99 research outputs found
The Packing Radius of a Code and Partitioning Problems: the Case for Poset Metrics
Until this work, the packing radius of a poset code was only known in the
cases where the poset was a chain, a hierarchy, a union of disjoint chains of
the same size, and for some families of codes. Our objective is to approach the
general case of any poset. To do this, we will divide the problem into two
parts.
The first part consists in finding the packing radius of a single vector. We
will show that this is equivalent to a generalization of a famous NP-hard
problem known as "the partition problem". Then, we will review the main results
known about this problem giving special attention to the algorithms to solve
it. The main ingredient to these algorithms is what is known as the
differentiating method, and therefore, we will extend it to the general case.
The second part consists in finding the vector that determines the packing
radius of the code. For this, we will show how it is sometimes possible to
compare the packing radius of two vectors without calculating them explicitly
Bounds for complexity of syndrome decoding for poset metrics
In this work we show how to decompose a linear code relatively to any given
poset metric. We prove that the complexity of syndrome decoding is determined
by a maximal (primary) such decomposition and then show that a refinement of a
partial order leads to a refinement of the primary decomposition. Using this
and considering already known results about hierarchical posets, we can
establish upper and lower bounds for the complexity of syndrome decoding
relatively to a poset metric.Comment: Submitted to ITW 201
Error-block codes and poset metrics
Let P = ({1, 2,..., n}, <=) be a poset, let V-1, V-2,...,V-n, be a family of finite-dimensional spaces over a finite field F-q and let V = V-1 circle plus V-2 circle plus ... V-n. In this paper we endow V with a poset metric such that the P-weight is constant on the non-null vectors of a component V-i, extending both the poset metric introduced by Brualdi et al. and the metric for linear error-block codes introduced by Feng et al.. We classify all poset block structures which admit the extended binary Hamming code [8; 4; 4] to be a one-perfect poset block code, and present poset block structures that turn other extended Hamming codes and the extended Golay code [24; 12; 8] into perfect codes. We also give a complete description of the groups of linear isometrics of these metric spaces in terms of a semi-direct product, which turns out to be similar to the case of poset metric spaces. In particular, we obtain the group of linear isometrics of the error-block metric spaces.Let P = ({1, 2,..., n}, <=) be a poset, let V-1, V-2,...,V-n, be a family of finite-dimensional spaces over a finite field F-q and let V = V-1 circle plus V-2 circle plus ... V-n. In this paper we endow V with a poset metric such that the P-weight is cons2195111FAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOsem informaçã
Block Codes on Pomset Metric
Given a regular multiset on , a partial order
on , and a label map defined by with , we define a pomset block metric
on the direct sum of
based on the pomset . This pomset block
metric extends the classical pomset metric which accommodate Lee
metric introduced by I. G. Sudha and R. S. Selvaraj, in particular, and
generalizes the poset block metric introduced by M. M. S. Alves et al, in
general, over . We find -perfect pomset block codes for both
ideals with partial and full counts. Further, we determine the complete weight
distribution for -space, thereby obtaining it for -space, and
pomset space, over . For chain pomset, packing radius and
Singleton type bound are established for block codes, and the relation of MDS
codes with -perfect codes is investigated. Moreover, we also determine the
duality theorem of an MDS -code when all the blocks have the same
length.Comment: 17 Page
Estimation of Sparsity via Simple Measurements
We consider several related problems of estimating the 'sparsity' or number
of nonzero elements in a length vector by observing only
, where is a predesigned test matrix
independent of , and the operation varies between problems.
We aim to provide a -approximation of sparsity for some constant
with a minimal number of measurements (rows of ). This framework
generalizes multiple problems, such as estimation of sparsity in group testing
and compressed sensing. We use techniques from coding theory as well as
probabilistic methods to show that rows are sufficient
when the operation is logical OR (i.e., group testing), and nearly this
many are necessary, where is a known upper bound on . When instead the
operation is multiplication over or a finite field
, we show that respectively and measurements are necessary and sufficient.Comment: 13 pages; shortened version presented at ISIT 201
Computational Approaches to Lattice Packing and Covering Problems
We describe algorithms which address two classical problems in lattice
geometry: the lattice covering and the simultaneous lattice packing-covering
problem. Theoretically our algorithms solve the two problems in any fixed
dimension d in the sense that they approximate optimal covering lattices and
optimal packing-covering lattices within any desired accuracy. Both algorithms
involve semidefinite programming and are based on Voronoi's reduction theory
for positive definite quadratic forms, which describes all possible Delone
triangulations of Z^d.
In practice, our implementations reproduce known results in dimensions d <= 5
and in particular solve the two problems in these dimensions. For d = 6 our
computations produce new best known covering as well as packing-covering
lattices, which are closely related to the lattice (E6)*. For d = 7, 8 our
approach leads to new best known covering lattices. Although we use numerical
methods, we made some effort to transform numerical evidences into rigorous
proofs. We provide rigorous error bounds and prove that some of the new
lattices are locally optimal.Comment: (v3) 40 pages, 5 figures, 6 tables, some corrections, accepted in
Discrete and Computational Geometry, see also
http://fma2.math.uni-magdeburg.de/~latgeo
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