Differential Geometry of Framed Curves 2022
General Information
Professor:
Hours:
Lectures and Exercises: Each week you should watch the videos of lectures *before* the class time. Then we meet in person each week [starting 26.09]: on Mondays 13h15 to 17h, in room MAA331, first to have an in person discussion of the video lectures and to answer any questions that arise. Then continue with the traditional Exercise sessions, where all the material is already available on the web page.
Assistant:
Course
Requirements
1st and 2nd year courses in math or physics, (or with teacher's permission)
Recommended
Ordinary Differential Equations, BA Math (MATH301)
Contents
This course will describe the classic differential geometry of curves, tubes and ribbons, and associated coordinate systems. We will prove various classic mathematical theorems such as the WeylHotelling formula for tube volumes, and the relation between Link, Twist and Writhe, which couples differential geometry and topological invariance for closed and knotted framed curves. While we will not consider applications explicitly in this course, much of the mathematical material that will be described is central in various problems of mechanics, including nanostructures and topological fluid mechanics.
Parts:
1) Framed Curvesbasic differential geometry of curves in the group SE(3) of rigid body displacements
2) The CalugareanuFullerWhite Theorem Lk = Tw + Wr. Topology meets vector analysis and nonuniform convergence
3) The geometry of Coordinates on SO(3) and 2pi vs 4pi. Euler angles, Cayley vectors, Euler parameters, and quaternions.
4) Fattened curves, Tubes and Ribbons. Contact framings, global radius of curvature, and ideal shapes. (As in previous years, due to first week off because of Jeûne Fédéral, this part is not included in the exam)
Lecture notes
These notes are meant to supplement your personal notes. They are to be understood as a first draft and as such may contain inaccuracies and mistakes. Be critical and do not hesitate to let us know if you find errors and/or typos. Furthermore, they make no pretence of being exhaustive. The material of the course is by definition what is exposed during lectures and exercise sessions. Finally, as the semester progresses, come back to the website and check frequently: pay attention to the version numbers. The notes will be edited as we progress during semester. The original document is a collection of chapters corresponding to the different lectures given last year. The horizontal red line indicates where the oral lecture is at.
There is an older polycopie associated with a DNA modelling masters course with some chapters, specifically chapters 8 and 9 on this page.
Weekbyweek correspondence
Week 1 (26.09)  Overview of course and physical demonstrations of the Calugareanu $Lk=Tw+ Wr$ formula. Basic vector and matrix notation. Arclength, curvature, (geometrical) torsion, and SerretFrenet equations for a space curve. You can download the recorded lesson by means of video (Corresponding notes of the first lesson are here, and prior JHM notes here).  
Week 2 (03.10)  Finish SerretFrenet equations for a space curve in R^3. The Lie groups $O(3)$, $SO(3)$, and $SE(3)$. Left and right actions in $SO(3)$ and $SE(3)$, both algebraic and geometric interpretation. Curves in $SO(3)$ and in $SE(3)$. You can download the recorded lesson by means of the following video . (Corresponding notes of the second lesson are here, and prior JHM notes here). Note that the first part of the video lectures introducing the SerretFrenet frame follow the text in Section 1.2 in the TeXed lecture notes available higher up in this page.  
Week 3 (10.10)  Darboux vector of a curve in $SE(3)$. Darboux vector of the Frenet frame. Framed curves, intrinsic, extrinsic, adapted or not. Factorisations of curves in $SO(3)$ and relations between their Darboux vectors. Relations between two adapted framings particularly important. Factored curves in $SE(3)$, and the special case of two offset curves in $\mathbb R^3$. You can download the recorded lesson by means of the following video. (Corresponding notes of the third lesson are here, and prior JHM notes here).  
Week 4 (17.10)  Statement of Calugareanu Theorem for a smoothly closed curve $x$ and a smoothly closed offset curve $y$. First definitions of a) Link $Lk$ of two closed nonintersecting curves, b) Total twist $Tw$ of a unit normal field about a curve $x$, and c) Writhe $Wr$ of a nonselfintersecting curve $x$. Start of discussion of the properties of Link. You can download the recorded lesson by means of the following video. (Corresponding notes of the 4th lesson are here, and prior JHM notes here).  
Week 5 (24.10)  Further properties of Link. Discussion on the zodiacus and its boundary. Interpretation of Link as as signed area on the unit sphere. Homotopy invariance of Link, with application to the Hopf Link (see also Exercise sheet 4). You can download the recorded lesson by means of the following video. (Corresponding notes of the 5th lesson are here, and prior JHM notes here).  
Week 6 (31.10)  Review of the properties of Link. Rule for computing Link by counting signed crossings. Example of the Whitehead Link (see also Exercise sheet 5) Connection between signed area integral definition of Link and counts of signed crossings in one specific (generic) projection Mathematica files and images used in the demonstration of Link as signed area are in this archive. You can download the recorded lesson by means of the following video. (Corresponding notes of the 6th lesson are here).  
Week 7 (07.11)  Curves lying on general surfaces, and the surface normal extrinsic adapted framing. The special case of curves lying on the unit sphere, and the particular case of the tangent indicatrix or tantrix of curve. Introduction to the next exercise series, tantrices of closed curves. You can download the recorded lesson by means of video1 and video2. (Corresponding notes of the 7th lesson are here, and prior JHM notes here).  
Week 8 (14.11)  Discussion of properties of Writhe. Interpretation as signed area of the zodiacus (now of one curve instead of two as was the case for Link), and singularities/discontinuities in the boundary of the zodiacus coming from the tantrix of the curve x. Interpretation of writhe in terms of global radius of curvature circles, namely as the signindefinite weighted sum of radii of circles that pass through two points of the curve and are tangent at one point. You can download the recorded lesson by means of the following video. (Corresponding notes of the 8th lesson are here, and prior JHM notes here).  
Week 9 (21.11)  Proof of the main CFW Theorem. Read through Chapter 7 of the polycopie. Note that Chapter 7 was originally written as a standalone document, so much of the first part of the Chapter is a rapid review of material that has already been treated in more detail in the current version of the polycopie. For an expanded version of the proof you can read the handwritten notes of Giulio Corazza here. You can download the recorded lesson by means of the following video.  
Week 10 (28.11)  Strand passages and curves with an even or odd 'number of turns'. Register angle between two framings. Discussion of particular framings of curves: FrenetSerret, surface, and parallel transport framings. You can download the recorded lesson by means of the following video. (Corresponding notes of the 10th lesson are here, and prior JHM notes here).  
Week 11 (05.12)  Continuation of special (throughout always assumed to be adapted) framings of closed curves: completion of natural framings, and introduction of the writhe framing (which is always closed and zero link for any smoothly closed curve). Interpretation of $Tw + Wr$ as the discontinuity angle for closed curves with open framings, and the lemma that framings of closed curves are closed iff $Tw + Wr$ is an integer. Open problem (not examinable): for framed open curves with a spherical closure between the pointtangent data pairs at each end (and with surface normal framing, and either for general curve closure or with biarcs) is there a simple geometrical interpretation of the sum $Tw + Wr$? You can download the recorded lesson by means of the following video. (Corresponding notes of the 11th lesson are here, and prior JHM notes here).  
Week 12 (12.12)  The midpoint rule applied to first order linear matrix ODE to motivate the Cayley transform. In the case of the matrix group $SO(3)$ connections between the Darboux vector and the Cayley vector of the Cayley transform (sometimes also called the Gibbs vector). Properties of the Cayley transform and the EulerRodrigues formula in terms of the Cayley vector. You can download the recorded lesson by means of the following video. (Corresponding notes of the 12th lesson are here together with the corrigendum. Prior JHM notes here).  
Week 13 (19.12)  EulerRodrigues Parameters as a double covering of $SO(3)$ to avoid singularities. EulerRodrigues formula in terms of the Cayley vector and connections to the EulerRodrigues parameters via stereographic projection. Discussion of the multiply covered circle in light of Euler parameters and tracking rotations mod $4\pi$ along curves of rotation matrices. Besides the polycopie (part III, chapter 9), see also these notes from Dichmann. Geometrical interpretation of a rotation as product of two reflections. Householder reflection matrices. Composition rule for EulerRodrigues parameters based on geometrical reflection decomposition. You can find the proof of the latter statement in Dichmann's notes. Composition rule for Cayley vectors exploiting the one for Euler parameters. It is also possible to recover the same composition rule starting from the definition of Cayley transform but the proof is somehow tedious and long. Introduction to quaternions. Who wants to give a look to the geometrical proof of the composition rule directly derived for Cayley vectors can read these notes by Thomas Zwahlen. Moreover, you may find more information about the parametrization of the rotation group in the article section of the page, where the 1840 original paper by Rodrigues is included. You can download the recorded lesson by means of video 1 and video 2. (Corresponding notes of the last year are here, and prior JHM notes here).  
Summary and description of the exercices
This document contains an overview and a description of all the exercises given so far.
Exercices
Note that there are 12 exercices sheets for 13 weeks. Exercice session 8 was dedicated to the proof of CFW theorem, thus from that week exercice sheet n is to be done on week n+1.Séries d'exercices  Corrigés 

Bibliography
There is no text book that we are aware of covering the material of this course. The first part on Frenet frames is however very standard and is discussed in any book on the Elementary Differential Geometry of Curves and Surfaces, of which there are many. One good one is by D. J. Struik, and another (from which some of the series questions were taken) is by M. P. Do Carmo.
The citations below are to research or survey articles concerning the material of the course. The citations have links to PDF versions of the articles but for copyright reasons the links are restricted to students in the class via password protection, login: frames, password as given in class.
Two articles, partially contradictory, regarding the history of the Gauss linking number :

Orbits of asteroids, a braid, and the first link invariant, M. Epple, Math. Intell., vol. 20 (1998), pp. 4552
DOI: 10.1007/BF03024400. 
Gauss' Linking Number Revisited, R. L. Ricca and B. Nipoti, J. Knot Theory Its Ramifications 20 (2011), pp. 13251343
DOI: 10.1142/S0218216511009261.
Three survey articles, some pages from a standard undergraduate text book, and an introductory monograph all discussing link, twist, and writhe, and the CalugareanuFullerWhite (CFW) theorem including connections with DNA

Questions de topologie en biologie moléculaire, C. Weber, Gazette des mathématiciens, Vol. 64 (1995), pp. 2942.
 This one is from a particular point of view (the author Pohl was White's PhD supervisor) DNA and differential geometry, W. F. Pohl, The Mathematical Intelligencer, vol. 3 (1980), pp. 2027.
 An article (there are many others) describing connexions to DNA:
DNA Topology: Fundamentals, S. M. Mirkin, Life Sciences, vol. 123 (2001), pp. 11
DOI: 10.1021/ja0156845.
 The CFW theorem in a standard biochemistry textbook for first year medical students
Biochemistry, D. Voet and J. Voet, (1994)
 An introductory monograph on knot theory in science New Scientific Applications of Geometry and Topology, De Witt L. Sumners et al. (1992)
An article on magnetohydrodynamics which relates helicity of a vector field to the CFW. The introduction contains a historical perspective of the CFW theorem.
 Helicity and the Calugareanu invariant, H. K. Moffat and R. Ricca Proc. R. Soc. Lond. A, vol 439 (1992), pp 411429.
An article on scroll waves, with an extensive discussion of the CFW theorem, including another historical perspective.
 The differential geometry of scroll waves, J. J. Tyson and S. H. Strogatz Int. Jour. Bifurcation and Chaos, vol 1 (1991), pp 723744.
A topology textbook which gives a friendly introduction to the subject. Chapters 10 and 14 are the most pertinent to the course.
 Knots, Molecules and the Universe: An Introduction to Topology, E. Flapan AMS NonSeries Monographs, vol 96 (2016)
A beautiful little book "A singular mathematical promenade" by Étienne Ghys. The most pertinent chapter for this course is the second to last one all about the Gauss linking number.
 A Singular Mathematical Promenade, E. Ghys ENS Éditions, hors collection (2017)
Some articles on the parametrization of the 3dimensional rotation group. In particular, the second one talks about the history of the composition rule and you can also find an explanation of the formula derived by Rodrigues in its original paper (third of the list).
 On the Parametrization of the ThreeDimensional Rotation Group, John Stuelpnagel, SIAM Review, Vol. 6, No. 4. (Oct., 1964), pp. 422430.
 An Historical Note on Finite Rotations, Hui Cheng  K. C.Gupta, Journal of Applied Mechanics, 1989, Vol. 56/145
 Des lois géométriques qui régissent les déplacements d’un système solide dans l’espace, et de la variation des coordonnées provenant de ces déplacements considérés indépendamment des causes qui peuvent les produire, M. Olinde Rodrigues, Journal de mathématiques pures et appliquées 1re série, tome 5 (1840), p. 380440.
More technical articles
The three original articles by Călugăreanu (the third one in Romanian): L'integrale de Gauss et l'analyse des noeuds tridimensionnels, G. Călugăreanu, Rev. Math. pures appl, vol. 4 (1959).
 Sur les classes d'isotopie des noeuds tridimensionnels et leurs invariants, G. Călugăreanu, Czechoslov. Math. J., vol. 11 (1961), pp. 588625.
 O Teorema Asupra Inlantuirilor Tridimensionale de Curbe Inchise, G. Călugăreanu, Comunicarile Academiei Republicii Populare Romine, (1961), pp. 829832.

The writhing number of a space curve, F. B. Fuller, Proc. Natl. Acad. Sci. U. S. A., vol. 68 (1971), pp. 815819
DOI: 10.1073/pnas.68.4.815.

How the writhing number of a curve depends on the curve, F. B. Fuller, Rev. Math. pures appl, vol. XVII (1972)

Decomposition of the linking number of a closed ribbon: A problem from molecular biology., F. B. Fuller, Proc. Natl. Acad. Sci. U. S. A., vol. 75 (1978), pp. 35573561
DOI: 10.1073/pnas.75.8.3557.

Formulae for the calculation and estimation of writhe, J. Aldinger, I. Klapper, and M. Tabor, J. Knot Theory and Its Ramifications, vol. 04 (1995), 343.

On White's formula, M.H. Eggar, J. Knot Theory Ramifications, vol. 09 (2000).

On the differential geometry of closed space curves, W. Fenchel, Bull. Am. Math. Soc., vol. 57 (1951), pp. 4455
DOI: 10.1090/S000299041951094409.
 The SelfLinking Number of a Closed Space Curve, W. F. Pohl, J. Math. Mech., vol. 17 (1968), pp. 975985.

Ribbons: Their Geometry and Topology, C. K. Au and T. C. Woo, ComputerAided Design and Applications, Vol. 1 (2004), pp. 16.
 An article on molecules with Möbius topology Möbius molecules with twists and writhes, S. R. Schaller and Rainer Herges, Chem. Commun., vol. 49 (2013), pp. 12541260.

Link, Twist, Energy, and the Stability of DNA Minicircles, K. A. Hoffman, R. S. Manning, and J. H. Maddocks, Biopolymers, vol. 70 (2003), pp. 145157.

Geometry of Călugăreanu theorem, M. R. Dennis and J. H. Hannay, Proc. Roy. Soc. A, vol. 461 (2005), pp. 32453254.
 The article that introduced global radius of curvature
Global curvature, thickness, and the ideal shapes of knots, O. Gonzalez and J. H. Maddocks, Proc. Natl. Acad. Sci. USA, vol. 96 (1994), pp. 47694773.

Best packing in proteins and DNA, A. Stasiak, and J. H. Maddocks, Nature, vol. 406 (2000), pp. 251253.

Optimal shapes of compact strings, A. Maritan, C. Micheletti, A. Trovato, and J. R. Banavar, Nature, vol. 406 (2000), pp. 287290.

On the writhe of nonclosed curves, E. L. Starostin, arXiv 0212095, (2002).

Computing the Writhing Number of a Polygonal Knot, P. K. Agarwal, H. E. Edelsbrunner, and Y. Wang, Discrete Comput Geom 32:37–53 (2004).

The writhe of open and closed curves, M. A. Berger and C. Prior, J. Phys. A: Math. Gen., vol 39, (2006), pp. 8321–8348.

The extended polar writhe: a tool for open curves mechanics, C. Prior and S. Neukirch (2015) hal01228386.

Writhing Geometry at Finite Temperature: Random Walks and Geometric phases for Stiff Polymers, A. C. Maggs.

Writhing geometry of open DNA, V. Rossetto and A. C. Maggs.

Computation of Writhe in Modeling of Supercoiled DNA, K. Klenin and J. Langowski, Biopolymers 54.5 (2000): 307317.
 White's orginal contribution in generalising "CFW" to higher dimensions
Selflinking and the gauss integral in higher dimensions., J.H. White, American Jour. Math., vol. 91 (1969), pp. 693728.
 A paper on generalising Link, Twist, and Writhe to nonEuclidean three dimensional spaces. Although it assumes prior knowledge of differential geometry on Lie groups, Section 2 is accessible by all and provides a nice historical background.
Electrodynamics and the Gauss linking integral on the 3sphere and in hyperbolic 3
space, D. DeTurck and H. Gluck, Jour. Math. Phys. vol 49 (2008), 023504.