Contents | 1 1 Introduction Is the “cable spaghetti” on the floor really knotted or is it enough to pull on both ends of the wire to completely unfold it? Since its invention, knot theory has always been a place of interdisciplinarity. The first knot tables composed by Tait have been motivated by Lord Kelvin’s theory of atoms. But it was not until a century ago that tools for rigorously distinguishing the trefoil and its mirror image as members of two different knot classes were derived. In the first half of the twentieth century, knot theory seemed to be a place mainly drivenby algebraic and combinatorial arguments, mentioning the contributions of Alexander, Reidemeister, Seifert, Schubert, and many others. Besides the development of higher dimensional knot theory, the search for new knot invariants has been a major endeav- our since about 1960. At the same time, connections to applications in DNA biology and statistical physics have surfaced. DNA biology had made a huge progress and it was well un- derstood that the topology of DNA strands matters: modeling of the interplay between molecules and enzymes such as topoisomerases necessarily involves notions of ‘knot- tedness’. Any configuration involving long strands or flexible ropes with a relatively small diameter leads to a mathematical model in terms of curves. Therefore knots appear almost naturally in this context and require techniques from algebra, (differential) geometry, analysis, combinatorics, and computational mathematics. The discovery of the Jones polynomial in 1984 has led to the great popularity of knot theory not only amongst mathematicians and stimulated many activities in this direction. Many deep connections between the Jones polynomial and quantum physics were found. In the 1990s, monographs of Adams, Kauffman, and Livingston addressed a wide audience of mathematicians, from undergraduate students to re- search professionals, disseminated new results and ideas and thereby added to the perception that knot theory is a vivid discipline having impact far beyond its roots. Today we find that knots appear in almost all mathematical disciplines, having important applications in the sciences and, most importantly, that concepts from one field have impact on problems in other areas. Even more, we will see thatapplica- tions do not only make use of existing theories developed in entirely theoretical frame- works, but that questions from the sciences also stimulate theoretical developments in turn. In this edition we focus on four aspects that thematically contour its basis and back- ground and these will be outlined below. Of course, this choice is not meant to be ex- haustive, e.g., we do not cover recent developments in the theory of low-dimensional 2 | Contents topology and restrict ourselves to one of the most thrilling and active parts of algebraic knot theory: Khovanov homology. Applications in the sciences As already pointed out, the initial development of knot theory has been driven by physical motivation. In the second half of the last century, many additional fields of application have emerged. We will illustrate this variety of them by commenting on some examples. At the beginning of the 1960s, it was realized that many molecules consist of long polymeric chains. The general aim is to investigate their structure and the relationship of the observed structures to biological features, their formation, and function. In this context, the question about the likeliness of knots in these structures arose almost naturally. In this time, Frisch and Wasserman as well as Delbrück stated that ring polymers would be knotted with probability increasing to one as their length tends to infin- ity. This conjecture stimulated the study of polygonal knots and could be settled only about thirty years later. An important point in this context is to identify and characterize “knotting” us- ing a topological concept that is applicable to open protein knots. Several approaches have been developed to achieve this goal. As polymers are often modeled by open poly- gons, there is a need to generalize quantities used for the characterization of closed curves, rods, and ropes (such as writhe, self-linking number, and linking number). In order to consider a large variety of possible states, random walks regarding polygons have successfully been investigated and are still a subject of recent activity. Certain knot invariants such as the Jones polynomial have been employed in algorithms to automatically determine the knot class of a randomly chosen polygon. In certain situations one can in fact work with closed curves. Since the 1960s it has been known that there exist some specific circular DNA molecules. In other cases the two sticky ends of the DNA are very close to each other, so modeling by a closed curve seems reasonable. According to experimental observations, molecules forming more complex knots migrate faster than those with less complex knots. The speed seems to be proportional to the average crossing number of the corresponding ideal knots. The latter ones min- imize the ropelength, i.e., the quotient of length over thickness of a given knot. While this definition seems to be conceptionally elementary, it becomes quite involved from an analytical point of view due to its non-smoothness. This is a major issue in geo- metric knot theory which will be addressed below. The average crossing number is a frequently used and studied functional in polymer topology and turned out to be a good measure of polymer compaction. Contents | 3 Long DNA chains are often packed compactly under extreme confining condi- tions. Although the general DNA packing mechanism still lacks a complete under- standing, there have been recent attempts to develop a corresponding DNA packing model and test whether the kind of knot spectrum observed in the experiments can be computationally reproduced by studying equilateral random polygons confined in a sphere. Knots and the concept of ropelength also appear in the context of mathematical physics, for instance in ideal magnetohydrodynamics, with potential applications in the study of astrophysical flows. Magnetic knots are tubular embeddings of the mag- netic field in some torus centered on a smooth oriented loop that is knotted inthefluid domain. They are driven by the Lorentz force to inflexion-free spiral knots and braids. The topological crossing number provides a lower bound on the minimum magnetic energy of knots. Combining results on magnetic energy relaxation and data on numer- ical knot tightening reveals relationships between ropelength and groundstate energy minima. Models of closed rods in elasticity theory exhibit knotted configurations when ap- plying a suitable amount of twist in the case where self-avoidance is not present. Im- posing repulsive forces leads to a variety of different shapes and bifurcation phenom- ena which has not been fully understood yet. Geometric knot theory In the 1990s, the notion of knot energies led to the definition of many functionals on a suitable space of knots, so calculus of variations came into play. Originating from an idea of Fukuhara, the general aim is to investigate geometric properties of a given knotted curve in order to gain information on its knot type. This new subfield emerged from the search of particularly “nice” shapes of knots. In a broader sense, it is part of geometric curvature energies which include geometric integrals measuring smooth- ness and bending for objects that a priori do not have to be smooth, also covering the higher-dimensional case. Moffatt speculated that the DNA molecule seeks to attain some minimum energy state. Up to now, the energy involved here has not been identified, but this idea is one of the main motivations to study repulsive functionals in geometric knot theory. There have been several attempts to employ self-avoiding energies for mathematical models in microbiology. In fact, knot energies basically model self-avoidance, i.e., the functionals blow up on embedded curves converging to a curve with a self-intersection. Following the neg- ative gradient flow of this functional should simultaneously “untangle” the curve and prevent it from leaving the ambient knot class. As long as there are no saddle points of the energy within the unknot class, any such flow is a candidate for a retraction of the unknot class onto the circles which exists due to the Smale conjecture. 4 | Contents In this context, questions of existence and regularity of minimizers are of great interest. The first knot energies defined on smooth curves by O’Hara (which containas a special case the famous “Möbius energy”) provide several interesting features. The definition is quite straight-forward: one penalizes distant strands of the curve witha small euclidean distance. It was the seminal discovery due to Freedman, He, and Wang that the Möbius energy is invariant under the full group of Möbius transformations in R3 which paved the way to proving deep theorems on existence and regularity of minimizers within certain knot classes essentially by geometric means. The conjecture that the Möbius energy of links is minimized by the stereographic projection of the standard Hopf link has been settled using the min-max theory of minimal surfaces which had been developed for proving the Willmore conjecture by Marques and Neves. There are still other open hypotheses concerning the Möbius en- ergy, for instance the non-existence of minimizers in composite knot classes as stated by Kusner and Sullivan. Ropelength, mentioned above, albeit conceptually being the most elementary knot energy one can think of, turns out to be analytically quite involved due to taking suprema which is a non-smooth operation. The existence of ideal knots (ropelength minimizers within a given knot class) has been established independently by several research groups. The problem of ropelength criticality is quite hard—due to the lack of an explicit analytical characterization of the shape of a (non-trivial) ideal knot, many contributions focus on discretization and numerical visualization.