702 research outputs found
Refined Complexity of PCA with Outliers
Principal component analysis (PCA) is one of the most fundamental procedures
in exploratory data analysis and is the basic step in applications ranging from
quantitative finance and bioinformatics to image analysis and neuroscience.
However, it is well-documented that the applicability of PCA in many real
scenarios could be constrained by an "immune deficiency" to outliers such as
corrupted observations. We consider the following algorithmic question about
the PCA with outliers. For a set of points in , how to
learn a subset of points, say 1% of the total number of points, such that the
remaining part of the points is best fit into some unknown -dimensional
subspace? We provide a rigorous algorithmic analysis of the problem. We show
that the problem is solvable in time . In particular, for constant
dimension the problem is solvable in polynomial time. We complement the
algorithmic result by the lower bound, showing that unless Exponential Time
Hypothesis fails, in time , for any function of , it is
impossible not only to solve the problem exactly but even to approximate it
within a constant factor.Comment: To be presented at ICML 201
Complexity of Token Swapping and its Variants
In the Token Swapping problem we are given a graph with a token placed on
each vertex. Each token has exactly one destination vertex, and we try to move
all the tokens to their destinations, using the minimum number of swaps, i.e.,
operations of exchanging the tokens on two adjacent vertices. As the main
result of this paper, we show that Token Swapping is -hard parameterized
by the length of a shortest sequence of swaps. In fact, we prove that, for
any computable function , it cannot be solved in time where is the number of vertices of the input graph, unless the ETH
fails. This lower bound almost matches the trivial -time algorithm.
We also consider two generalizations of the Token Swapping, namely Colored
Token Swapping (where the tokens have different colors and tokens of the same
color are indistinguishable), and Subset Token Swapping (where each token has a
set of possible destinations). To complement the hardness result, we prove that
even the most general variant, Subset Token Swapping, is FPT in nowhere-dense
graph classes.
Finally, we consider the complexities of all three problems in very
restricted classes of graphs: graphs of bounded treewidth and diameter, stars,
cliques, and paths, trying to identify the borderlines between polynomial and
NP-hard cases.Comment: 23 pages, 7 Figure
East African topography and volcanism explained by a single, migrating plume
Anomalous topographic swells and Cenozoic volcanism in east Africa have been associated with mantle plumes. Several models involving one or more fixed plumes beneath the northeastward migrating African plate have been suggested to explain the space-time distribution of magmatism in east Africa. We devise paleogeographically constrained global models of mantle convection and, based on the evolution of flow in the deepest lower mantle, show that the Afar plume migrated southward throughout its lifetime. The models suggest that the mobile Afar plume provides a dynamically consistent explanation for the spatial extent of the southward propagation of the east African rift system (EARS), which is difficult to explain by the northeastward migration of Africa over one or more fixed plumes alone, over the last ≈45 Myrs. We further show that the age-progression of volcanism associated with the southward propagation of EARS is consistent with the apparent surface hotspot motion that results from southward motion of the modelled Afar plume beneath the northeastward migrating African plate. The models suggest that the Afar plume became weaker as it migrated southwards, consistent with trends observed in the geochemical record
Axiomatic Digital Topology
The paper presents a new set of axioms of digital topology, which are easily
understandable for application developers. They define a class of locally
finite (LF) topological spaces. An important property of LF spaces satisfying
the axioms is that the neighborhood relation is antisymmetric and transitive.
Therefore any connected and non-trivial LF space is isomorphic to an abstract
cell complex. The paper demonstrates that in an n-dimensional digital space
only those of the (a, b)-adjacencies commonly used in computer imagery have
analogs among the LF spaces, in which a and b are different and one of the
adjacencies is the "maximal" one, corresponding to 3n\"i1 neighbors. Even these
(a, b)-adjacencies have important limitations and drawbacks. The most important
one is that they are applicable only to binary images. The way of easily using
LF spaces in computer imagery on standard orthogonal grids containing only
pixels or voxels and no cells of lower dimensions is suggested
Metallic nanoparticles meet Metadynamics
We show how standard Metadynamics coupled with classical Molecular Dynamics
can be successfully ap- plied to sample the configurational and free energy
space of metallic and bimetallic nanopclusters via the implementation of
collective variables related to the pair distance distribution function of the
nanoparticle itself. As paradigmatic examples we show an application of our
methodology to Ag147, Pt147 and their alloy AgshellPtcore at 1:1 and 2:1
chemical compositions. The proposed scheme is not only able to reproduce known
structural transformation pathways, as the five and the six square-diamond
mechanisms both in pure and core-shell nanoparticles but also to predict a new
route connecting icosahedron to anti-cuboctahedron.Comment: 7 pages, 8 figure
Double Scaling in Tensor Models with a Quartic Interaction
In this paper we identify and analyze in detail the subleading contributions
in the 1/N expansion of random tensors, in the simple case of a quartically
interacting model. The leading order for this 1/N expansion is made of graphs,
called melons, which are dual to particular triangulations of the D-dimensional
sphere, closely related to the "stacked" triangulations. For D<6 the subleading
behavior is governed by a larger family of graphs, hereafter called cherry
trees, which are also dual to the D-dimensional sphere. They can be resummed
explicitly through a double scaling limit. In sharp contrast with random matrix
models, this double scaling limit is stable. Apart from its unexpected upper
critical dimension 6, it displays a singularity at fixed distance from the
origin and is clearly the first step in a richer set of yet to be discovered
multi-scaling limits
The double scaling limit of random tensor models
Tensor models generalize matrix models and generate colored triangulations of
pseudo-manifolds in dimensions . The free energies of some models have
been recently shown to admit a double scaling limit, i.e. large tensor size
while tuning to criticality, which turns out to be summable in dimension less
than six. This double scaling limit is here extended to arbitrary models. This
is done by means of the Schwinger--Dyson equations, which generalize the loop
equations of random matrix models, coupled to a double scale analysis of the
cumulants.Comment: 37 pages, 13 figures; several references were added. A new subsection
was added to first present all the results (before the technical proofs which
will follow). A misprint was correcte
- …