9,707 research outputs found
Analysis of Spatially-Coupled Counter Braids
A counter braid (CB) is a novel counter architecture introduced by Lu et al.
in 2007 for per-flow measurements on high-speed links. CBs achieve an
asymptotic compression rate (under optimal decoding) that matches the entropy
lower bound of the flow size distribution. Spatially-coupled CBs (SC-CBs) have
recently been proposed. In this work, we further analyze single-layer CBs and
SC-CBs using an equivalent bipartite graph representation of CBs. On this
equivalent representation, we show that the potential and area thresholds are
equal. We also show that the area under the extended belief propagation (BP)
extrinsic information transfer curve (defined for the equivalent graph),
computed for the expected residual CB graph when a peeling decoder equivalent
to the BP decoder stops, is equal to zero precisely at the area threshold.
This, combined with simulations and an asymptotic analysis of the Maxwell
decoder, leads to the conjecture that the area threshold is in fact equal to
the Maxwell decoding threshold and hence a lower bound on the maximum a
posteriori (MAP) decoding threshold. Finally, we present some numerical results
and give some insight into the apparent gap of the BP decoding threshold of
SC-CBs to the conjectured lower bound on the MAP decoding threshold.Comment: To appear in the IEEE Information Theory Workshop, Jeju Island,
Korea, October 201
Conley: Computing connection matrices in Maple
In this work we announce the Maple package conley to compute connection and
C-connection matrices. conley is based on our abstract homological algebra
package homalg. We emphasize that the notion of braids is irrelevant for the
definition and for the computation of such matrices. We introduce the notion of
triangles that suffices to state the definition of (C)-connection matrices. The
notion of octahedra, which is equivalent to that of braids is also introduced.Comment: conley is based on the package homalg: math.AC/0701146, corrected the
false "counter example
Evolution of a barotropic shear layer into elliptical vortices
When a barotropic shear layer becomes unstable, it produces the well known
Kelvin-Helmholtz instability (KH). The non-linear manifestation of KH is
usually in the form of spiral billows. However, a piecewise linear shear layer
produces a different type of KH characterized by elliptical vortices of
constant vorticity connected via thin braids. Using direct numerical simulation
and contour dynamics, we show that the interaction between two
counter-propagating vorticity waves is solely responsible for this KH
formation. We investigate the oscillation of the vorticity wave amplitude, the
rotation and nutation of the elliptical vortex, and straining of the braids.
Our analysis also provides possible explanation behind the formation and
evolution of elliptical vortices appearing in geophysical and astrophysical
flows, e.g. meddies, Stratospheric polar vortices, Jovian vortices, Neptune's
Great Dark Spot and coherent vortices in the wind belts of Uranus.Comment: 7 pages, 4 figures, Accepted in Physical Review
Estimating topological entropy from the motion of stirring rods
Stirring a two-dimensional viscous fluid with rods is often an effective way
to mix. The topological features of periodic rod motions give a lower bound on
the topological entropy of the induced flow map, since material lines must
`catch' on the rods. But how good is this lower bound? We present examples from
numerical simulations and speculate on what affects the 'gap' between the lower
bound and the measured topological entropy. The key is the sign of the rod
motion's action on first homology of the orientation double cover of the
punctured disk.Comment: 10 pages, 20 figures. IUTAM Procedia style (included). Submitted to
volume "Topological Fluid Dynamics II.
Evolution method and "differential hierarchy" of colored knot polynomials
We consider braids with repeating patterns inside arbitrary knots which
provides a multi-parametric family of knots, depending on the "evolution"
parameter, which controls the number of repetitions. The dependence of knot
(super)polynomials on such evolution parameters is very easy to find. We apply
this evolution method to study of the families of knots and links which include
the cases with just two parallel and anti-parallel strands in the braid, like
the ordinary twist and 2-strand torus knots/links and counter-oriented 2-strand
links. When the answers were available before, they are immediately reproduced,
and an essentially new example is added of the "double braid", which is a
combination of parallel and anti-parallel 2-strand braids. This study helps us
to reveal with the full clarity and partly investigate a mysterious
hierarchical structure of the colored HOMFLY polynomials, at least, in
(anti)symmetric representations, which extends the original observation for the
figure-eight knot to many (presumably all) knots. We demonstrate that this
structure is typically respected by the t-deformation to the superpolynomials.Comment: 31 page
- …