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SN~ (~6) lff 2It 3k_~ , 5 ",,x_J {b)

SN~ (~6) lff 2It 3k_~ , 5 ",,x_J {b)

The Center Manifold for

The Center Manifold for Delay Equations in the Light of Suns and Stars Odo Diekmann and Stephan A. van Gils Abstract We state and prove the center manifold theorem for retarded functional differential equations. The method of proof is based on the variation-of-constants formula in the framework of dual semigroups. As an application we deal with Hopf bifurcation. Keywords ~ Phrases: strongly continuous semlgroups, weak • continuous semigroups, dual semigroups, variation-of-constants formula, retarded functional differential equations, center manifold, Hopf bifurcation. 1980 Mathematics Snbjecl Classification: 47D05, 47H20, 34K15. 1 Introduction Center manifold theory plays a key role in the description and understanding of the dynamics of nonlinear systems. Especially for infinite dimensional systems it provides us with a very powerful tool. If the center manifold is finite dimensional, the reduction leads to the relatively easy setting of an ordinary differential equation. Hence results about stability and bifurcations are readily available. Since the introduction of the center manifold some twenty years ago by Pliss [Pli64] and Kelley [Ke167], many papers have been published which consider the reduction process in different contexts. An readable presentation in finite dimensions is given by Vanderbanwhede [VanS9]. There are two different methods to prove a center manifold theorem. The method of graph transforms, see for instance [HPS77], is a geometric construction. The other method is more analytical and uses the variation-of-constants formula. This goes at least back to Perron [Per30], as was pointed out by Duistermaat [Dui76]. For bounded nonlinearities the proof makes no difference between the finite - and the infinite dimensional case provided the spectral projection corresponding to the ima~nary axis has finite rank (see [Sca89] for the case of infinite rank). For partial differential equations one can employ subtle ways to express and exploit the relative boundedness of the nonlinearity in terms of fractional power and/or interpolation spaces; see [VI89, preprint] and the references given there. For retarded functional differential equations the nonlinearity becomes bounded once a convenient framework is introduced. The convolution part of the variation-of-constants for- mula involves the so-called fundamental solution; see Hale [Hal77, Chapter 7]. The fundamen- tal solution does not belong to the state space of continuous functions, but the convolution

123 produces a continuous function. The framework of dual semigroups is suitable to give a gen- eral functional analytic description of this phenomenon and the rele~aat perturbation theory has been worked out in a series of papers [CDG+87a, CDG+87b, CDG+89a, CDG+89b], while the application to delay equations is presented in IDle87]. Without this framework one can also prove the existence of invariant manifolds, see for instance [ChaT1, Ste84]. However, the proof becomes technically more involved because of the lack of a true variation-of-constants formula and a true adjoint. The aim of the present paper is to formulate and prove the center manifold theorem for retarded functional differential equations (R.FDE). The method of proof is based on the variation-of-constants formula and we shall exploit the framework of dual semigroups to be able to consider the nonlinearity as bounded. As an application we deal with the Hopf bifurcation theorem. 2 Dual semigroups Let {T(t)} be a strongly continuous semigroup on a Banach space X, with infinitesimal generator A. Then {T*(t)} is a weak * continuous semigroup on the dual space X*. In general {T*(t)} is not strongly continuous. The maximal subspace (of X*) of strong continuity is denoted by X ® (pronounced as X-sun). Actually one can prove, see [HP57], that X o = D(A*). Let {T°(t)} be the restriction of {T*(t)} to the invariant subspace X o, then {T°(t)} is a strongly continuous semigroup on the Banach space X °. So we can repeat this process of taking duals and considering suitable restrictions. We thus introduce x ®0 = {,0" E X ®* : 1}~llTO'(t)z ®" -z®'ll = 0}. Since {T(t)} is strongly continuous on X we have that X C X e® (if we identify X with its natural embedding into X®*). Definition 2.1 X is called Q-reflexlve with respect to A iff X o® -~ X. Theorem 2.2 Let f : [0, oo) -., X ®* be norm continuous, then t ~ f~ Te*(t - r)f(r) dr is a norm continuous X ®® valued function. Remark 2.3 The integral is a weak, integral. This means that by definition f~ T®*(t - r)f(r) dr is the element in X ®* defined by (z®' ~o' T®*(t - r)f(r) dr) = ~j (T®(t - T)x ®, f(r)) dr. Let {T0(t)} be a strongly continuous semigroup on X with generator Ao. These we refer to as the unperturbed semigroup and generator. A bounded perturbation is defined, on the level of the generator as a bounded linear operator from X into X ®*.

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