## Most smooth closed space curves contain an approximate solution to the N-body problem

Most smooth closed space curves contain an approximate solution to the N-body problem

The determination of the exact trajectories of mutually interacting masses (the *n*-body problem) is apparently intractable for *n* greater than or equal to 3, when the generic solutions become chaotic. A few special solutions are known, which require the masses to be in certain initial positions; these are known as ‘central configurations’ (an example is the equilateral triangle formed by the Sun, Jupiter and Trojan asteroids). The configurations are usually found by symmetry arguments. Here I report a generalization of the central-configurationapproach which leads to large continuous families of approximate solutions. I consider the uniform motion of equidistributed masses on closed space curves, in the limit when the number of particles tends to infinity. In this situation, the gravitational force on each particle is proportional to the local curvature, and may be calculated using an integral closely related to theBiot–Savart integral. Approximate solutions are possible for certain (constant) values of the particle speed, determined by equating this integral to the mass times the centrifugal acceleration. Most smooth, closed space curves contain such approximate solutions, because only the local curvature is involved. Moreover, the theory also holds for sets of closed curves, allowing approximate solutions for knotted and linked configurations.

## DNA disentangling by type-2 topoisomerases

DNA disentangling by type-2 topoisomerases

or Buck:Zech

G. Buck and E. Zechiedrich

A type-2 topoisomerase cleaves a DNA strand, passes another through the break, and then rejoins the severed ends. Because it appears that this action is as likely to increase as to decrease entanglements, the question is: how are entanglements removed? We argue that type-2 topoisomerases have evolved to act at ‘hooked’ juxtapositions of strands (where the strands are curved toward each other). This type of juxtaposition is a natural consequence of entangled long strands. Our model accounts for the observed preference for unlinking and unknotting of short DNA plasmids by type-2 topoisomerases and well explains experimental observations.

## The Wondrous Mathematics of Winter

The Wondrous Mathematics of Winter

When winter comes I think of Plato’s theory of ideal forms. If I say circle or square, you know what I mean, though in some sense you have never seen a circle—the shape in the plane where all the points are exactly equidistant from the center—because in reality everything is always at least a little off. Plato thought truth ought to work this way, that what we understand as truth is always an approximation to an ideal form.

Winter is the Platonist’s season…

## Accessibility and occlusion of biopolymers, ray tracing of radiating tubes, and the temperature of a tangle

G. Buck, R. Scharein, J.Schnick, J. Simon

We introduce a measure of complexity, an energy, for any conformation of filaments. It is the occlusion, the portion hidden when viewed from an arbitrary exterior point. By inverting we get the exposure, a first approximation of the accessibility of the filaments. Assuming the filament is a source, we get the self-irradiation, which leads to both an interpretation as the temperature and a visualization technique: ray tracing as a virtual laboratory. There is a wide variety of applications, from enzyme action on and radiation damage of biopolymers, to the geometry of light bulb filaments. Energy minimization provides automatic detangling, resulting in symmetric and pleasing conformations.

## A Mathematician Goes to the Beach

A Mathematician Goes to the Beach

Like everyone else I know, when I go to the beach I think mathematics. Archimedes, my favorite mathematician, did too — his perhaps best-known work is the ‘Sand Reckoner,’ wherein he counts the grains of sand it would take to fill the universe. He must have been at the beach when he thought how to do it. I’m not sure Archimedes picked the most interesting thing about the beach for analysis, but he might be forgiven — because the ancient Greeks didn’t wear swimsuits.

Swimsuits often remind me of Carl Friedrich Gauss, a superb nineteenth-century mathematician who really understood a curved surface…

## The spectrum of filament entanglement complexity and an entanglement phase transition

The spectrum of filament entanglement complexity and an entanglement phase transition

G. Buck and J. Simon

DNA, hair, shoelaces, vortex lines, rope, proteins, integral curves, thread, magnetic flux tubes, cosmic strings and extension cords; filaments come in all sizes and with diverse qualities. Filaments tangle, with profound results: DNA replication is halted, field energy is stored, polymer materials acquire their remarkable properties, textiles are created and shoes stay on feet. We classify entanglement patterns by the rate with which entanglement complexity grows with the length of the filament. We show which rates are possible and which are expected in arbitrary circumstances. We identify a fundamental phase transition between linear and nonlinear entanglement rates. We also find (perhaps surprising) relationships between total curvature, bending energy and entanglement.