99 research outputs found
On the Linear Extension Complexity of Regular n-gons
In this paper, we propose new lower and upper bounds on the linear extension
complexity of regular -gons. Our bounds are based on the equivalence between
the computation of (i) an extended formulation of size of a polytope ,
and (ii) a rank- nonnegative factorization of a slack matrix of the polytope
. The lower bound is based on an improved bound for the rectangle covering
number (also known as the boolean rank) of the slack matrix of the -gons.
The upper bound is a slight improvement of the result of Fiorini, Rothvoss and
Tiwary [Extended Formulations for Polygons, Discrete Comput. Geom. 48(3), pp.
658-668, 2012]. The difference with their result is twofold: (i) our proof uses
a purely algebraic argument while Fiorini et al. used a geometric argument, and
(ii) we improve the base case allowing us to reduce their upper bound by one when for some integer . We conjecture that this new upper bound
is tight, which is suggested by numerical experiments for small . Moreover,
this improved upper bound allows us to close the gap with the best known lower
bound for certain regular -gons (namely, and ) hence allowing for the first time to determine their extension
complexity.Comment: 20 pages, 3 figures. New contribution: improved lower bound for the
boolean rank of the slack matrices of n-gon
Algorithms for Positive Semidefinite Factorization
This paper considers the problem of positive semidefinite factorization (PSD
factorization), a generalization of exact nonnegative matrix factorization.
Given an -by- nonnegative matrix and an integer , the PSD
factorization problem consists in finding, if possible, symmetric -by-
positive semidefinite matrices and such
that for , and . PSD
factorization is NP-hard. In this work, we introduce several local optimization
schemes to tackle this problem: a fast projected gradient method and two
algorithms based on the coordinate descent framework. The main application of
PSD factorization is the computation of semidefinite extensions, that is, the
representations of polyhedrons as projections of spectrahedra, for which the
matrix to be factorized is the slack matrix of the polyhedron. We compare the
performance of our algorithms on this class of problems. In particular, we
compute the PSD extensions of size for the
regular -gons when , and . We also show how to generalize our
algorithms to compute the square root rank (which is the size of the factors in
a PSD factorization where all factor matrices and have rank one)
and completely PSD factorizations (which is the special case where the input
matrix is symmetric and equality is required for all ).Comment: 21 pages, 3 figures, 3 table
Heuristics for Exact Nonnegative Matrix Factorization
The exact nonnegative matrix factorization (exact NMF) problem is the
following: given an -by- nonnegative matrix and a factorization rank
, find, if possible, an -by- nonnegative matrix and an -by-
nonnegative matrix such that . In this paper, we propose two
heuristics for exact NMF, one inspired from simulated annealing and the other
from the greedy randomized adaptive search procedure. We show that these two
heuristics are able to compute exact nonnegative factorizations for several
classes of nonnegative matrices (namely, linear Euclidean distance matrices,
slack matrices, unique-disjointness matrices, and randomly generated matrices)
and as such demonstrate their superiority over standard multi-start strategies.
We also consider a hybridization between these two heuristics that allows us to
combine the advantages of both methods. Finally, we discuss the use of these
heuristics to gain insight on the behavior of the nonnegative rank, i.e., the
minimum factorization rank such that an exact NMF exists. In particular, we
disprove a conjecture on the nonnegative rank of a Kronecker product, propose a
new upper bound on the extension complexity of generic -gons and conjecture
the exact value of (i) the extension complexity of regular -gons and (ii)
the nonnegative rank of a submatrix of the slack matrix of the correlation
polytope.Comment: 32 pages, 2 figures, 16 table
A Homotopy-based Algorithm for Sparse Multiple Right-hand Sides Nonnegative Least Squares
Nonnegative least squares (NNLS) problems arise in models that rely on
additive linear combinations. In particular, they are at the core of
nonnegative matrix factorization (NMF) algorithms. The nonnegativity constraint
is known to naturally favor sparsity, that is, solutions with few non-zero
entries. However, it is often useful to further enhance this sparsity, as it
improves the interpretability of the results and helps reducing noise. While
the -"norm", equal to the number of non-zeros entries in a vector, is a
natural sparsity measure, its combinatorial nature makes it difficult to use in
practical optimization schemes. Most existing approaches thus rely either on
its convex surrogate, the -norm, or on heuristics such as greedy
algorithms. In the case of multiple right-hand sides NNLS (MNNLS), which are
used within NMF algorithms, sparsity is often enforced column- or row-wise, and
the fact that the solution is a matrix is not exploited. In this paper, we
first introduce a novel formulation for sparse MNNLS, with a matrix-wise
sparsity constraint. Then, we present a two-step algorithm to tackle
this problem. The first step uses a homotopy algorithm to produce the whole
regularization path for all the -penalized NNLS problems arising in
MNNLS, that is, to produce a set of solutions representing different tradeoffs
between reconstruction error and sparsity. The second step selects solutions
among these paths in order to build a sparsity-constrained matrix that
minimizes the reconstruction error. We illustrate the advantages of our
proposed algorithm for the unmixing of facial and hyperspectral images.Comment: 20 pages + 7 pages supplementary materia
Yard traffic and congestion in container terminals
In container terminals, the yard is the zone where containers are stored: it serves as a buffer for loading, unloading and transshipping containers. Yard is usually the bottleneck of the terminal, as traffic, congestion and capacity issues mainly originate from there. A berth&yard allocation plan assigns ships to berths and containers to yard blocks: a typical objective is to minimize the total distance traveled by the carriers to transfer containers from the quayside to the yard. Although congestion issues are usually disregarded, operations are often slowed down because of overloaded areas in the yard. In particular, we are interested in evaluating and reducing traffic congestion induced by the berth&yard allocation plan at the operational level. In this work, we firstly present quantitative indicators we devised to measure congestion in the terminal network and to capture its effects on terminals efficiency. These measures are then combined to provide new objective functions, other than traveled distance, which are used in the design of berth&yard templates; a local search algorithm which minimizes congestion is presented. Computational results are analyzed and discussed
Sunlight refraction in the mesosphere of Venus during the transit on June 8th, 2004
Many observers in the past gave detailed descriptions of the telescopic
aspect of Venus during its extremely rare transits across the Solar disk. In
particular, at the ingress and egress, the portion of the planet's disk outside
the Solar photosphere has been repeatedly perceived as outlined by a thin,
bright arc ("aureole"). Those historical visual observations allowed inferring
the existence of Venus' atmosphere, the bright arc being correctly ascribed to
the refraction of light by the outer layers of a dense atmosphere. On June 8th,
2004, fast photometry based on electronic imaging devices allowed the first
quantitative analysis of the phenomenon. Several observers used a variety of
acquisition systems to image the event -- ranging from amateur-sized to
professional telescopes and cameras -- thus collecting for the first time a
large amount of quantitative information on this atmospheric phenomenon. In
this paper, after reviewing some elements brought by the historical records, we
give a detailed report of the ground based observations of the 2004 transit.
Besides confirming the historical descriptions, we perform the first
photometric analysis of the aureole using various acquisition systems. The
spatially resolved data provide measurements of the aureole flux as a function
of the planetocentric latitude along the limb. A new differential refraction
model of solar disk through the upper atmosphere allows us to relate the
variable photometry to the latitudinal dependency of scale-height with
temperature in the South polar region, as well as the latitudinal variation of
the cloud-top layer altitude. We compare our measurements to recent analysis of
the Venus Express VIRTIS-M, VMC and SPICAV/SOIR thermal field and aerosol
distribution. Our results can be used a starting point for new, more optimized
experiments during the 2012 transit event.Comment: Icarus, in pres
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