Cretaceous Coastal Batholith of Peru and its Role in South American Orogenesis and Hemispheric Subduction Flip
We examined the temporal and spatial relations of rock units within the Western Cordillera of Peru where two Cretaceous basins, the Huarmey-Cañete and the West Peruvian Trough, were considered by previous workers to represent western and eastern parts respectively of the same marginal basin. The Huarmey-Cañete Trough, which sits on Mesoproterozoic basement of the Arequipa block, was filled with up to 9 km of Tithonian to Albian tholeiiticâcalc-alkaline volcanic and volcaniclastic rocks. It shoaled to subaerial eastward. At 105â101 Ma the rocks were tightly folded and intruded during and just after the deformation by a suite of 103 ± 2 Ma mafic intrusions, and later in the interval 94â82 Ma by probable subduction-related plutons of the Coastal batholith. The West Peruvian Trough, which sits on Paleozoic metamorphic basement, comprised a west-facing siliciclastic-carbonate platform and adjacent basin filled with up to 5 km of sandstone, shale, marl and thinly bedded limestone deposited continuously throughout the Cretaceous. Rocks of the West Peruvian Trough were detached from their basement, folded and thrust eastward during the Late CretaceousâEarly Tertiary. Because the facies and facing directions of the two basins are incompatible, and their development and subjacent basements also distinct, the two basins could not have developed adjacent to one another.    Based on thickness, composition and magmatic style, we interpret the magmatism of the Huarmey-Cañete Trough to represent a magmatic arc that shut down at about 105 Ma when the arc collided with an unknown terrane. We relate subsequent magmatism of the early 103 ± 2 Ma syntectonic mafic intrusions and dyke swarms to slab failure. The Huarmey-Cañete-Coastal batholithic block and its Mesoproterozoic basement remained offshore until 77 ± 5 Ma when it collided with, and was emplaced upon, the partially subducted western margin of South America to form the east-vergent Marañon foldâthrust belt. A major pulse of 73â62 Ma plutonism and dyke emplacement followed terminal collision and is interpreted to have been related to slab failure of the west-dipping South American lithosphere. Magmatism, 53 Ma and younger, followed terminal collision and was generated by eastward subduction of Pacific oceanic lithosphere beneath South America.   Similar spatial and temporal relations exist over the length of both Americas and represent the terminal collision of an arc-bearing ribbon continent with the Americas during the Late CretaceousâEarly Tertiary Laramide event. It thus separated long-standing westward subduction from the younger period of eastward subduction characteristic of today. We speculate that the Cordilleran Ribbon Continent formed during the Mesozoic over a major zone of downwelling between Tuzo and Jason along the boundary of Panthalassic and Pacific oceanic plates.SOMMAIRENous avons Ă©tudiĂ© les relations spatiales et temporales des unitĂ©s de roches dans la portion ouest de la CordillĂšre du PĂ©rou, oĂč deux bassins crĂ©tacĂ©s, la fosse dâaccumulation de Huarmey-Cañete et la fosse dâaccumulation pĂ©ruvienne de lâouest, ont Ă©tĂ© perçues par des auteurs prĂ©cĂ©dents comme les portions ouest et est dâun mĂȘme bassin de marge. La fosse de Huarmey-Cañete, qui repose sur le socle mĂ©soprotĂ©rozoĂŻque du bloc dâArequipa, a Ă©tĂ© comblĂ©e par des couches de roches volcaniques tholĂ©itiques â calco-alcalines de lâAlbien au Thithonien atteignant 9 km dâĂ©paisseur. Vers lâest, lâensemble a fini par former des hauts fonds. Vers 105 Ă 101 Ma, les roches ont Ă©tĂ© plissĂ©es fortement puis recoupĂ©es par une suite dâintrusions vers 103 ± 2 Ma, durant et juste aprĂšs la dĂ©formation, et plus tard dans lâintervalle 94 â 82 Ma, probablement par des plutons de subduction du batholite cĂŽtier. Quant Ă la fosse dâaccumulation pĂ©ruvienne de lâouest, elle repose sur un socle mĂ©tamorphique palĂ©ozoĂŻque, et elle est constituĂ©e dâune plateforme silicoclastique â carbonate Ă pente ouest et dâun bassin contigu comblĂ© par des grĂšs, des schistes, des marnes et des calcaires finement laminĂ©s atteignant 5 km dâĂ©paisseur et qui se sont dĂ©posĂ©s en continu durant tout le CrĂ©tacĂ©. Les roches de la fosse dâaccumulation pĂ©ruvienne de lâouest ont Ă©tĂ© dĂ©collĂ©es de leur socle, plissĂ©es et charriĂ©es vers lâest durant la fin du CrĂ©tacĂ© et le dĂ©but du Tertiaire. Parce que les facies et les profondeurs de sĂ©dimentation de ces deux fosses dâaccumulation dont incompatibles, et que leur dĂ©veloppement et leur socle sont diffĂ©rents, ces deux fosses ne peuvent pas sâĂȘtre dĂ©veloppĂ©es cĂŽte Ă cĂŽte.    à cause de lâĂ©paisseur accumulĂ©e, de sa composition et du style de son magmatisme, nous pensons que la fosse dâaccumulation de Huarmey-Cañete reprĂ©sente un arc magmatique qui sâest Ă©teinte vers 105 Ma, lorsque lâarc est entrĂ© en collision avec un terrane inconnu. Nous pensons que le magmatisme subsĂ©quent aux premiĂšres intrusions mafiques syntectoniques et aux rĂ©seaux de dykes de 103 ± 2 Ma sont Ă mettre au compte dâune rupture de plaque. Le bloc Huarmey-Cañete-batholitique cĂŽtier et son socle mĂ©soprotĂ©rozoĂŻque sont demeurĂ©s au large jusquâĂ Â 77 ± 5 Ma, moment oĂč il est entrĂ© en collision et a Ă©tĂ© poussĂ© par-dessus la marge ouest sud-amĂ©ricaine partiellement subduite, pour ainsi former la zone de chevauchement de vergence est de Marañon. Nous croyons que la sĂ©quence majeure de plutonisme et dâintrusion de dykes qui a succĂ©dĂ© Ă la collision finale Ă 73â62 Ma doit ĂȘtre reliĂ©e Ă une rupture de la plaque lithosphĂ©rique sud-amĂ©ricaine Ă pendage ouest. Le magmatisme de 53 Ma et plus rĂ©cent qui a succĂ©dĂ© Ă la collision finale, a Ă©tĂ© gĂ©nĂ©rĂ© par la subduction vers lâest de la lithosphĂšre ocĂ©anique du Pacifique sous lâAmĂ©rique du Sud.    Des relations temporelles et spatiales similaires qui existent tout le long des deux AmĂ©riques reprĂ©sentent la collision terminale dâun ruban continental dâarcs avec les AmĂ©riques durant la phase tectonique laramienne de la fin du CrĂ©tacĂ©âdĂ©but du Tertiaire. Elle a donc sĂ©parĂ© la subduction vers lâouest de longue date de la pĂ©riode de subduction vers lâest plus jeune caractĂ©risant la situation actuelle. Nous considĂ©rons que le ruban continental de la CordillĂšre sâest constituĂ© durant le MĂ©sozoĂŻque au-dessus dâune zone majeure de convection descendante entre Tuzo et Jason, le long de la limite entre les plaques ocĂ©aniques Panthalassique et Pacifique
- âŠ