4,165 research outputs found
Symmetric ribbon disks
We study the ribbon discs that arise from a symmetric union presentation of a
ribbon knot. A natural notion of symmetric ribbon number is introduced and
compared with the classical ribbon number. We show that the gap between these
numbers can be arbitrarily large by constructing an infinite family of ribbon
knots with ribbon number 2 and arbitrarily large symmetric ribbon number. The
proof is based on a particularly simple description of symmetric unions in
terms of certain band diagrams which leads to an upper bound for the Heegaard
genus of their branched double covers.Comment: 9 pages, 10 figures. Few typos corrected. Final version published in
JKT
Automated Discovery of Internet Censorship by Web Crawling
Censorship of the Internet is widespread around the world. As access to the
web becomes increasingly ubiquitous, filtering of this resource becomes more
pervasive. Transparency about specific content that citizens are denied access
to is atypical. To counter this, numerous techniques for maintaining URL filter
lists have been proposed by various individuals and organisations that aim to
empirical data on censorship for benefit of the public and wider censorship
research community.
We present a new approach for discovering filtered domains in different
countries. This method is fully automated and requires no human interaction.
The system uses web crawling techniques to traverse between filtered sites and
implements a robust method for determining if a domain is filtered. We
demonstrate the effectiveness of the approach by running experiments to search
for filtered content in four different censorship regimes. Our results show
that we perform better than the current state of the art and have built domain
filter lists an order of magnitude larger than the most widely available public
lists as of Jan 2018. Further, we build a dataset mapping the interlinking
nature of blocked content between domains and exhibit the tightly networked
nature of censored web resources
Knot concordance and homology sphere groups
We study two homomorphisms to the rational homology sphere group. If
denotes the inclusion homomorphism from the integral homology sphere group,
then using work of Lisca we show that the image of intersects trivially
with the subgroup of the rational homology sphere group generated by lens
spaces. As corollaries this gives a new proof that the cokernel of is
infinitely generated, and implies that a connected sum of 2-bridge knots is
concordant to a knot with determinant 1 if and only if is smoothly slice.
Furthermore, if denotes the homomorphism from the knot concordance
group defined by taking double branched covers of knots, we prove that the
kernel of contains a summand by analyzing the
Tristram-Levine signatures of a family of knots whose double branched covers
all bound rational homology balls.Comment: 14 pages, 6 figures. Version 3 contains minor changes. This is
essentially the version accepted for publication by International Mathematics
Research Notices (IMRN
Rational approximation to the fractional Laplacian operator in reaction-diffusion problems
This paper provides a new numerical strategy to solve fractional in space
reaction-diffusion equations on bounded domains under homogeneous Dirichlet
boundary conditions. Using the matrix transform method the fractional Laplacian
operator is replaced by a matrix which, in general, is dense. The approach here
presented is based on the approximation of this matrix by the product of two
suitable banded matrices. This leads to a semi-linear initial value problem in
which the matrices involved are sparse. Numerical results are presented to
verify the effectiveness of the proposed solution strategy
Rational cobordisms and integral homology
We consider the question of when a rational homology 3-sphere is rational
homology cobordant to a connected sum of lens spaces. We prove that every
rational homology cobordism class in the subgroup generated by lens spaces is
represented by a unique connected sum of lens spaces whose first homology
embeds in any other element in the same class. As a first consequence, we show
that several natural maps to the rational homology cobordism group have
infinite rank cokernels. Further consequences include a divisibility condition
between the determinants of a connected sum of 2-bridge knots and any other
knot in the same concordance class. Lastly, we use knot Floer homology combined
with our main result to obstruct Dehn surgeries on knots from being rationally
cobordant to lens spaces.Comment: 19 pages, final version to appear in Compositio Mathematic
Embedding 3-manifolds in spin 4-manifolds
An invariant of orientable 3-manifolds is defined by taking the minimum
such that a given 3-manifold embeds in the connected sum of copies of , and we call this the embedding number of the 3-manifold. We
give some general properties of this invariant, and make calculations for
families of lens spaces and Brieskorn spheres. We show how to construct
rational and integral homology spheres whose embedding numbers grow arbitrarily
large, and which can be calculated exactly if we assume the 11/8-Conjecture. In
a different direction we show that any simply connected 4-manifold can be split
along a rational homology sphere into a positive definite piece and a negative
definite piece.Comment: 27 pages, 14 figures. This is the final version. We made several
corrections and small improvements, some suggested by the referee. This paper
has been accepted for publication by the Journal of Topolog
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