17 research outputs found
Intrinsic symmetry groups of links with 8 and fewer crossings
We present an elementary derivation of the "intrinsic" symmetry groups for
knots and links of 8 or fewer crossings. The standard symmetry group for a link
is the mapping class group \MCG(S^3,L) or \Sym(L) of the pair .
Elements in this symmetry group can (and often do) fix the link and act
nontrivially only on its complement. We ignore such elements and focus on the
"intrinsic" symmetry group of a link, defined to be the image of
the natural homomorphism \MCG(S^3,L) \rightarrow \MCG(S^3) \cross \MCG(L).
This different symmetry group, first defined by Whitten in 1969, records
directly whether is isotopic to a link obtained from by permuting
components or reversing orientations.
For hyperbolic links both \Sym(L) and can be obtained using the
output of \texttt{SnapPea}, but this proof does not give any hints about how to
actually construct isotopies realizing . We show that standard
invariants are enough to rule out all the isotopies outside for all
links except , and where an additional construction
is needed to use the Jones polynomial to rule out "component exchange"
symmetries. On the other hand, we present explicit isotopies starting with the
positions in Cerf's table of oriented links which generate for each
link in our table. Our approach gives a constructive proof of the
groups.Comment: 72 pages, 66 figures. This version expands the original introduction
into three sections; other minor changes made for improved readabilit
A new cohomological formula for helicity in reveals the effect of a diffeomorphism on helicity
The helicity of a vector field is a measure of the average linking of pairs
of integral curves of the field. Computed by a six-dimensional integral, it is
widely useful in the physics of fluids. For a divergence-free field tangent to
the boundary of a domain in 3-space, helicity is known to be invariant under
volume-preserving diffeomorphisms of the domain that are homotopic to the
identity. We give a new construction of helicity for closed -forms on a
domain in -space that vanish when pulled back to the boundary of the
domain. Our construction expresses helicity in terms of a cohomology class
represented by the form when pulled back to the compactified configuration
space of pairs of points in the domain. We show that our definition is
equivalent to the standard one. We use our construction to give a new formula
for computing helicity by a four-dimensional integral. We provide a Biot-Savart
operator that computes a primitive for such forms; utilizing it, we obtain
another formula for helicity. As a main result, we find a general formula for
how much the value of helicity changes when the form is pushed forward by a
diffeomorphism of the domain; it relies upon understanding the effect of the
diffeomorphism on the homology of the domain and the de Rham cohomology class
represented by the form. Our formula allows us to classify the
helicity-preserving diffeomorphisms on a given domain, finding new
helicity-preserving diffeomorphisms on the two-holed solid torus, and proving
that there are no new helicity-preserving diffeomorphisms on the standard solid
torus. We conclude by defining helicities for forms on submanifolds of
Euclidean space. In addition, we provide a detailed exposition of some standard
`folk' theorems about the cohomology of the boundary of domains in .Comment: 51 pages, 5 figures. For v2, references updated, typos corrected, and
a new appendix explaining how the Hodge Decomposition Theorem for forms on
manifolds with boundary affects our theorems added. For v3, corrected an
error in the caption to Figure 3 and updated reference
The Biot-Savart operator and electrodynamics on subdomains of the three-sphere
We study steady-state magnetic fields in the geometric setting of positive
curvature on subdomains of the three-dimensional sphere. By generalizing the
Biot-Savart law to an integral operator BS acting on all vector fields, we show
that electrodynamics in such a setting behaves rather similarly to Euclidean
electrodynamics. For instance, for current J and magnetic field BS(J), we show
that Maxwell's equations naturally hold. In all instances, the formulas we give
are geometrically meaningful: they are preserved by orientation-preserving
isometries of the three-sphere.
This article describes several properties of BS: we show it is self-adjoint,
bounded, and extends to a compact operator on a Hilbert space. For vector
fields that act like currents, we prove the curl operator is a left inverse to
BS; thus the Biot-Savart operator is important in the study of curl
eigenvalues, with applications to energy-minimization problems in geometry and
physics. We conclude with two examples, which indicate our bounds are typically
within an order of magnitude of being sharp.Comment: 24 pages (was 28 pages) Revised to include a new introduction, a
detailed example, and results about helicity; other changes for readabilit
An infinite family of convex Brunnian links in
This paper proves that convex Brunnian links exist for every dimension by constructing explicit examples. These examples are three-component
links which are higher-dimensional generalizations of the Borromean rings.Comment: 10 pages, 4 figure
The Shapes of Tight Composite Knots
We present new computations of tight shapes obtained using the constrained
gradient descent code RIDGERUNNER for 544 composite knots with 12 and fewer
crossings, expanding our dataset to 943 knots and links. We use the new data
set to analyze two outstanding conjectures about tight knots, namely that the
ropelengths of composite knots are at least 4\pi-4 less than the sums of the
prime factors and that the writhes of composite knots are the sums of the
writhes of the prime factors.Comment: Summary text file of tight knot lengths and writhing numbers stored
in anc/ropelength_data.txt. All other data freely available at
http:://www.jasoncantarella.com/ and through Data Conservanc
A pathway-specific microarray analysis highlights the complex and co-ordinated transcriptional networks of the developing grain of field-grown barley
The aim of the study was to describe the molecular and biochemical interactions associated with amino acid biosynthesis and storage protein accumulation in the developing grains of field-grown barley. Our strategy was to analyse the transcription of genes associated with the biosynthesis of storage products during the development of field-grown barley grains using a grain-specific microarray assembled in our laboratory. To identify co-regulated genes, a distance matrix was constructed which enabled the identification of three clusters corresponding to early, middle, and late grain development. The gene expression pattern associated with the clusters was investigated using pathway-specific analysis with specific reference to the temporal expression levels of a range of genes involved mainly in the photosynthesis process, amino acid and storage protein metabolism. It is concluded that the grain-specific microarray is a reliable and cost-effective tool for monitoring temporal changes in the transcriptome of the major metabolic pathways in the barley grain. Moreover, it was sensitive enough to monitor differences in the gene expression profiles of different homologues from the storage protein families. The study described here should provide a strong complement to existing knowledge assisting further understanding of grain development and thereby provide a foundation for plant breeding towards storage proteins with improved nutritional quality
A NEW COHOMOLOGICAL FORMULA FOR HELICITY IN R 2k+1 REVEALS THE EFFECT OF A DIFFEOMORPHISM ON HELICITY
Abstract. The helicity of a vector field is a measure of the average linking of pairs of integral curves of the field. Computed by a six-dimensional integral, it is widely useful in the physics of fluids. For a divergence-free field tangent to the boundary of a domain in 3-space, helicity is known to be invariant under volume-preserving diffeomorphisms of the domain that are homotopic to the identity. We give a new construction of helicity for closed (k + 1)-forms on a domain in (2k + 1)-space that vanish when pulled back to the boundary of the domain. Our construction expresses helicity in terms of a cohomology class represented by the form when pulled back to the compactified configuration space of pairs of points in the domain. We show that our definition is equivalent to the standard one. We use our construction to give a new formula for computing helicity by a four-dimensional integral. We provide a Biot-Savart operator that computes a primitive for such forms; utilizing it, we obtain another formula for helicity. As a main result, we find a general formula for how much the value of helicity changes when the form is pushed forward by a diffeomorphism of the domain; it relies upon understanding the effect of the diffeomorphism on the homology of the domain and the de Rham cohomology class represented by the form. Our formula allows us to classify the helicity-preserving diffeomorphisms on a given domain, finding new helicity-preserving diffeomorphisms on the two-holed solid torus and proving that there are no new helicity-preserving diffeomorphisms on the standard solid torus. We conclude by defining helicities for forms on submanifolds of Euclidean space. In addition, we provide a detailed exposition of some standard āfolk ā theorems about the cohomology of the boundary of domains in R 2k+1. 1
Graduate Group ChairpersonAcknowledgements
Most graduate students search for one good advisor; I am fortunate to have two wonderful advisors, Dennis DeTurck and Herman Gluck. Maybe I needed twice the guidance of an average student. They both have been extremely generous with their time, insights, and patience. Herman taught me that math simply cannot be done without tea and cookies. He also taught me that, no matter how hectic academic life may be, there is always time to play tennis or at least talk wistfully about it. Dennis is probably the most efficient person I know; he is excellent at multi-tasking. Dennis taught me a lot about math when I was teaching with him and taught me a lot about teaching when I was working on math with him. My years at Penn would not nearly have been as pleasant or productive without the great office staff here. Janet Burns has gone out of her way to assist me whenever I needed it; she encouraged me and kept me out of trouble for six years, even as I procrastinated my way past deadline after deadline. Janet, I appreciate everything you have done for me! Paula Airey, Monica Pallanti, and Robin Toney have been terrifically helpful; Paula and Monica can cheer up even the dreariest day. I want to acknowledge the continued support of my parents, who have always believed in me and helped me succeed. There are many others at Penn whose wisdom and camaraderie are much appreciated. Among the faculty, I especially want to mention Chris Croke, John Etnyre, and Steve Shatz. So many graduate students have been encouraging, including a great list of officemates: Darren Glass, Nadia Masri, Nakia Rimmer, Sukhendu Mehrotra, and Fred Butler. My girlfriend Sarah has inspired me, comforted me, kept me sane, and forced me to enjoy life during a hectic time. Thanks for all your understanding Sarah! The Biot-Savart operator and electrodynamics on bounded subdomains of the three-spher