Philip J. Aston

On the unfolding of a blowout bifurcation

Abstract Suppose a chaotic attractor A in an invariant subspace loses stability on varying a parameter. At the point of loss of stability, the most positive Lyapunov exponent of the natural measure on A crosses zero at what has been called a ‘blowout’ bifurcation. We introduce the notion of an essential basin of an attractor A . This is the set of points x such that accumulation points of the sequence of measures 1 n ∑n − 1 k = 0 δ f k (x) are supported on A . We characterise supercritical and subcritical scenarios according to whether the Lebesgue measure of the essential basin of A is positive or zero. We study a drift-diffusion model and a model class of piecewise linear mappings of the plane. In the supercritical case, we find examples where a Lyapunov exponent of the branch of attractors may be positive (‘hyperchaos’) or negative, depending purely on the dynamics far from the invariant subspace. For the mappings we find asymptotically linear scaling of Lyapunov exponents, average distance from the subspace and basin size on varying a parameter. We conjecture that these are general characteristics of blowout bifurcations.

Mathematical analysis of the pharmacokinetic-pharmacodynamic (PKPD) behaviour of monoclonal antibodies: predicting in vivo potency.

We consider the relationship between the target affinity of a monoclonal antibody and its in vivo potency. The dynamics of the system is described mathematically by a target-mediated drug disposition model. As a measure of potency, we consider the minimum level of the free receptor following a single bolus injection of the ligand into the plasma compartment. From the differential equations, we derive two expressions for this minimum level in terms of the parameters of the problem, one of which is valid over the full range of values of the equilibrium dissociation constant K(D) and the other which is valid only for a large drug dose or for a small value of K(D). Both of these formulae show that the potency achieved by increasing the association constant k(on) can be very different from the potency achieved by decreasing the dissociation constant k(off). In particular, there is a saturation effect when decreasing k(off) where the increase in potency that can be achieved is limited, whereas there is no such effect when increasing k(on). Thus, for certain monoclonal antibodies, an increase in potency may be better achieved by increasing k(on) than by decreasing k(off).

Nonlinear Mathematics and Its Applications

At the University of Surrey in 1995 an EPSRC Spring School was held in Applied Nonlinear Mathematics for postgraduate students in mathematics, engineering, physics or biology; this volume contains the bulk of the lectures given there by a team of internationally distinguished scientists. The aim of the school was to introduce students to current topics of research interest at an appropriate level. The majority of the courses are in the area of nonlinear dynamics with application to fluid dynamics, boundary layer transition, driven oscillators and waves. However, there are also lectures considering problems in nonlinear elasticity and mathematical biology. The articles have all been edited so that the book forms a coherent and accessible account of recent advances in nonlinear mathematics.

SYMMETRY BREAKING BIFURCATIONS OF CHAOTIC ATTRACTORS

In an array of coupled oscillators, synchronous chaos may occur in the sense that all the oscillators behave identically although the corresponding motion is chaotic. When a parameter is varied this fully symmetric dynamical state can lose its stability, and the main purpose of this paper is to investigate which type of dynamical behavior is expected to be observed once the loss of stability has occurred. The essential tool is a classification of Lyapunov exponents based on the symmetry of the underlying problem. This classification is crucial in the derivation of the analytical results but it also allows an efficient computation of the dominant Lyapunov exponent associated with each symmetry type. We show how these dominant exponents determine the stability of invariant sets possessing various instantaneous symmetries, and this leads to the idea of symmetry breaking bifurcations of chaotic attractors. Finally, the results and ideas are illustrated for several systems of coupled oscillators.

Targeting in systems with discontinuities, with applications to power electronics

Targeting methods appropriate for systems with discontinuities are considered. The multivalued inverse function is used to generate multiple preimages of the target region which quickly cover the attractor. This method is applied to the current-controlled boost converter in order to jump between two controlled states. A significant reduction in the length of the target orbit is observed when compared with targeting methods for invertible maps.

The computation of Lyapunov exponents via spatial integration with application to blowout bifurcations

We propose a new method for the numerical approximation of the largest Lyapunov exponent. This method is based on the computation of a spatial average with respect to an underlying (natural) invariant measure rather than on a long-term simulation of the dynamical system. This approach is particularly advantageous for the detection of blowout bifurcations of a synchronous chaotic state, and we illustrate this fact for a system of two coupled Duffing oscillators.

Is radioactive decay really exponential

Radioactive decay of an unstable isotope is widely believed to be exponential. This view is supported by experiments on rapidly decaying isotopes but is more difficult to verify for slowly decaying isotopes. The decay of 14C can be calibrated over a period of 12550 years by comparing radiocarbon dates with dates obtained from dendrochronology. It is well known that this approach shows that radiocarbon dates of over 3000 years are in error, which is generally attributed to past variation in atmospheric levels of 14C. We note that predicted atmospheric variation (assuming exponential decay) does not agree with results from modelling, and that theoretical quantum mechanics does not predict exact exponential decay. We give mathematical arguments that non-exponential decay should be expected for slowly decaying isotopes and explore the consequences of non-exponential decay. We propose an experimental test of this prediction of non-exponential decay for 14C. If confirmed, a foundation stone of current dating methods will have been removed, requiring a radical reappraisal both of radioisotope dating methods and of currently predicted dates obtained using these methods.

Bifurcations of the horizontally forced spherical pendulum

Abstract Various planar motions of the horizontally forced damped spherical pendulum are considered and, in particular, their stability to non-planar perturbations. By making a careful choice of coordinates, all solutions of the planar pendulum can be considered including small amplitude periodic solutions, running oscillations and chaotic solutions. The full nonlinear equations in the chosen coordinates are derived and the symmetries of the system are described. Bifurcation diagrams for various types of solutions are presented. Stability of the chaotic solutions is determined by considering a normal Lyapunov exponent.

Symmetry and chaos in the complex Ginzburg-Landau equation: II. Translational symmetries

Abstract The complex Ginzburg–Landau (CGL) equation on a one-dimensional domain with periodic boundary conditions has a number of different symmetries, and solutions of the equation may or may not be fixed by the action of these symmetries. We investigate the stability of chaotic solutions that are spatially periodic with period L with respect to subharmonic perturbations that have a spatial period kL for some integer k>1. This is done by considering the isotypic decomposition of the space of solutions and finding the dominant Lyapunov exponent associated with each isotypic component. We find a region of parameter space in which there exist chaotic solutions with spatial period L and homogeneous Neumann boundary conditions that are stable with respect to perturbations of period 2L. On varying the parameters it is possible to arrange for this solution to become unstable to perturbations of period 2L while remaining chaotic, leading to a supercritical subharmonic blowout bifurcation. For a large number of parameter values checked, chaotic solutions with spatial period L were found to be unstable with respect to perturbations of period 3L. We conclude that while periodic boundary conditions are often convenient mathematically, we would not expect to see chaotic, spatially periodic solutions forming starting with an arbitrary, non-periodic initial condition.

Blowout bifurcations of codimension two

Abstract We consider examples of loss of stability of chaotic attractors in invariant subspaces (blowouts) that occur on varying two parameters, i.e. codimension-two blowout bifurcations. Such bifurcations act as organising centres for nearby codimension-one behaviour, analogous to the case for codimension-two bifurcations of equilibria. We consider examples of blowout bifurcations showing change of criticality, blowouts that occur into two different invariant subspaces and interact, blowouts that occur with onset of hyperchaos, interaction of blowout and symmetry increasing bifurcations and collision of blowout bifurcations. As in the case of bifurcation of equilibria, there are many cases in which one can infer the presence and form of secondary bifurcations and associated branches of attractors. There is presently no generic theory of such higher codimension blowouts (there is not even such a theory for codimension-one blowouts). We want to present a number of examples that would need to be covered by such a theory.