# Tracing-Problems in Dynamic Geometry

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1 Tracing-Problems in Dynamic Geometry (Pfadverfolgungsprobleme aus der Dynamischen Geometrie) Dissertation zur Erlangung des Doktorgrades vorgelegt am Fachbereich Mathematik und Informatik der Freien Universität Berlin 2008 von Britta Denner-Broser Institut für Informatik Freie Universität Berlin Takustraße Berlin zum Teil gefördert durch die DFG im Rahmen des Europäischen Graduiertenkollegs Combinatorics, Geometry, and Computation (GRK 588/3)

2

3 Betreuer: Prof. Dr. Helmut Alt Institut für Informatik Freie Universität Berlin Takustraße Berlin Germany Gutachter: Prof. Dr. Helmut Alt Prof. Dr. Ulrich Kortenkamp Institut für Informatik Freie Universität Berlin Institut für Mathematik und Informatik Pädagogischen Hochschule Schwäbisch Gmünd Takustraße 9 Oberbettringer Straße Berlin Schwäbisch Gmünd Deutschland Deutschland Vorlage zur Begutachtung: 29. Juli 2008 Termin der Disputation: 1. Dezember 2008

4 c Britta Denner-Broser, 2008.

5 Preface Quo vadis This famous Latin verse goes back to the Bible 1 and can be translated as Where are you going?. It perfectly reflects the subject of this thesis: Given is a geometric construction possibly done with a geometry software on a computer. In general, a geometric construction consists of independent objects, which are placed at arbitrary positions at the beginning, and constructed elements like a line connecting two given points. If an independent object from the construction is moved, then an important question is what happens with the remaining geometric objects? Where are they going? This problem is essential to make geometry dynamic: After making a geometric construction with a geometry software, we can move the construction by dragging independent objects with the mouse, expecting that the whole construction adjusts automatically. As usual, the devil is in the details: The two words adjusts automatically make the field of Dynamic Geometry difficult and thus interesting. If angular bisectors, circles or even conics are involved, then ambiguities arise. Adjust automatically now means to choose the right possibility among a certain number of choices at each time of the motion. But which choice is the right one? This answer is easy: The right choice is the one the user of a Dynamic Geometry Software expects. But the user could be grabby and could expect Continuity: In the motion of the construction, no jumps occur; Conservatism: Reversing moves leads back to the initial configuration; Determinism: For each placement of the free objects there is at most one configuration of the construction; and Consistency: Degenerate situations are treated consistently and, thus, the behavior at degeneracies is somehow predictable. 1 e.g. Genesis 16:8: dixit ad eam Agar ancilla Sarai unde venis et quo vadis quae respondit a facie Sarai dominae meae ego fugio; He said, Hagar, servant of Sarai, where have you come from, and where are you going? She replied, I m running away from my mistress, Sarai., Judges 19:17, or John 13:36. i

6 ii PREFACE These are four wishes, and unfortunately it is not possible to satisfy all of them simultaneously. In my opinion, the first wish is the most important one since unmotivated and unpredictable jumps are difficult to handle. The last wish Consistency is related to Continuity. In this thesis, I consider the beautiful model for Dynamic Geometry that has been developed by Kortenkamp and Richter-Gebert as the foundation of the interactive Geometry Software Cinderella. The continuity of the induced motions is the natural center of the mathematical model. The consistent treatment of critical points is achieved by the creative idea to allow complex coordinates for the geometric objects. In my thesis, I investigate an algebraic variant of this model for Dynamic Geometry. I greatly enjoyed to see that this nice model perfectly fits into the areas of pure and applied mathematics! Based on this knowledge, we (Ulli and myself) found a reliable algorithm for a continuous realization of the drag mode. Ulli told me about Dynamic Geometry at a party at Stefan Felsner s, and I am really glad that Stefan invited both of us! This party was the beginning of my research in the field of Dynamic Geometry. I would like to thank Ulli and Helmut for their friendly, persistent and very constructive support! I would like to say a special thank you to Ulli for all the interesting, motivating and helpful discussions, he always took time for me when I needed some help! He directed my attention to the area of Dynamic Geometry and proposed the subject of my thesis together with Helmut. I would like to give sincere thanks to Helmut for the fruitful discussions, he suggested to consider Tarski-formulas. This idea led to our proof of the decidability of the Tracing Problem and the Reachability Problem by considering semi-algebraic sets. This result was an important step of my thesis. Helmut and Ulli always encouraged and supported me to run my family and to write my thesis! Their attitude and support was and is very helpful for my whole family! I would like to thank all members of the research group Theoretical Computer Science of FU-Berlin for the very nice research atmosphere, special thanks to Géraldine, Tobias and Jens who shared and share the office with me! Particular thanks to Domique Michelucci (Université de Bourgogne) and Martin Weiser (Zuse Institut Berlin), who proposed independently to use interval arithmetic for solving the Tracing Problem! I greatly appreciate Hanfried Lenz (born 1916), he immediately answered my and recommended to look at Bieberbach s book about geometric constructions. I would like to thank the members of the Department of Mathematics and Computer Science of FU-Berlin. I had the opportunity to borrow a large number of famous old books from the library of our department. At this point, I would particularly like to thank Elmar Vogt and Karin Gatermann, my mathematical knowledge is based on their lectures, seminars, and my master s thesis (Diplomarbeit)! I am very sad that Karin died on the first of January I would like to thank Dirk and Elke for very helpful proofreading! I also would like to say a special thank you to Marco, Stephan and Beni for their

7 friendship and for all the lectures and seminars we attended together! Grit gives me whenever necessary a chance to escape the stressful everyday life, thanks a lot! I would like to thank Dirk for very careful proofreading! Special thanks to my whole family, who supported me all the time. My parents abandoned one week of their holidays to enable me to finish this thesis. My grandmother (born 1921) took care of Lisa when she was ill, my parents in law always walk the dog with Lisa and Svenja when the girls feel too sick for visiting preschool. My sister Tanja supports me with her amazing educational background, she found the Bible verses for this preface. Similar to our situation, my brother Matthias and his friend Felissa manage their (PhD-) studies and a family, as well. This common background is very helpful! At this point, I would like to commemorate my three grandparents, who have passed away. Opa Waldemar would have been very happy to witness the success of my PhD-studies at FU-Berlin. In particular, I would like to thank my husband Philip and our children Lisa and Svenja, they had to put a lot of effort in my thesis, as well. During the last months, Philip had to combine his full-time position in a pharmaceutical company with two children and a wife, who was always busy with her thesis. Lisa and Svenja had to abandon a lot of time 2 with me and had a very impatient mother in the last months! To compensate this lack of attention, Lisa visited us almost every night. iii 2 Lisa: Mama, ich fahre jetzt ganz langsam, weil ich noch ein bisschen bei Dir sein möchte. Heute ist doch der letzte Tag, an dem Du nicht nach Hause kommst! ; Svenja: Mama Uni!

8 iv PREFACE

9 Zusammenfassung Unter dynamischer Geometrie versteht man das interaktive Erstellen von geometrischen Konstruktionen am Computer. Ein Dynamisches Geometrie System ist ein Geometriesystem, in dem es möglich ist, geometrische Konstruktionen durchzuführen, und das einen Zugmodus hat. Im Zugmodus können geometrische Objekte, die mindesten einen Freiheitsgrad haben, mit der Maus bewegt werden. Dabei paßt sich die gesamte geometrische Konstruktion der Bewegung an, indem der Computer das entstehende Pfadverfolgungsproblem löst. In dem von uns verwendeten Modell für dynamische Geometrie steht die Stetigkeit der resultierenden Bewegungen im Vordergrund, es wurde von Kortenkamp und Richter- Gebert entwickelt und ist die Grundlage für die Geometriesoftware Cinderella. Wir arbeiten den Zusammenhang dieses Modells zu Riemannschen Flächen algebraischer Funktionen heraus. Im Rahmen dieser Doktorarbeit zeigen wir, wie sich eine algebraische Variante des Modells für Dynamische Geometrie von Kortenkamp und Richter-Gebert sowohl in die angewandte als auch in die reine Mathematik einfügt. Daraus resultiert ein numerisches Verfahren für das Tracing Problem, das auf einer allgemeinen Prediktor-Korrektor-Methode aufbaut. Wie bei den meisten numerischen Verfahren gibt es hierbei keine Garantie dafür, dass die Schrittweite klein genug gewählt ist, um auf dem richtigen Lösungsweg zu bleiben. Das bedeutet, dass ein korrekter Umgang mit Mehrdeutigkeiten nicht garantiert werden kann. Wir haben einen weiteren Algorithmus entwickelt, bei dem die Schrittweite mit Hilfe von Intervallrechnung so gewählt wird, dass die Korrektheit der Lösung garantiert ist. Kritische Punkte werden durch Umwege umgangen, bei denen die geometrischen Objekte bzw. die entsprechenden Variablen in einem algebraischen Modell komplexe Koordinaten haben können. Dabei hängt die erreichte Konfiguration wesentlich von dem gewählten Umweg ab. Diese Idee von Kortenkamp und Richter-Gebert führt zu einer konsistenten Behandlung von kritischen Punkten und kommt in der interaktiven Geometriesoftware Cinderella zum Einsatz. v

10 vi ZUSAMMENFASSUNG

11 Abstract Dynamic Geometry is the field of interactively doing geometric constructions using a computer. Usually, the classical ruler-and-compass constructions are considered. The available tools are simulated by the computer. A Dynamic Geometry System is a system to do geometric constructions that has a drag mode. In the drag mode, geometric elements with at least one degree of freedom can be moved, and the remaining part of the geometric construction adjusts automatically. Thus, the computer has to trace the paths of the involved geometric objects during the motion. In this thesis, we focus on the beautiful model by Kortenkamp and Richter-Gebert that is the foundation of the geometry software Cinderella. We embed an algebraic variant of this model into different fields of pure and applied mathematics, which leads to different approaches for realizing the drag mode practically. We develop a numerical method to solve the Tracing Problem that is based on a generic Predictor-Corrector method. Like most numerical methods, this method cannot guarantee the correctness of the computed solution curve, hence ambiguities are not treated satisfactorily. To overcome this problem, we develope a second algorithm that uses interval analysis. This algorithm is robust, and the computed step length is small enough to break up all ambiguities. Critical points are bypassed by detours, where the geometric objects or the corresponding variables in the algebraic model can have complex coordinates. Here, the final configuration depends essentially on the chosen detour, but this procedure due to Kortenkamp and Richter-Gebert leads to a consistent treatment of degeneracies. We investigate the connection of the used model for Dynamic Geometry to Riemann surfaces of algebraic functions. vii

12 viii ABSTRACT

13 Contents Preface i Zusammenfassung v Abstract vii 1 Introduction What is Dynamic Geometry? A Model for Dynamic Geometry Properties and Advantages of the Chosen Model Scientific Contribution of this Thesis Organization of the Thesis Geometry versus Algebra The Geometric Situation The Algebraic Situation Linear Constructions Quadratic Constructions Cubic Constructions Using Complex Coordinates Some Basic Concepts from Dynamic Geometry Geometric Straight-Line Programs ix

14 x CONTENTS 3.2 Critical Points Continuity The Tracing Problem and the Reachability Problem The Derivative of a GSP with Respect to Time GSPs and Interval Arithmetic Decidability Aspects in Dynamic Geometry The Reachability Problem The Tracing Problem An Algorithm for the Reachability Problem for Complex GSPs with one Free Variable and a Finite Number of Critical Points The Tracing Problem and Continuation Methods Continuous Evaluations as Implicit Curves Numerical Solutions for the Tracing Problem Reliable Algorithms for the Tracing Problem Main Idea An Algorithm for the Tracing Problem An Algorithm for the Tracing Problem Using Γ The Cone Algorithm Computation of the First Intersection of the Cone C with the t-axis Improvements of the Cone Algorithm The Problem of Overestimation Robustness Cubic and Higher Order Roots Is Affine Arithmetic a Better Choice? Detection and Treatment of Critical Points 105

15 CONTENTS xi 7.1 Detection of Critical Points Treatment of Critical Points Consequences of Detouring How to Approximate all Critical Points of a GSP with one free variable in a Box Using Interval Analysis Algebraic Functions A Brief Introduction Resultants, Discriminants, and their Applications to Algebraic Functions Formal Definition of the Riemann Surface of an Algebraic Function Algebraic Functions and GSPs Algebraic Functions and Continuous Evaluations Continuous Evaluations and Coverings Coverings of Riemann Surfaces A Interval Arithmetic 151 A.1 Real Interval Arithmetic A.2 Complex Interval Arithmetic A.2.1 Rectangular Interval Arithmetic A.2.2 Circular Interval Arithmetic A.3 Rounded Interval Arithmetic A.4 Interval Extensions of Real Functions A.5 About Solving Square Systems of Equations Using Interval Analysis171 A.6 Affine Arithmetic A.6.1 Affine Forms versus Intervals A.6.2 Computing with Affine Arithmetic A.6.3 Problem: Inclusion Monotonicity Bibliography 183

16 xii CONTENTS Index 187 Curriculum Vitae 193

17 Chapter 1 Introduction 1.1 What is Dynamic Geometry? Dynamic Geometry is the field of interactively doing geometric constructions using a computer. Usually, the classical ruler-and-compass constructions are considered. The available tools are simulated by the computer. A Dynamic Geometry System (DGS) is a system to do geometric constructions that has a drag mode (German: Zugmodus). In the drag mode, geometric elements with at least one degree of freedom can be moved, and the remaining part of the geometric construction adjusts automatically; see Figure 1.1. Thus, the computer has to trace the paths of the geometric objects involved during the motion. The main application of Dynamic Geometry is in math education. Doing geometric constructions with a computer avoids inaccuracies due to an imprecise usage of the construction tools like ruler, compass and the used pen. The drag mode allows to develop a better intuition for geometric coherences and helps to understand whether a conjectured theorem is true or false. If you start a geometric construction with an arbitrary quadrilateral, then most people draw an almost square. Without noticing it, the drawn shape has some kind of symmetry. This phenomenon makes it difficult to decide whether an observed property of a geometric construction that is based on this particular shape is a universal property of the shape or not. If we consider the two diagonals of the drawn quadrilateral, we could observe that they bisect each other. This is a property of parallelograms that does not hold for general quadrilaterals; see Figure 1.2. Dynamic Geometry is an important tool for scientists, as well. One application is the investigation of rigid motions of mechanisms [24, 42]. The enhancements of the Dynamic Geometry Systems enlarge the range of use. The additional functionality includes the treatment of conics, plotting loci of constructed points, 1

18 2 CHAPTER 1. INTRODUCTION Q γ 1 S γ 2 l M 1 M 2 P g P Figure 1.1: The left figure shows a geometric construction. The line g connects the two points P and Q. The intersection point M 2 of g and the line l is the center of the circle γ 2. The point S is the upper intersection point of the two circles. The right figure visualizes the drag mode: If the point P is moved, then the entire construction follows the motion of P. dealing with transformations, doing (simple) simulations of physical laws, the connection to computer-algebra, or the possibility to switch to other geometries [42, 41]. To implement the drag mode in a Dynamic Geometry System is a challenging task. The main problem is to deal with the ambiguities due to the involvement of circles or angular bisectors. For example, a circle and a line intersect in two, one or no points, two intersecting lines have two angular bisectors, which are orthogonal. These ambiguities lead to the necessity to choose one of the two possible intersection points or angular bisectors. In the initial drawing, the user of the Dynamic Geometry Software makes these choices. In the drag mode, the computer has to make these decisions autonomously. At each time of the motion, the computer should come to the same decision as the user of the software would come to in the current configuration. Thus, we expect that there are no jumps in the motions of the geometric objects, and we prefer a continuous behavior of the Dynamic Geometry System. In fact, to realize the drag mode, we have to solve a path tracking problem. This problem is called the Tracing Problem from Dynamic Geometry. We are given a starting configuration that was fixed by the user. Additionally, we are given an abstract description of the geometric construction and paths of the free objects; they implicitly define the resulting motion of the dependent elements. The starting configuration acts as an initial value.

19 1.2. A MODEL FOR DYNAMIC GEOMETRY 3 Figure 1.2: Different quadrilaterals with their diagonals are shown. The field of Dynamic Geometry has been founded in the late eighties and the beginning of the nineties of the twentieth century. At this time, the first geometry softwares Cabri Géomètre and Geometer s Sketchpad were developed that realize the drag mode. Up to now, several software packages for Dynamic Geometry have been designed. They all have the functionality to do geometric constructions and provide a drag mode. Additionally, they have different features as described above. However, their drag modes are implemented in different ways and have different properties that arise from the underlying mathematical models. In this thesis, we focus on the beautiful model by Kortenkamp and Richter- Gebert [40, 43] that is the foundation of the geometry software Cinderella [42, 41] and that leads to a continuous behavior of the drag mode. We embed this model into different fields of pure and applied mathematics, which leads to different approaches for realizing the drag mode practically. 1.2 A Model for Dynamic Geometry The results of this thesis are based on the model for Dynamic Geometry developed by Kortenkamp and Richter-Gebert [40, 43], which is the theoretical foundation of the geometry software Cinderella [42, 41]. We explain this model and some resulting problems in this section. A detailed description is given in Chapter 3. In Dynamic Geometry, the step-by-step-procedure of doing geometric constructions can be represented by Geometric Straight-Line Programs (GSP). A GSP is a sequence of instructions that describe the single construction steps or introduce new, independent points; see Figure 1.3. These independent points are called free points. The remaining geometric objects are called dependent elements, they are defined by instructions that belong to a construction step like the computation of a line connecting two points, the intersection point of two lines, or one of the at most two intersection points of a line and a circle. Throughout this thesis, let k be the number of free points in the GSP and n the number of dependent elements. An instance of a GSP is an assignment of fixed values to all free parameters and choices; see [43, 44]. Thus, instances of a GSP correspond to concrete drawings

20 4 CHAPTER 1. INTRODUCTION of the underlying geometric construction. To deal with singularities like the intersection of two identical circles or of a circle and a tangent is important, as well. The corresponding point in the configuration space is called a critical point. Since we work on Dynamic Geometry, we have to formalize movements of constructions. This is done via continuous evaluations [43]: Continuous paths p l (t), t [0, 1] and l = k + 1,...,0, of the free points are given. A continuous evaluation under the movement {p l } is an assignment of continuous paths v i, i = 1,...,n, to all the dependent elements, such that the objects (p k+1 (t),...,p 0 (t),v 1 (t),...,v n (t)) form an instance of the GSP for all times t [0, 1]. Thus a Dynamic Geometry System that is based on the notion of continuous evaluations supports continuous motions of constructions. There are two problems arising naturally from this setup: Problem 1. (Reachability Problem) Two instances A and B of a GSP are given, where A is called a starting instance and B a final instance. Decide whether there are paths {p l } of the free points for which a continuous evaluation from A to B exists. Problem 2. (Tracing Problem) As in the Reachability Problem, we are given a starting instance A and a final instance B. Let p A be the position of the free points at instance A, and p B their position at B. Furthermore, a movement {p l } of the free points from p A to p B is given. We assume that the resulting continuous evaluation starting at A exists. Decide: Does this continuous evaluation end at the given instance B? In [43], Kortenkamp and Richter-Gebert show by a reduction of 3-SAT that these two problems are NP-hard (in R). In [13], decidability aspects are discussed. In this work, we focus on the Tracing Problem and develop different approaches. We give an algorithmic solution with interval analysis [14]; see [1, 50] for standard references for interval arithmetic. We have to solve the Tracing Problem to realize the drag mode. Assume that a user of a Dynamic Geometry software has done a geometric construction. Then what is seen on the screen is an instance A of the GSP describing the underlying construction. Now the user drags a free point with his mouse. This describes a continuous path of the free points: The path of the dragged point is piecewise linear whereas the paths of the other free points are constant. The Dynamic Geometry software has to decide which instance B should be drawn after the motion (and at all intermediate positions).

21 1.2. A MODEL FOR DYNAMIC GEOMETRY 5 P FREE l x JOIN((0, 0), ( 1, 0)) l y JOIN((0, 0), (0, 1)) l JOIN((0, 1),P) Q MEET(l,l x ) C 1 CIRCLE((0, 0), 1 2 ) C 2 CIRCLE(Q, 1 2 ) M MEET(C 1,C 2 ) ( 1, 0) l C 1 0 l y (0, 1) M Q C 2 P l x Figure 1.3: A GSP Γ and an instance of Γ are shown. By construction, Q has to stay on the line l x. A solution to the Reachability Problem could help in automated theorem proving [62, 45]. Here a geometric problem is given as an instance A of a certain GSP. If a probabilistic approach is used as in [40], we have to create instances B of the geometric problem at random. For generating an instance, we have to assign fixed values to all free parameters and to the choices; e.g. we have to specify which intersection point of a line and a circle is chosen. If we can reach the instance B with a continuous evaluation starting at A, then we either have found a counterexample and know that the conjecture is wrong or a positive example, which increases the probability that the conjecture holds; see [40]. Allowing Complex Coordinates. In some situations, considering complex coordinates of the objects is useful as well. The complex Tracing Problem occurs when we are dealing with singularities like the intersection of two identical circles or of a circle and a tangent: Depending on the path p of the free variables, tracing along p might force us to consider such degenerate situations; see Figures 1.3 and 1.4: Let the free point P move on the linear path p(t) = ( ) t ( 3 ) 0. Then at time t 0 = 1 the point P lies on the y-axis l 2 y. Hence, Q reaches the origin, and the two circles are identical. Thus we have a degenerate situation and the intersection point M is not defined. To avoid a singularity S = p(t 0 ), we might bypass it with a detour by modifying p in a neighborhood of t 0. Unfortunately, this might be impossible as our example shows; see Figures 1.3 and 1.4: By construction, the dependent point Q is always incident to the line l x, and the singularity occurs when Q is moved to the origin. Since Q has to stay on l x by construction, the singularity S cannot be avoided by modifying the path p of the free point P. A way out of this problem is to consider detours for that the coordinates of the free points may have non-real coordinates.

22 6 CHAPTER 1. INTRODUCTION M Q Q M p(t) P P Figure 1.4: A singularity occurs if the free point P moves on the path p(t) across the y-axis. Now the line l x of our example becomes a two-dimensional object, and the point Q might bypass S without leaving l x. Additionally, if the singularity is removable, the instance being reached after finishing the detour does not depend on the detour itself (as long as the detour does not catch other singularities). This is an application of the complex Tracing Problem since we have to trace the complex detour instead of the original real path p of the free point P. The complex Reachability Problem seems to be useful for automated theorem proving. This is due to the fact that if a theorem holds over C then it also holds over R. Concerning the Tracing Problem, switching to complex coordinates does not cause many changes since we are tracing given paths. In contrast to this, the Reachability Problem over R and the complex Reachability Problem are somehow different problems; see Figure 1.4: In the real situation, the right instance in Figure 1.4 cannot be reached from the left one, whereas it can be reached via a complex path as described previously. The consequences of using complex coordinates are discussed in Section 2.6. Switching to an Algebraic Model. The geometric situation translates easily into an algebraic model where the objects are numbers (real or complex) and the operations are addition, subtraction, multiplication, division, and taking square roots; see Chapter 2. In this thesis, we discuss algorithms for the Tracing Problem and the Reachability Problem in this algebraic model. We focus on the Tracing Problem. Model of Computation. In our algorithms, we assume that each arithmetic operation and each interval operation is executed exactly and in constant time.

23 1.3. PROPERTIES AND ADVANTAGES OF THE CHOSEN MODEL Properties and Advantages of the Chosen Model for Dynamic Geometry In the chosen model for Dynamic Geometry, the focus lies on a continuous behavior of the Dynamic Geometry System (DGS) and on a consistent treatment of degeneracies. A DGS is a system to do geometric constructions that has a drag mode (German: Zugmodus). The drag mode allows the user of a Dynamic Geometry Software to move geometric objects with at least one degree of freedom. The resulting path of the picked object induces a motion of the entire construction. In fact, the motion of the entire construction is composed by several paths: For every geometric object, we have exactly one path that describes the motion of the corresponding object. Non-moving elements belong to constant paths. At each time of the motion, the constraints of the geometric construction must be fulfilled. Additionally, we prefer a continuous behavior of the DGS: If the picked object is moved on a continuous path, then the induced motion of the construction should be continuous as well. The concept of continuity avoids jumps of the construction. In our model for Dynamic Geometry that has been developed by Kortenkamp and Richter-Gebert [40, 43], the movements of geometric constructions are modeled via continuous evaluations. This notion formalizes and combines the concepts explained above and guarantees a continuous behavior of the DGS. If a free object is moved on a continuous path, then the remaining objects have to move on continuous paths as well, and at each time the given constraints are fulfilled; see page 4 and Section 3.3. The only constraints that are fixed in our model are the relations between the geometric objects. We neither bound nor cut the ranges of the geometric objects. In particular, we consider the configuration space as a whole and do not split it up into regions that are for example separated by degeneracies. These cuts conflict with a global continuous behavior of the DGS, since we might be in trouble if the given motion crosses some of these cuts. The idea of detouring around critical points as introduced by Kortenkamp [40] leads to a consistent treatment of degeneracies; see Section 7.2. Consistency is achieved since all degeneracies that occur at a certain time in the motion are treated simultaneously in the same manner. The idea behind this concept is that the path p(t) of the driving free element is changed by replacing the time parameter t with t(s): [0, 1] C resp. t(s): [t 1,t 2 ] C s 1 eiπs s t eiπs (t 2 2 t 1 ) ( ) We describe the concept of detouring around critical points in Example Note that here only a single degeneracy occurs.

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