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Abstract and Applied Analysis Fixed points and periodic points of semiflows of holomorphic maps
Fixed points and periodic points of semiflows of holomorphic maps
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Volume:
2003
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2003
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english
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Abstract and Applied Analysis
DOI:
10.1155/s1085337503203109
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FIXED POINTS AND PERIODIC POINTS OF SEMIFLOWS OF HOLOMORPHIC MAPS EDOARDO VESENTINI Received 16 September 2001 Let φ be a semiflow of holomorphic maps of a bounded domain D in a complex Banach space. The general question arises under which conditions the existence of a periodic orbit of φ implies that φ itself is periodic. An answer is provided, in the first part of this paper, in the case in which D is the open unit ball of a J ∗ algebra and φ acts isometrically. More precise results are provided when the J ∗ algebra is a Cartan factor of type one or a spin factor. The second part of this paper deals essentially with the discrete semiflow φ generated by the iterates of a holomorphic map. It investigates how the existence of fixed points determines the asymptotic behaviour of the semiflow. Some of these results are extended to continuous semiflows. 1. Introduction Let D be a bounded domain in a complex Banach space Ᏹ and let φ : R+ × D → D be a continuous semiflow of holomorphic maps acting on D. Under which conditions does the existence of a periodic point of φ (with a positive period) imply that the semiflow φ itself is periodic? An answer to this question was provided in [22] in the case in which Ᏹ is a complex Hilbert space and D is the open unit ball of Ᏹ, showing that, if the orbit of the periodic point spans a dense linear subspace of Ᏹ, then φ is the restriction to R+ of a continuous periodic flow of holomorphic automorphisms of D. In the first part of this paper, a somewhat similar result will be established in the more general case in which Ᏹ is a J ∗ algebra and D is the open unit ball B of Ᏹ. The main result in this direction can be stated more easily in the case in which the periodic point is the center 0 of B. It will be shown that, if the points of the orbit of 0 which are collinear to extreme points of the closure B of B span a dense linear subspace of Ᏹ, then the same conclusion of [22] holds, Copyright © 2003 Hindawi Publishing Corporation Abstract and Applied Analysis 2003:4 (2003) 21; 7–260 2000 Mathematics Subject Classification: 17C65, 32M15, 46G20 URL: http://dx.doi.org/10.1155/S1085337503203109 218 Periodicity of holomorphic maps that is, φ is the restriction to R+ of a continuous periodic flow of holomorphic automorphisms of B. If the J ∗ algebra Ᏹ is a Cartan factor of type one—that is, it is the Banach space ᏸ(Ᏼ,) of all bounded linear operators acting on a complex Hilbert space Ᏼ with values in a complex Hilbert space —it was shown by Franzoni in [4] that any holomorphic automorphism of B is essentially associated to a linear continuous operator preserving a Kreı̆n space structure defined on the Hilbert space direct sum ⊕ Ᏼ; a situation that has been further explored in [19, 20] in the case in which ⊕ Ᏼ carries the structure of a Pontryagin space. Starting from a strongly continuous group T : R → ᏸ( ⊕ Ᏼ), inducing a continuous flow φ of holomorphic automorphisms of B, it will be shown that, if φ has a periodic point x0 , and if the orbit of x0 is “suﬃciently ample,” a rescaled version of T is periodic. A theorem of Bart [1] yields a complete description of the spectral structure of the infinitesimal generator X of T. The particular case in which Ᏼ C and B is the open unit ball of , which was initially explored in [22], will be revisited, showing that the periodic flow φ fixes some point of B and that, if φ is eventually diﬀerentiable, the dimension of is finite. As was shown in [17, 19], in the case in which ⊕ Ᏼ carries the structure of Pontryagin space, a Riccati equation defined on B is canonically associated to X. The periodicity of φ implies then the periodicity of the integrals of this Riccati equation. A similar investigation to the one carried out in Sections 3 and 4 for a Cartan factor of type one is developed in Section 5 in the case in which Ᏹ is a spin factor. In this case, the norm in Ᏹ is equivalent to a Hilbert space norm. Assuming again, for the sake of simplicity, that the periodic point is the center 0 of D, a hypothesis leading to the periodicity of φ, consists in supposing that the points of the orbit of 0 which are collinear to scalar multiples of selfadjoint unitary operators acting on Ᏹ span a dense linear submanifold of this latter space. The case of fixed points of the semiflow φ acting on the bounded domain D is considered in the second part of this paper, where, among other things, some results which were announced in [16] for discrete semiflows generated iterating a holomorphic map f : D → D are established in the general case. (One of the basic tools in this investigation was the EarleHamilton theorem (see [2] or, e.g., [5, 6, 9]). This theorem, coupled with the theory of complex geodesics for the Carathéodory distance, was also used by several authors (see, e.g., [10, 11, 15, 16, 23, 24, 25, 26, 27]) to investigate the geometry of the set of fixed points of f . Further references to fixed points of holomorphic maps can be found in [13].) Our main purpose is to obtain some information on the asymptotic behaviour of φ in terms of “local” properties. In this direction, extending to the continuous case a result announced in [16] for the iteration of a holomorphic map, it is shown that, if there is a sequence {tν } in R+ diverging to infinity and such that {φtν } converges, for the topology of local Edoardo Vesentini 219 uniform convergence, to a function mapping D into a set completely interior to D, then there exists a unique point x0 ∈ D which is fixed by the semiflow φ; moreover, φs (x) tends to x0 as s → +∞, for all x ∈ D. If some point x0 ∈ D is fixed by the continuous semiflow φ, the map t → dφt (x0 ), where dφt (x0 ) ∈ ᏸ(Ᏹ) is the Fréchet diﬀerential of φt (x) at x = x0 , defines a strongly continuous semigroup of bounded linear operators acting on Ᏹ. Some situations are explored in which the behaviour of this semigroup determines the asymptotic behaviour of the semiflow φ. It is shown in Sections 7 and 8 that, if the spectral radius ρ(dφt (x0 )) of dφt (x0 ) is ρ(dφt (x0 )) < 1 for some t > 0, then, as s → +∞, φs converges to the constant map x → x0 for the topology of local uniform convergence. The case in which ρ(dφt (x0 )) = 1 at some t > 0 is considered in Sections 9 and 10, under the additional hypothesis that dφt (x0 ) is an idempotent of ᏸ(Ᏹ). As is well known, the spectrum σ(dφt (x0 )) of dφt (x0 ) consists of two eigenvalues in 0 and in 1 at most. If σ dφt x0 = { 0 }, (1.1) then dφs (x0 ) = {0} for all s ≥ t. As a consequence of Sections 7 and 8, if s → +∞, φs converges to the constant map x → x0 for the topology of local uniform convergence. If σ dφt x0 = { 1 }, (1.2) then φ is the restriction to R+ of a periodic flow of holomorphic automorphisms of D. Finally, if 1 ∈ σ dφt x0 , (1.3) and if there is some t > 0, with t /t ∈ Q, such that also dφt (x0 ) is an idempotent of ᏸ(Ᏹ), then the semiflow φ is constant, that is, φt = id (the identity map) for all t ≥ 0. 2. The general case of a J ∗ algebra Let Ᏹ be a complex Banach space, let D be a domain in Ᏹ, and let φ : R+ × D −→ D (2.1) 220 Periodicity of holomorphic maps be a semiflow of holomorphic maps of D into D, that is, a map such that φ0 = id, (2.2) φt1 +t2 = φt1 φt2 , (2.3) φt ∈ Hol(D), (2.4) for all t,t1 ,t2 ∈ R+ , where Hol(D) is the semigroup of all holomorphic maps D → D. A point x ∈ D is said to be a periodic point of φ with period τ > 0 if φτ (x) = x and φt (x) = x for all t ∈ (0,τ). The semiflow φ will be said to be periodic with period τ if φτ = id and, whenever 0 < t < τ, φt is not the identity map. We begin by establishing the following elementary lemma, which is a consequence of Cartan’s uniqueness theorem (see, e.g., [5]) and which might have some interest in itself. Let D be a hyperbolic domain in the Banach space Ᏹ (or, more in general, a domain in Ᏹ on which either the Carathéodory or the Kobayashi distances define equivalent topologies to the relative topology) and let x0 ∈ D be a fixed point of the semiflow φ, that is, φt (x0 ) = x0 for all t ∈ R+ . Lemma 2.1. If there is a vector ξ ∈ Ᏹ\{0}, for which the map t → dφt (x0 )ξ of R+ into ᏸ(Ᏹ) is periodic with period τ > 0, and there is a set K ⊂ (0,τ) such that {dφt (x0 )ξ : t ∈ K } spans a dense aﬃne subspace K̃ of Ᏹ, then φτ = id. Proof. Let x0 = 0. Since dφτ (0) dφt (0)ξ = dφτ+t (0)ξ = dφt (0)ξ ∀t ≥ 0, (2.5) then dφτ (0) = id on K̃ and therefore on Ᏹ. Cartan’s identity theorem yields the conclusion. Let Ᏼ and be complex Hilbert spaces and let ᏸ(Ᏼ,) be the complex Banach space of all continuous linear operators Ᏼ → , endowed with the operator norm. For A ∈ ᏸ(Ᏼ,), A∗ ∈ ᏸ(,Ᏼ) will denote the adjoint of A. A J ∗ algebra [7] is a closed linear subspace Ꮽ of ᏸ(Ᏼ,) such that A ∈ Ꮽ =⇒ AA∗ A ∈ Ꮽ. (2.6) The roles of Ᏹ and D will now be played by a J ∗ algebra Ꮽ and by the open unit ball B of Ꮽ. Let S be the set of all extreme points of the closure B of B. As was noted by Harris in [7], if Ꮽ is weakly closed in ᏸ(Ᏼ,), then S = ∅. Lemma 2.2. Let S = ∅. If 0 is a periodic point of the semiflow φ : R+ × B → B, with period τ > 0, and if there is a set K ⊂ (0,τ) such that, for every t ∈ K, φt (0) is collinear to some point of S, and the set {φt (0) : t ∈ K } spans a dense linear subspace of Ꮽ, then the semiflow φ is periodic with period τ. Edoardo Vesentini 221 Proof. Let ∆ be the open unit disc of C. For t ∈ K, ∆ ζ −→ ζ φt (0) φt (0) (2.7) is, up to parametrization, the unique complex geodesic whose support contains both 0 and φt (0). (For the Kobayashi or Carathéodory metrics on B, for the basic notions concerning complex geodesics, see, e.g., [14, 15].) Since φτ (0) = 0 and φτ φt (0) = φt φτ (0) = φt (0), (2.8) then φτ is the identity on the support of the complex geodesic (2.7). Hence dφτ (0) φt (0) = φt (0) ∀t ∈ K, (2.9) and therefore dφτ (0) = IᏭ . Thus dφτ (0) maps the set S onto itself. By Harris’ Schwarz lemma [7, Theorem 10], φτ = dφτ (0) = id. Let now x0 ∈ B be a periodic point of φ with period τ > 0. As was shown in [7], the Moebius transformation Mx0 is a holomorphic automorphism of B which maps any x ∈ B to the point Mx0 (x) = I − x0 x0 ∗ −1/2 x + x 0 I + x0 ∗ x −1 I − x0 ∗ x0 1/2 1/2 −1 1/2 = x0 + I − x0 x0 ∗ x I + x0 ∗ x I − x0 ∗ x0 . (2.10) Furthermore, Mx0 (0) = x0 , M x 0 −1 = M −x 0 , (2.11) and Mx0 is the restriction to B of a holomorphic function on an open neighbourhood of B in Ꮽ, mapping ∂B onto itself. Applying Lemma 2.2 to the semiflow t → ψt = M−x0 φt Mx0 , we obtain the following theorem. Theorem 2.3. If x0 ∈ B is a periodic point of φ with period τ > 0 and if there is a set K ⊂ (0,τ) such that (i) for any t ∈ K, M−x0 (φt (x0 )) is collinear to some point in S; (ii) the set {φt (x0 ) : t ∈ K } spans a dense aﬃne subspace of Ꮽ (as was shown by Harris in [7, Corollary 8], B is the closed convex hull of S), then the semiflow φ is periodic with period τ. 222 Periodicity of holomorphic maps Remark 2.4. Under the hypotheses of Theorem 2.3, setting ψt = φt when t ≥ 0, and ψt = φ−t when t ≤ 0, one defines a flow ψ : R × B → B of holomorphic automorphisms of B, whose restriction to R+ is φ. The flow ψ is continuous if and only if the semiflow φ is continuous, that is, the map φ : R+ × B → B is continuous. In the case in which n = dimC Ꮽ < ∞, a similar statement to Theorem 2.3 holds for a discrete semiflow, that is to say, for the semiflow generated by the iterates f m = f ◦ f ◦ · · · ◦ f (m = 1,2,...) of a holomorphic map f : B → B. Theorem 2.5. If f has a periodic point x0 ∈ B, with period p > n (i.e., f p (x0 ) = x0 , f q (x0 ) = x0 if q = 1,..., p − 1), if M−x0 ( f q (x0 )) is collinear to some point in the Shilov boundary of B for q = 1,..., p − 1, and if the orbit { f q (x0 ) : q = 1,..., p − 1} of x0 spans Ꮽ, then f is periodic with period p. For example, let f1 : z → e2πi/3 z and let f2 be another holomorphic function ∆ → ∆ such that f2 (0) = 0 but f2 ≡ 0. Let f : ∆ × ∆ → ∆ × ∆ be the holomorphic map defined by f z1 ,z2 = f1 z1 , f2 z2 , z1 ,z2 ∈ ∆ . (2.12) If f2 has a periodic point in ∆\{0}, and therefore is periodic, f is periodic with period ≥ 3. If f2 is not periodic, f is not periodic. However, every point (z1 ,0) with z1 ∈ ∆\{0} is a periodic point of f with period 3. 3. Cartan domains of type one Let the J ∗ algebra Ꮽ be a Cartan factor of type one, Ꮽ = ᏸ(Ᏼ,). Let I J= 0 0 , −IᏴ (3.1) and let Γ(J) be the group of all linear continuous operators A on ⊕ Ᏼ which are invertible in ᏸ( ⊕ Ᏼ) and such that A∗ JA = J. (3.2) It was shown by Franzoni in [4] that the group of all holomorphic automorphisms of the unit ball B of Ꮽ, which is called a Cartan domain of type one, is isomorphic to a quotient of Γ(J), up to conjugation when dimC Ᏼ = dimC . To avoid conjugation, we will consider now the case in which ∞ ≥ dimC Ᏼ = dimC ≤ ∞. Let T : R → ᏸ( ⊕ Ᏼ) be a strongly continuous group such that T(t)∗ JT(t) = J, (3.3) Edoardo Vesentini 223 or equivalently T(t)JT(t)∗ = J, (3.4) for all t ∈ R. If T11 (t) T12 (t) T(t) = T21 (t) T22 (t) (3.5) is the representation of T(t) in ⊕ Ᏼ, with T11 (t) ∈ ᏸ(), T12 (t) ∈ ᏸ(Ᏼ,), T21 (t) ∈ ᏸ(,Ᏼ), and T22 (t) ∈ ᏸ(Ᏼ), then (3.3) and (3.4) are equivalent to T11 (t)∗ T11 (t) − T21 (t)∗ T21 (t) = I , T22 (t)∗ T22 (t) − T12 (t)∗ T12 (t) = IᏴ , ∗ (3.6) ∗ T12 (t) T11 (t) − T22 (t) T21 (t) = 0, T11 (t)T11 (t)∗ − T12 (t)T12 (t)∗ = I , T22 (t)T22 (t)∗ − T21 (t)T21 (t)∗ = IᏴ , ∗ (3.7) ∗ T21 (t)T11 (t) − T22 (t)T21 (t) = 0. Here T11 (t)∗ ∈ ᏸ(), T12 (t)∗ ∈ ᏸ(,Ᏼ), T21 (t)∗ ∈ ᏸ(Ᏼ,), and T22 (t)∗ ∈ ᏸ(Ᏼ) are the adjoint operators of T11 (t), T12 (t), T21 (t), and T22 (t). From now on, in this section, latin letters x and y indicate elements of ᏸ(Ᏼ,) and greek letters ξ and η indicate vectors in Ᏼ and . It was shown in [4], that, if x ∈ B, T21 (t)x + T22 (t) ∈ ᏸ(Ᏼ) is invertible in defined on B by ᏸ(Ᏼ), and the function T(t), : x −→ T11 (t)x + T12 (t) T21 (t)x + T22 (t) T(t) −1 , (3.8) is, for all t ∈ R, a holomorphic automorphism of B. Setting φt = T(t) (3.9) for t ∈ R, we define a continuous flow φ of holomorphic automorphisms of B. If x0 ∈ B is a periodic point of φ with period τ > 0, and if the hypotheses of Theorem 2.3 are satisfied, φ is periodic with period τ. = id, then Since T(τ) T11 (τ)x + T12 (τ) = xT21 (τ)x + xT22 (τ) ∀x ∈ ᏸ(Ᏼ,), (3.10) whence T12 (τ) = 0, T21 (τ) = 0, (3.11) 224 Periodicity of holomorphic maps and therefore, by (3.6), T11 (τ)∗ T11 (τ) = T11 (τ)T11 (τ)∗ = I , T22 (τ)∗ T22 (τ) = T22 (τ)T22 (τ)∗ = IᏴ , (3.12) that is, T11 (τ) and T22 (τ) are unitary operators in the Hilbert spaces and Ᏼ. Furthermore, (3.10) becomes T11 (τ)x = xT22 (τ) ∀x ∈ ᏸ(Ᏼ,). (3.13) Since T22 (τ) is unitary, every point eiθτ (θ ∈ R) in the spectrum σ(T22 (τ)) of T22 (τ) is contained either in the point spectrum or in the continuous spectrum. In both cases, there exists a sequence {ξν } in Ᏼ (which may be assumed to be constant if eiθτ is an eigenvalue), with ξν = 1, such that lim T22 (τ)ξν − eiθτ ξν = 0. ν→+∞ (3.14) Since, by the Schwarz inequality, T22 (τ)ξν ξν − eiθτ = T22 (τ)ξν − eiθτ ξν ξν ≤ T22 (τ)ξν − eiθτ ξν , (3.15) then lim T22 (τ)ξν ξν = eiθτ . (3.16) ν→+∞ Hence, letting, for any η ∈ , xν = η ⊗ ξν ∈ ᏸ(Ᏼ,), then xν (ξν ) = η and lim xν T22 (τ)ξν = lim T22 (τ)ξν ξν η = eiθτ η. ν→+∞ ν→+∞ (3.17) Thus, by (3.13), T11 (τ)η = lim T11 (τ) xν ξν ν→+∞ = lim xν T22 (τ)ξν = eiθτ η ν→+∞ (3.18) for all η ∈ . Therefore, T11 (τ) = eiθτ I , (3.19) T22 (τ) = eiθτ IᏴ . (3.20) and (3.13) yields Edoardo Vesentini 225 In conclusion, T(τ) = eiθτ I⊕Ᏼ . (3.21) Thus, the rescaled group L : R → ᏸ( ⊕ Ᏼ), defined by L(t) = e−iθt T(t), (3.22) is periodic with period τ. Note that L(t)∗ JL(t) = J ∀t ∈ R. (3.23) If L11 (t) L12 (t) L(t) = L21 (t) L22 (t) (3.24) is the representation of L(t) in ⊕ Ᏼ, with L11 (t) ∈ ᏸ(), L12 (t) ∈ ᏸ(Ᏼ,), L21 (t) ∈ ᏸ(,Ᏼ), and L22 (t) ∈ ᏸ(Ᏼ), then Lα,β (t) = e−iθt Tα,β (t) (3.25) for α,β = 1,2. Therefore, setting, for x ∈ B, L(t)(x) : x −→ L11 (t)x + L12 (t) L21 (t)x + L22 (t) −1 , (3.26) then = φt L(t) ∀t ∈ R. (3.27) If X : Ᏸ(X) ⊂ ⊕ Ᏼ → ⊕ Ᏼ is the infinitesimal generator of the group T, the operator X − iθI⊕Ᏼ , with domain Ᏸ(X), generates the group L. The structure of the spectrum σ(X − iθI⊕Ᏼ ) is described in [1] by a theorem of Bart, whereby (i) σ(X − iθI⊕Ᏼ ) ⊂ i(2π/τ)Z; (ii) σ(X − iθI⊕Ᏼ ) consists of simple poles of the resolvent function ζ → (ζI⊕Ᏼ − (X − iθI⊕Ᏼ ))−1 ; (iii) the eigenvectors of X − iθI⊕Ᏼ span a dense linear subspace of ⊕ Ᏼ. According to [1], if X is the infinitesimal generator of a strongly continuous group T, and if conditions (i), (ii), and (iii) hold, the group L defined by (3.22) is periodic with period τ. Summing up, in view of Theorem 2.3, the following result has been established. 226 Periodicity of holomorphic maps Theorem 3.1. If there is a periodic point x0 ∈ B for φ, with period τ > 0, and if there is a set K ⊂ (0,τ) such that, for any t ∈ K, M−x0 (φt (x0 )) is collinear to some point of S, and the set {φt (x0 ) : t ∈ K } spans a dense aﬃne subspace of ᏸ(Ᏼ,), then there exist a strongly continuous group T : R → ᏸ(Ᏼ,) and a real number θ such that the rescaled group R t → L(t) is a periodic group with period τ. If X : Ᏸ(X) ⊂ ⊕ Ᏼ → ⊕ Ᏼ is the infinitesimal generator of the group T, conditions (i), (ii), and (iii) characterize the periodicity of L with period τ. Thus, if X generates a strongly continuous group T, and if conditions (i), (ii), and (iii) hold, the group L defined by (3.22) is periodic with period τ. As was proved in [19, Proposition 4.1], the group T satisfies (3.3) for all t ∈ R if and only if the operator iJX is selfadjoint. If that is the case, setting ⊕ 0 = ( ⊕ 0) ∩ Ᏸ(X), 0 ⊕ Ᏼ = (0 ⊕ Ᏼ) ∩ Ᏸ(X), (3.28) [19, Lemma 5.3] implies that the linear spaces and Ᏼ are dense in and Ᏼ. We consider now the case in which the semigroup TR+ is eventually diﬀerentiable (i.e., there is t 0 ≥ 0 such that the function t → T(t)x is diﬀerentiable in (t 0 ,+∞) for all x ∈ ⊕ Ᏼ). By (3.22), also LR+ is eventually diﬀerentiable. According to a theorem by Pazy (see, e.g., [12]), there exist a ∈ R and b > 0 such that the set ζ ∈ C : ζ ≥ a − b log ζ  (3.29) is contained in the resolvent set of X − iθI⊕Ᏼ . Thus, the intersection of σ(X − iθI⊕Ᏼ ) with the imaginary axis is bounded. Condition (i) implies then that σ(X − iθI⊕Ᏼ ) is finite. But then, by [1, Proposition 3.2], X − iθI⊕Ᏼ ∈ ᏸ( ⊕ Ᏼ), and therefore X ∈ ᏸ( ⊕ Ᏼ), proving thereby the following proposition. Proposition 3.2. Under the hypotheses of Theorem 3.1, if moreover the semigroup TR+ is eventually diﬀerentiable, the group T is uniformly continuous. Remark 3.3. The above argument holds for any strongly continuous semigroup T of linear operators, which is periodic, showing that, if T is eventually diﬀerentiable, then T is uniformly continuous. If T is eventually norm continuous, then (see, e.g., [3]) its infinitesimal generator X is such that, for every r ∈ R, the set ζ ∈ σ(X) : ζ ≥ r (3.30) is bounded. At this point, [1, Proposition 3.2] implies that, if T is also periodic, then the operator X is bounded, and therefore T is uniformly continuous. This conclusion holds, for example, if the periodic semigroup T is eventually compact. Edoardo Vesentini 227 4. The unit ball of a Hilbert space Theorem 3.1 has been established in [22] in the case in which B is the open unit ball of the Hilbert space (i.e., when Ᏼ = C). In this case, T11 (t) ∈ ᏸ() is invertible in ᏸ(), T12 (t)∈ , T21 (t)=(•T12 (t)), and T22 (t) ∈ C are characterized by the equations T22 (t)2 − T12 (t)2 = 1, 1 ∗ ∗ • T11 (t) T12 (t) T11 (t) T12 (t). T22 (t)2 T11 (t)∗ T11 (t) = I+ (4.1) As was shown in [22], there is a neighbourhood U of B such that xT11 (t)∗ T12 (t) + T22 (t) = 0 ∀x ∈ U, t ∈ R. (4.2) The orbit of x0 ∈ B is described by 1 x0 = T11 (t)x0 + T12 (t) . φt x0 = T(t) ∗ x0 T11 (t) T12 (t) + T22 (t) (4.3) The infinitesimal generator X of T is represented in ⊕ C by the matrix X11 X12 , X= iX22 • X12 (4.4) where X12 ∈ , X22 ∈ R, iX11 is a selfadjoint operator, and the domains Ᏸ(X) and Ᏸ(X11 ) of X and of X11 are related by Ᏸ(X) = Ᏸ X11 ⊕ C. (4.5) Since φτ is the identity, by [17, Proposition 7.3] and by (3.27), the set Fix φ = x ∈ B : φt (x) = x ∀t ∈ R (4.6) is nonempty. The ball B being homogeneous, there is no restriction in assuming 0 ∈ Fix φ. Thus, by (3.8), T12 (t) = 0 for all t ∈ R, and therefore X12 = 0. Furthermore, as a consequence of (4.1), T22 (t) = eiX22 t , (4.7) and the skewselfadjoint operator X11 generates the strongly continuous group T11 : t → T11 (t) of unitary operators in . 228 Periodicity of holomorphic maps Equation (3.9), which now reads φt (x) = e−iX22 t T11 (t), (4.8) yields the following lemma. Lemma 4.1. The set Fix φ is the intersection of B with a closed aﬃne subspace of . Because of (3.21), X22 = θ + 2nπ τ (4.9) for some n ∈ Z, and therefore φt (x) = e−(2nπ/τ)it L11 (t)x (4.10) for all x ∈ B and some n ∈ Z. The strongly continuous periodic group L11 : t → L11 (t), with period τ, of unitary operators in is generated by Y11 := X11 − iθI : Ᏸ X11 ⊂ −→ . (4.11) By [1], σ(Y11 ) ⊂ i(2π/τ)Z consists entirely of eigenvalues, and the corresponding eigenspaces, which are mutually orthogonal, span a dense linear subspace of . For m ∈ Z, let Pm be the orthogonal spectral projector associated with (2π/ τ)mi. By [1, (3)], L11 is expressed by e(2mπ/τ)it Pm x L11 (t)x = (4.12) m for all x ∈ and all t ∈ R. Thus L11 (t) leaves invariant every space Pm (), and acts on it by the rotation x −→ e(2mπ/τ)it x. (4.13) Hence, the following lemma follows. Lemma 4.2. If the orbit of x0 ∈ B spans a dense aﬃne subspace of , then dimC Pm () ≤ 1 for all m ∈ Z. Since, by (3.25), σ Y11 = σ X11 − iθI (4.14) if σ(X11 ) is finite, also σ(Y11 ) is finite. A similar argument to that leading to Proposition 3.2 yields now the following theorem. Edoardo Vesentini 229 Theorem 4.3. If the continuous flow φ of holomorphic automorphisms of the open unit ball B of defined by a strongly continuous group T : R → ᏸ( ⊕ C) has a periodic point whose orbit spans a dense aﬃne subspace of , and if moreover T is eventually diﬀerentiable, then dimC < ∞. According to [17, Theorem VII], for any γ > 0 and every choice of x0 ∈ B ∩ Ᏸ(X11 ), the function φ• x0 [0,γ] : [0,γ] −→ Ᏸ X11 , (4.15) defined by (4.3) for 0 ≤ t ≤ γ, is the unique continuously diﬀerentiable map [0,γ] → with x([0,γ]) ⊂ Ᏸ(X11 ), which is continuous for the graph norm x −→ x + X11 x (4.16) on Ᏸ(X11 ), and satisfies the Riccati equation d φt x0 = X11 φt x0 − φt x0 X12 + iX22 φt x0 + X12 dt (4.17) with the initial condition φ0 (x0 ) = x0 ∈ B ∩ Ᏸ(X11 ). Hence, Theorem 3.1 can be rephrased. Proposition 4.4. If the Riccati equation (4.17) has a periodic integral which spans a dense aﬃne subspace of , (4.17) is periodic (i.e., all integrals of (4.17) satisfying the above regularity conditions are periodic). We consider now the case in which one of the two spaces and Ᏼ has a finite dimension, and therefore J defines in ⊕ Ᏼ the structure of a Pontryagin space. Assuming ∞ > dimC Ᏼ < dimC ≤ ∞, (4.18) the extreme points of B are all the linear isometries Ᏼ → ; by [19, Theorem III], X is represented by the matrix X11 X= X12 ∗ X12 , iX22 (4.19) where X11 : Ᏸ(X11 ) ⊂ → and X22 ∈ ᏸ() are skewselfadjoint, X12 ∈ ᏸ(Ᏼ,), and Ᏸ(X) = Ᏸ(X11 ) ⊕ Ᏼ. 230 Periodicity of holomorphic maps The Riccati equation (4.17) is replaced in [19] by the operatorvalued Riccati equation d x(t) = X11 x(t)−x(t)X22 − x(t)X22 − x(t)X12 ∗ x(t) + X12 dt (4.20) acting on C 1 maps of [0,γ] into Ď = x ∈ ᏸ(Ᏼ,) : xξ ∈ Ᏸ X11 ∀ξ ∈ Ᏼ (4.21) which are continuous for the norm (4.16). For any γ > 0, any choice of u invertible in ᏸ(Ᏼ) and of v ∈ Ď such that x0 = vu−1 ∈ B, the function t → x(t) expressed by (3.8), with x = x0 , for t ∈ [0,γ] is the unique solution of (4.20) satisfying the conditions stated above, with the initial condition x(0) = x0 . Theorem 3.1 yields then the following proposition. Proposition 4.5. Let the integral t → x(t) be periodic with period τ > 0, and let there be a set K ⊂ (0,τ) such that x(K) spans a dense aﬃne subspace of ᏸ(Ᏼ,). If, for any t ∈ K, M−x0 (x(t)) is collinear to some linear isometry of Ᏼ into , the Riccati equation (4.20) is periodic. 5. Spin factors Similar results to some of those of Section 3 will now be established in the case in which the J ∗ algebra Ꮽ is a spin factor. In this section, is, as before, a complex Hilbert space, and C ∗ is the adjoint of C ∈ ᏸ(). A Cartan factor of type four, also called a spin factor, is a closed linear subspace Ꮽ of ᏸ() which is ∗ invariant and such that C ∈ Ꮽ implies that C 2 is a scalar multiple of I . Since, for C1 ,C2 ∈ Ꮽ, C1 C2 ∗ + C2 ∗ C1 is a scalar multiple, 2(C1 C2 )I , of the identity, then C1 ,C2 → (C1 C2 ) is a positivedefinite scalar product, with respect to which Ꮽ is a complex Hilbert space. (For more details concerning spin factors, see, e.g., [7, 18, 21].) Denoting by  ·  and by · the operator norm and the Hilbert space norm on Ꮽ, then 2 C 2 = C 2 + C 4 − C C ∗ ∀C ∈ Ꮽ. (5.1) The open unit ball B for the norm  · , also expressed by 2 1 + C C ∗ B = C ∈ Ꮽ : C < <1 , 2 2 (5.2) is called a Cartan domain of type four. The set S of all extreme points of B is the set of all multiples, by a constant factor of modulus one, of all selfadjoint unitary operators acting on the Hilbert space , which are contained in Ꮽ [7, 21]. Edoardo Vesentini 231 Changing again notations, we denote by x, y elements of the spin factor Ꮽ, and x → x stands for the conjugation defined by the adjunction in the Hilbert space Ꮽ. For any M ∈ ᏸ(Ꮽ), M t will indicate the transposed of M. The same notation will be used to indicate the canonical transposition in C2 and the transposition in Ꮽ ⊕ C2 . According to [7, 21], any holomorphic automorphism f of B can be described as follows. Let I J= 0 0 , −IC2 (5.3) and let Λ be the semigroup consisting of all A ∈ ᏸ(Ꮽ ⊕ C2 ) such that At JA = J. (5.4) Every A ∈ Λ is represented by a matrix M q1 e11 e21 A = • r1 • r2 q2 e12 , e22 (5.5) where M ∈ ᏸ(Ꮽ) is a real operator, q1 , q2 , r1 , and r2 are real vectors in Ꮽ, and e11 E := e21 e12 e22 (5.6) is a real 2 × 2 matrix such that detE > 0, and M t M − Rt R = IᏭ , (5.7) t t (5.8) t t (5.9) M Q − R E = 0, E E − Q Q = IC2 . Here R : Ꮽ → C2 and Q : C2 → Ꮽ are defined by xr1 ∈ C2 Rx = xr2 Qz = z1 q1 + z2 q2 ∀z = ∀x ∈ Ꮽ, (5.10) z1 ∈ C2 . z2 It was shown in [18] that the set Λ0 = {A ∈ Λ : detE > 0} is a subsemigroup of Λ. For x ∈ Ꮽ, let δ(A,x) = 2 xr1 − r2 + e11 − e22 + i e12 + e21 (xx) + e11 + e22 + i e21 − e12 . (5.11) 232 Periodicity of holomorphic maps One shows (see [18, 21]) that, if A ∈ Λ0 , δ(A,x) = 0 for all x in an open neighbourhood U of B. Hence, the map Â : U x −→ 1 2Mx + 1 + (xx) q1 − i 1 − (xx) q2 δ(A,x) (5.12) is holomorphic in U. Its restriction to B, which will be denoted by the same symbol Â, is the most general holomorphic isometry for the CarathéodoryKobayashi metric of B [21]. This isometry is a holomorphic automorphism of B if, and only if, A is invertible in ᏸ(Ꮽ ⊕ C2 ). If Â(0) = 0, then q1 − iq2 = 0, and therefore q1 = q2 = 0 because q1 and q2 are real vectors; (5.9) reads now E ∈ SO(2), and (5.8), which now becomes Rt E = 0, yields r1 = r2 = 0. Thus, by (5.7), M is a real linear isometry of Ꮽ. Setting cosα − sinα E= sinα cosα (5.13) for some α ∈ R, then Â(x) = eiα Mx ∀x ∈ B. (5.14) As a consequence, e−iα IᏭ Â(x) = x ∀x ∈ B ⇐⇒ A = 0 0 0 0 cosα − sinα . sinα cosα (5.15) Now, let T : R+ → ᏸ(Ꮽ ⊕ C2 ) be a strongly continuous semigroup such that T(t) ∈ Λ0 for all t ≥ 0. Setting φt = T(t) (5.16) for t ≥ 0, one defines a continuous semiflow φ : R+ × B → B of holomorphic isometrics B → B. If x0 ∈ B is a periodic point of φ with period τ > 0, and if the hypotheses of Theorem 2.3 are satisfied, then (i) φ is the restriction to R+ of a continuous flow R × B → B, which will be denoted by the same symbol φ; (ii) T is the restriction to R+ of a strongly continuous group R → ᏸ(Ꮽ ⊕ C2 ), which will be denoted by the same symbol T; (iii) (5.16) holds for all t ∈ R. Since, T(τ)(x) = x for all x ∈ B, by (5.15), there is some α ∈ R such that T(τ) = F(τ), (5.17) Edoardo Vesentini 233 where F(τ) = e−iατ IᏭ 0 0 0 0 cos(ατ) − sin(ατ) . sin(ατ) cos(ατ) (5.18) Thus, σ T(τ) = σ F(τ) . (5.19) Setting L− = (ζ,iζ) : ζ ∈ C , L+ = (ζ, −iζ) : ζ ∈ C , (5.20) if ατ ∈ π Z, σ(T(τ)) consists of the eigenvalue e−iατ , with the eigenspace Ꮽ ⊕ L− ⊂ Ꮽ ⊕ C2 , and of the eigenvalue eiατ , with the eigenspace 0 ⊕ L+ ⊂ Ꮽ ⊕ C2 . If ατ ∈ π Z, T(τ) = IᏭ⊕C2 when ατ/π is even , and T(τ) = −IᏭ⊕C2 when ατ/π is odd. In conclusion, the following theorem has been established. Theorem 5.1. If there is a periodic point x0 ∈ B for φ, with period τ > 0, and if there is a set K ⊂ (0,τ) such that, for any t ∈ K, M−x0 (φt (x0 )) is collinear to a multiple, by a constant factor of modulus one, of a selfadjoint unitary operator which acts on the Hilbert space and is contained in Ꮽ, and the set {φt (x0 ) : t ∈ K } spans a dense aﬃne subspace of Ꮽ, then there exist a strongly continuous group T : R → ᏸ(Ꮽ ⊕ C2 ) and a real number α for which (5.17) and (5.18) hold. The infinitesimal generator X : Ᏸ(X) ⊂ Ꮽ ⊕ C2 −→ Ꮽ ⊕ C2 (5.21) of the group T has a pure point spectrum, consisting of at least one and at most two distinct eigenvalues. If ατ ∈ π Z, σ(T(τ)) consists of the eigenvalue e−iατ , with the eigenspace Ꮽ ⊕ L− , and of the eigenvalue eiατ with the onedimensional eigenspace 0 ⊕ L+ . If ατ ∈ π Z, the group T is periodic with period τ when ατ/π is even, and period 2τ when ατ/π is odd. According to [18, Theorem 4.1], Ᏸ(X) = Ᏸ ⊕ C2 , where Ᏸ is a dense linear subspace of Ꮽ, and X is expressed by the matrix X 11 X = • X12 • X13 X12 0 −X23 X13 X23 , 0 (5.22) where X23 ∈ R, X12 and X13 are real vectors in Ꮽ, and X11 is a real, skewselfadjoint operator on Ꮽ with domain Ᏸ. 234 Periodicity of holomorphic maps Similar results to those established in Propositions 4.4 and 4.5 for (4.17) and (4.20) hold for the Riccati equation 1 d φt x0 = X11 + iX23 I φt x0 + X12 + iX13 φt x0 φt x0 dt 2 1 − φt x0 X12 − iX13 φt x0 + X12 − iX13 2 (5.23) with initial conditions φ0 (x0 ) = x0 ∈ B ∩ Ᏸ(X11 ). 6. Fixed points of semiflows The next sections will be devoted to investigating the fixed points of a continuous semiflow φ : R+ × D → D of holomorphic maps of a bounded domain D in a complex Banach space Ᏹ, that is to say, the points x ∈ D such that φt (x) = x for all x ∈ R+ . Actually, some of the results we are going to establish hold under slightly weaker conditions. Namely, φ will be a map of R∗+ × D into D satisfying (2.3) and (2.4) for all t,t1 ,t2 ∈ R∗+ and such that the map t → φt (y) is continuous on R∗ + for all y ∈ Ᏹ. A set S ⊂ D is said to be completely interior to D, in symbols S D if inf {x − y : x ∈ D, y ∈ Ᏹ\D} > 0. Since φt+s = φt φs (D) ⊂ φt (D) ∀t,s > 0, (6.1) if φt (D) D, (6.2) then φr (D) D ∀r ≥ t. (6.3) Let φt0 (D) D for some t0 > 0, and let t ≥ t0 . By the EarleHamilton theorem (see [2] or, e.g., [5, Theorem V.5.2]), there is a unique point xt ∈ D such that φt (xt ) = xt . Hence xt is the unique point in D such that φnt xt = xt ∀n = 1,2.... (6.4) Moreover, by the EarleHamilton theorem, lim φnt (x) = xt n→+∞ ∀x ∈ D. (6.5) Edoardo Vesentini 235 Let p, q be positive integers, with p ≥ q. There is a unique point x(p/q)t ∈ D such that φ(p/q)t x(p/q)t = x(p/q)t . (6.6) Since φn(p/q)t x(p/q)t = x(p/q)t (6.7) for n = 1,2,..., choosing n = mq, m = 1,2,... yields φmpt x(p/q)t = x(p/q)t . (6.8) Since, by (6.5), lim φmpt x(p/q)t = xt , (6.9) x(p/q)t = xt (6.10) m→+∞ then for all positive integers p ≥ q = 1,2,.... The continuity of t → φt (y) implies that φrt xt = xt (6.11) for all real numbers r ≥ 1. Hence there is a point x0 ∈ D which is the unique fixed point of φt for every t ≥ t0 . Let t0 > 0 and choose s ∈ (0,t0 ) and t ≥ t0 . Then φs x0 = φs φt x0 = φt+s x0 = x0 (6.12) because t + s > t0 . In conclusion, the first part of the following theorem has been established. Theorem 6.1. Let φ : R∗+ × D → D satisfy (2.3) and (2.4), and be such that t → φt (x) is continuous on R∗+ for all x ∈ D. If D is bounded, and if φt (D) D for some t > 0, there exists x0 ∈ D which is the unique fixed point of φs for every s > 0, and lim φs (x) = x0 s→+∞ ∀x ∈ D. (6.13) Proof. Let kD be the Kobayashi distance in D. To complete the proof of the theorem note that, given x ∈ D and s > 0, for every > 0 there exists a positive 236 Periodicity of holomorphic maps integer n0 such that, whenever n ≥ n0 , kD x0 ,φns (x) < . (6.14) If n ≥ n0 and t > ns, kD x0 ,φt (x) = kD x0 ,φns+t−ns (x) = kD φt−ns x0 ,φt−ns φns (x) ≤ kD x0 ,φns (x) < . (6.15) Corollary 6.2. Under the hypotheses of Theorem 6.1, x0 is the only ωstable point of φ. (That means that, for every > 0 and every τ > 0, there is some t ≥ τ for which kD (x0 ,φt (x0 )) < .) Theorem 6.3. Let D be bounded and let φ : R∗+ × D → D satisfy the hypotheses of Theorem 6.1. If there exist a sequence {tν } ⊂ R∗+ diverging to +∞ and a map g : D → D such that limν→+∞ φtν = g for the topology of local uniform convergence and if g(D) D, then there exists a unique point x0 ∈ D such that φt (x0 ) = x0 for all t > 0 and limt→+∞ φt (x) = x0 for all x ∈ D. Proof. Since g is holomorphic and g(D) D, the EarleHamilton theorem implies that there is a unique point x0 ∈ D which is fixed by g. If φt (y) = y for some y ∈ D and some t > 0, then, if s > t, φs (y) = φs−t+t (y) = φs−t φt (y) = φs−t (y), (6.16) and therefore φt φs (y) = φt φs−t (y) = φs (y). (6.17) g(y) = lim φtν (y) = y, (6.18) But then ν→+∞ and therefore y = x0 . Hence, either Fix φt = ∅ for all t > 0, or Fix φt = {x0 } when t 0. Let R > 0 be such that B x0 ,R D. (6.19) Since the Kobayashi distance kD and · are equivalent on B(x0 ,R), there exist real constants c > b > 0 such that bx − y ≤ kD (x, y) ≤ cx − y ∀x, y ∈ B x0 ,R . (6.20) Edoardo Vesentini 237 Let r > 0 be such that BkD x0 ,r ⊂ B x0 ,R . (6.21) For every > 0, there is ν0 such that ∀x ∈ B x0 ,R ν ≥ ν0 =⇒ φtν (x) − g(x) < (6.22) (because the sequence {φtν } converges to g for the topology of local uniform convergence). Since g(D) D, there exists a ∈ (0,1) such that kD φtν (x),x0 ≤ kD φtν (x),g(x) + kD g(x),x0 ≤ cφtν (x) − g(x) + akD x,x0 (6.23) < c + ar. Let ∈ (a,1) and be such that 0<< −a r. c (6.24) Then c + ar < ( − a)r + ar = r, (6.25) and therefore φtν BkD x0 ,r ⊂ BkD x0 ,r ∀ν ≥ ν0 . (6.26) It turns out that BkD x0 ,r BkD x0 ,r . (6.27) Indeed, if x ∈ BkD (x0 ,r) and y ∈ B(x0 ,R)\BkD (x0 ,r), x − y ≥ 1 − 1 1 kD (x, y) ≥ kD y,x0 − kD x0 ,x > r. c c c (6.28) As a consequence of (6.27), φtν BkD x0 ,r BkD x0 ,r ∀ν ≥ ν0 . (6.29) 238 Periodicity of holomorphic maps If t > tν0 , φt BkD x0 ,r = φt−tν0 +tν0 BkD x0 ,r = φtν0 φt−tν0 BkD x0 ,r ⊂ φtν0 BkD x0 ,r BkD x0 ,r . (6.30) Hence, Fix φt = x0 ∀t ≥ tν0 . (6.31) Thus, lim φt (x) = x0 t →+∞ (6.32) for all x ∈ BkD (x0 ,r). In particular, lim φtν (x) = x0 ν→+∞ (6.33) for all x ∈ BkD (x0 ,r). Hence, g(x) = x0 on BkD (x0 ,r) and therefore also on D (because the open set D is connected and g is holomorphic on D), and (6.32) holds for all x ∈ D. 7. Convergence of iterates and its consequences The following theorem was announced in [16] without proof. Theorem 7.1. Let D be a bounded domain in the complex Banach space Ᏹ, and let f : D → D be a holomorphic map fixing a point x0 ∈ D. If the sequence { f n } of the iterates of f converges for the topology of local uniform convergence on D, then either ⊂∆ (7.1) = {1} ∪ ∆ ∩ σ df x0 , (7.2) σ df x0 or σ df x0 and 1 is an isolated point of σ(df (x0 )) at which the resolvent function (•I − df (x0 ))−1 has a pole of order one. Since df n (x0 ) = (df (x0 ))n for n = 0,1,..., and {df n (x0 )} converges in the operator topology, Theorem 7.1 is a consequence of the following proposition, also announced in [16] without proof. Proposition 7.2. Let A and P be elements of ᏸ(Ᏹ). If lim An − P = 0, n→+∞ (7.3) Edoardo Vesentini 239 there exists k ∈ R∗+ , for which, n A ≤ k ∀n = 1,2,..., (7.4) and therefore the spectral radius of A is ρ(A) ≤ 1. (7.5) If ρ(A) < 1, then P = 0. If ρ(A) = 1, then σ(A) ∩ ∂∆ = {1}, (7.6) and 1 is an isolated point of σ(A) which is a pole of order one of the resolvent function (•I − A)−1 . Furthermore, P is the projector associated to the spectral set {1} in the spectral resolution of A. Proof. For any integer m ≥ 0, Am P = PAm = P, (7.7) P 2 = lim Am P = P, (7.8) and therefore m→+∞ that is, P is an idempotent of ᏸ(Ᏹ). For m = 1, (A − I)P = 0, and this fact, together with (7.3), yields ker(A − I) = RanP. (7.9) Thus, P = 0 if, and only if, 1 is an eigenvalue of A. Since n A − P ≤ An − P , (7.10) (7.3) implies (7.4), for a finite constant k > 0, and therefore implies (7.5) as well. Recall that σ(P) ⊂ {0,1} and that σ(P) = {0} if, and only if, P = 0, σ(P) = {1} if, and only if, P = I. By the upper semicontinuity of the spectrum, for any open neighbourhood U of σ(P), there is an integer n0 ≥ 0 such that, whenever n ≥ n0 , σ(An ) ⊂ U, and therefore the image of σ(A) by the map ζ → ζ n is contained in U. Hence, P = 0 =⇒ ρ(A) < 1, and if 1 ∈ σ(P), then (7.3) and the upper semicontinuity imply (7.6). (7.11) 240 Periodicity of holomorphic maps Choosing a neighbourhood U of the pair {0,1} consisting of two mutually disjoint open discs ∆(0,r1 ) and ∆(1,r2 ) centered at the points 0 and 1, with radii r1 > 0 and r2 > 0, and using again the upper semicontinuity of the spectrum, we see that 1 is an isolated point of σ(A) and σ(A) = {1} ∪ σ(A) ∩ ∆ . (7.12) What is left to prove is the final part of the proposition. (a) It will be shown first that, for any open, relatively compact neighbourhood U in C of {0,1} and for any compact set K ⊂ C such that K ∩ U = ∅, there exist a constant k1 > 0 and an integer n1 ≥ 1 such that −1 sup ζI − An : ζ ∈ K, n ≥ n1 ≤ k1 . (7.13) Let now r1 and r2 be such that 0 < r1 < r1 + r2 < 1, so that ∆ 0,r1 ∪ ∆ 1,r2 ⊂ U. (7.14) There is n2 ≥ n1 such that σ An ∩ ∆ ⊂ ∆ 0,r1 ∀n ≥ n 2 . (7.15) Given n ≥ n2 , choose r3 ∈ (0,r2 ) so small that the image by the map ζ → ζ n of ∆(1,r3 ) be contained in ∆(1,r2 ). Then, for any ζ ∈ K, ζI − An −1 = 1 2πi τ =r1 + 1 (τI − A)−1 dτ ζ − τn τ −1=r3 1 ζ − τn (7.16) −1 (τI − A) dτ . Let d be the Euclidean distance in C. If ζ ∈ K, then ζ  >r1 and, for any τ  = r1 , ζ − τ n ≥ ζ  − τ n = ζ  − τ n ≥ ζ  − τ  ≥ d ζ,∆ 0,r1 ≥ d(K,U). (7.17) If τ ∈ ∆(1,r3 ), then ζ − τ n ≥ d ζ,∆ 1,r2 ≥ d(K,U). (7.18) Edoardo Vesentini 241 Thus, (7.16) yields ζI − An −1 ≤ 2 sup (τI − A)−1 : τ ∈ U d(K,U) (7.19) for all ζ ∈ K and all n ≥ n1 , proving thereby (7.13). (b) Let k2 = sup ζI − P : ζ ∈ K . (7.20) For ζ ∈ K, ζI − An −1 − (ζI − P)−1 = ζI − An −1 ζI − P − ζI − An (ζI − P)−1 −1 n A − P (ζI − P)−1 = ζI − An −1 n A − P (ζI − P)−1 ≤ ζI − An ≤ k1 k2 An − P . (7.21) In the following, K = ∂∆(1,r), and r ∈ (0,1) will be chosen in such a way that ∆(1,r) ∩ σ(A) = ∅. (7.22) Let ζI − An −1 +∞ = (ζ − 1)ν An ν , (7.23) ν=−∞ with An ν ∈ ᏸ(Ᏹ), be the Laurent expansion of (ζI − An )−1 at 1. Let Pν ∈ ᏸ(Ᏹ) be the coeﬃcient of (ζ − 1)ν in the Laurent expansion of (ζI − −1 P) at 1. Then, by (7.21), for ν ≥ 1, n ν −1 n −1 −1 A −ν − P −ν ≤ 1 (ζ − 1) ζI − A − (ζI − P) dζ 2π ζ −1=r −1 1 (7.24) ≤ ζ − 1ν−1 ζI − An − (ζI − P)−1 dζ 2π ≤r ν −1 ζ −1=r k1 k2 An − P , and therefore lim An −ν − P−ν = 0 n→+∞ (7.25) 242 Periodicity of holomorphic maps for ν = 1,2,.... But, since (ζI − P)−1 = 1 1 P + (I − P), ζ −1 ζ (7.26) P−1 = P and P−ν = 0 for ν ≥ 2. Hence, lim An −1 − P = 0, (7.27) n→+∞ lim An −ν = 0 (7.28) n→+∞ for ν = 2,3,.... (c) Choose r1 and r2 in such a way that 0 < r1 < r1 + r2 < 1, and σ(A) ∩ ∆ ⊂ ∆(0,r1 ). For any n ≥ 1, choose r3 such that 0 < r3 < r2 and that the image of ∆(1,r3 ) by the map ζ → ζ n be contained in ∆(1,r2 ). For any ν ≥ 1, Dunford’s integral and Fubini’s theorem yield An −ν = 1 (2πi)2 ζ −1=r2 (ζ − 1)ν−1 × τ =r1 + 1 = (2πi)2 τ =r1 1 τ −1=r3 ζ −1=r2 + 1 (τI − A)−1 dτ ζ − τn τ −1=r3 ζ −τ (τI − A)−1 dτ dζ n (ζ − 1)ν−1 dζ (τI − A)−1 dτ ζ − τn ζ −1=r2 (7.29) (ζ − 1)ν−1 dζ (τI − A)−1 dτ . ζ − τn For τ  = r1 , the function ζ −→ (ζ − 1)ν−1 ζ − τn (7.30) is holomorphic in a neighbourhood of ∆(1,r2 ). Hence, by the Cauchy integral theorem, ζ −1=r2 (ζ − 1)ν−1 dζ = 0. ζ − τn (7.31) On the other hand, the Cauchy integral formula yields 1 2πi ζ −1=r3 ν−1 (ζ − 1)ν−1 dζ = τ n − 1 . ζ − τn (7.32) Edoardo Vesentini 243 Hence, for ν ≥ 1, An −ν = 1 2πi τ −1=r3 τn − 1 ν−1 (τI − A)−1 dτ = An − I ν−1 A−1 , (7.33) and (7.27) yields An −1 = P for n = 1,2,.... (7.34) Since A−ν = (A − I)ν−1 P ∀ν = 1,2,..., (7.9) yields A−ν = 0 for ν = 2,3,.... (7.35) A part of Proposition 7.2 follows also from the following lemma. Lemma 7.3. If (7.4) holds, if ∂∆ ∩ σ(A) eiθ for some θ ∈ R, and if eiθ is an isolated point of σ(A) which is a pole of the resolvent function (•I − A)−1 , then eiθ is a pole of order one. Proof. There is no restriction in assuming eiθ = 1. If n > 0 is the order of the pole, the resolvent function is represented in a neighbourhood of 1 by the Laurent series (ζI − A)−1 = +∞ (ζ − 1)ν Aν , (7.36) ν=−n and the range Ran(A−1 ) of A−1 is related to ker(I − A)m by Ran A−1 = ker(I − A)m for m = n,n + 1,.... (7.37) Being ker(I − A) ⊂ ker(I − A)2 ⊂ · · · , (7.38) (7.37) holds for m = 1 if, and only if, Ax = x ∀x ∈ Ran A−1 . (7.39) To see that this latter condition actually holds, assume that there is some y ∈ Ran(A−1 ) such that (A − I)y = 0, and let λ be a continuous linear form on Ᏹ such that (A − I)y,λ = 0. (7.40) 244 Periodicity of holomorphic maps By (7.35), (A − I)n y = 0, and therefore N N N A y = (A − I + I) y = p =0 n −1 N N (A − I) p y = (A − I) p y p p p =0 (7.41) for all N ≥ n. Thus N n −1 A y,λ = p =0 N (A − I) p y,λ , p (7.42) and therefore lim AN y,λ = ∞, N →+∞ (7.43) contradicting the fact that, in view of (7.4), N A y,λ ≤ λAN y ≤ k λ y for all N > 0. Thus (7.39) holds, and (7.35) yields A−ν = 0 for ν = 2,3,.... (7.44) If the hypotheses of Lemma 7.3 are satisfied with eiθ = 1, σ(A) splits as the union of the two disjoint spectral sets {1} and σ(A) ∩ ∆. The corresponding spectral projectors are P = A−1 and I − P; moreover, (A − I)P = 0. Setting C = A(I − P) = A − P, (7.45) then σ(C) = (σ(A) ∩ ∆) ∪ {0}. Since CP = PC, then An = P + C n for n = 1,2,.... (7.46) Being ρ(C) < 1, there exist ∈ (0,1) and n0 ≥ 1 such that n 1/n C ≤ 1 − , (7.47) that is, n C ≤ (1 − )n and therefore, by (7.46), (7.3) holds. ∀n ≥ n 0 , (7.48) Edoardo Vesentini 245 In conclusion, the following proposition has been established. Proposition 7.4. If (7.4) and (7.6) hold and if 1 is an isolated point of σ(A) which is also a pole of the resolvent function (•I − A)−1 , then (7.3) holds, where P is the spectral projector associated to the spectral set {1} in the spectral resolution of A. It will be shown in Section 8 that, if (7.1) holds, Theorem 7.1 can be inverted. 8. Suﬃcient conditions for the convergence of iterates Let D be a bounded domain in the complex Banach space Ᏹ, and let f : D → D be a holomorphic map fixing a point x0 ∈ D. As was noticed already, since D is bounded, σ(df (x0 )) ⊂ ∆ (see [5]). Theorem 8.1. If σ(df (x0 )) ⊂ ∆, the sequence { f n } of the iterates of f converges to the constant map x → x0 for the topology of local uniform convergence on D. Obviously, there is no restriction in assuming D to be a bounded, connected, open neighbourhood of x0 = 0. Let R > 0 be such that D ⊂ B(0,R). (8.1) f (x) = Ax + A2 (x,x) + · · · + AN (x,...,x) + · · · (8.2) Let be the power series expansion of f in 0, where A ∈ ᏸ(Ᏹ) and AN is a continuous, homogeneous, polynomial of degree N = 2,3,... on Ᏹ, with values in Ᏹ, that is, the restriction to the diagonal of Ᏹ × · · · × Ᏹ (n times) of a continuous Nlinear symmetric map, which will be denoted by the same symbol AN , of Ᏹ × · · · × Ᏹ into Ᏹ. If r = inf y : y ∈ D , (8.3) the power series (8.2) converges uniformly on B(0,s) whenever 0 < s < r. The nth iterate f n (n = 2,3,...) of f has a power series expansion in 0 which converges uniformly on B(0,s) and is expressed by f n (x) = An x + C2(n) (x,x) + · · · + CN(n) (x,...,x) + · · · , (8.4) where CN(n) is a continuous homogeneous polynomial of degree N = 2,3,... on Ᏹ with values in Ᏹ. 246 Periodicity of holomorphic maps An induction argument on n will show now that, for all x ∈ Ᏹ, N = 2,3,... and n = 2,3,..., CN(n) (x,...,x) = n −1 Aq AN An−q−1 x,...,An−q−1 x q =0 n −1 N −1 + m=1 q=2 (q,N) (8.5) Cq(m) A p1 An−m−1 x,...,An−m−1 x ,..., A pq An−m−1 x,...,An−m−1 x , where x ∈ Ᏹ, Cq(1) = Aq , and the sum (q,N) is extended to all positive integers p1 ,..., pq such that p1 + · · · + pq = N. First of all, a simple induction on n yields C2(n) (x,x) = n −1 Aq A2 An−q−1 x,An−q−1 x , (8.6) q =0 which coincides with (8.5) when N = 2. Assuming (8.5) to hold, then CN(n+1) (x,...,x) = An AN (x,...,x) + n = A AN (x,...,x) N −1 N Cq(n) A p1 (x,...,x),...,A pq (x,...,x) q=2 (q,N) + CN(n) (Ax,...,Ax) + q=2 (q,N) Cq(n) A p1 (x,...,x),...,A pq (x,...,x) n −1 q n − q −1 A AN A Ax,...,An−q−1 Ax = An AN (x,...,x) + q =0 n −1 N −1 + m=1 q=2 (q,N) Cq(m) A p1 An+1−m−1 x,...,An+1−m−1 x ,..., A pq An+1−m−1 x,...,An+1−m−1 x N −1 + q=2 (q,N) n+1−1 = Cq(n) A p1 (x,...,x),...,A pq (x,...,x) Aq AN An+1−q−1 Ax,...,An+1−q−1 Ax q =0 n+1−1 N −1 + m=1 q=2 (q,N) Cq(m) A p1 An+1−m−1 x,...,An+1−m−1 x ,..., A pq An+1−m−1 x,...,An+1−m−1 x . (8.7) Edoardo Vesentini 247 This inductive argument shows that (8.5) holds for N = 2,3,... and n = 2,3,.... Lemma 8.2. If A < 1, for N = 2,3,..., there is a positive constant cN such that (n) C ≤ cN An−N+1 N ∀n ≥ N − 1. (8.8) Here, CN(n) is the norm of the continuous polynomial x → CN(n) (x,...,x) (n) C = sup C (n) (x,...,x) : x ≤ 1 , N N (8.9) and is related to the norm (n) C = sup C (n) (x,..., y) : x ≤ 1,..., y ≤ 1 N N (8.10) of the continuous, symmetric Nlinear map (x,..., y) → CN(n) (x,..., y) by the inequalities (see, e.g., [5]) (n) (n) N N (n) C ≤ C ≤ C . N N N! (8.11) N Proof of Lemma 8.2. By (8.5), C2(n) (x,x) = n −1 Aq A2 An−q−1 x,An−q−1 x , (8.12) q =0 and therefore (n) n −1 C (x,x) ≤ A2 A2n−2q−2+q x2 2 q =0 n −1 = A2 An−1 An−q+1 x2 q =0 1 − An x 2 = A2 An−1 1 − A An−1 x 2 . ≤ A2 1 − A (8.13) 248 Periodicity of holomorphic maps Assuming the lemma to hold for q = 2,3,...,N − 1, and choosing n ≥ N − 1, then (n) C (x,...,x) N n −1 ≤ AN Aq AN(n−q−1) q =0 n −1 N −1 + m=1 q=2 qq C (m) A p · · · A p AN(n−m−1) xN q 1 q q! (q,N) 1 − An(N −1) ≤ AN An−1 1 − AN −1 n −1 N −1 + m=1 q=2 qq cq Am−q+1 A p1 · · · A pq AN(n−m−1) xN q! (q,N) 1 − An(N −1) 1 − A(n−1)(N −1) = AN An−1 + 1 − AN −1 1 − AN −1 N −1 qq A p · · · A p x N + cq An−q q =2 q 1 q! (q,N) N −1 q q n −1 ≤ AN A + cq q =2 q! A p · · · A p An−q 1 q (q,N) x N . 1 − AN −1 (8.14) Since A < 1, then An−q ≤ An−N+1 for q = 1,2,...,N − 1. (8.15) Hence, (n) C (x,...,x) ≤ cN An−N+1 xN , N (8.16) with N −1 q q cq cN = AN + q =2 q! A p · · · A p 1 q (q,N) 1 . 1 − AN −1 (8.17) In view of (8.1), the Cauchy inequalities yield (n) C ≤ R N rn ∀N ≥ 1, n ≥ 1. (8.18) Edoardo Vesentini 249 Hence, if s ∈ (0,1) is suﬃciently small, in such a way that B(0,s) ⊂ D, and if x ∈ B(0,s/2), n ≥ 1, and N0 ≥ 2, +∞ n n (n) f (x) ≤ A x + C (x,x) + · · · + C (n) (x,...,x) + R 2 N0 (n) (n) ≤ An x + C (x,x) + · · · + C (x,...,x) 2 +R x s N0 +1 N =N0 +1 x N s N0 1 1 − x/s ≤ A x + c2 An−1 x2 + · · · + cN0 An−N0 +1 xN0 + n R . 2 N0 (8.19) Let c = max{1,c2 ,...,cN0 }. Then n f (x) ≤ An−N0 +1 AN0 −1 + AN0 −2 + · · · + 1 s + R 2 N0 ≤c An−N0 +1 1 − A s+ R . 2 N0 (8.20) For > 0, choosing N0 0 and n0 0 in such a way that Rr N0 +1 < , 1−r 2 c An−N0 +1 r< 1 − A 2 ∀n ≥ n 0 , (8.21) then n f (x) < s ∀x ∈ B 0, , ∀n ≥ n 0 . 2 (8.22) That proves the following lemma. Lemma 8.3. If A < 1, for any > 0 and any s ∈ (0,1) such that B(0,s) ⊂ D, there is n0 ≥ 1 such that (8.22) holds. Proposition 8.4. If σ(A) ⊂ ∆, for any > 0 and any s ∈ (0,1) such that B(0,s) ⊂ D, there is n0 ≥ 1 such that (8.22) holds. Proof. There is n1 ≥ 1 such that An1 < 1. By Lemma 8.3, there is n2 ≥ 1 such that nn f 1 (x) < s ∀x ∈ B 0, , n ≥ n2 . 2 (8.23) 250 Periodicity of holomorphic maps Let ω be the Poincaré distance in ∆. Since holomorphic maps contract the Kobayashi distance, for m ≥ 1, n ≥ n2 , and x ∈ B(0,s/2), then ω 0, n n+m f 1 (x) R = kB(0,R) 0, f n1 n+m (x) ≤ kD 0, f n1 n+m (x) ≤ kD 0, f n1 n (x) ≤ kB(0,s) 0, f n1 n (x) f n1 n (x) < ω 0, . = ω 0, s (8.24) s Thus, the sequence { f n } converges to 0 uniformly on B(0,s/2), and therefore converges to zero everywhere on D by Vitali’s theorem [8, Theorem 3.18.1]. The convergence being uniform on B(0,s/2), the sequence { f n } tends to zero for the topology of local uniform convergence on D [5, page 104]. The proof of Theorem 8.1 is complete. As in Section 6, let φ be a map of R∗+ × D into D satisfying (2.3) and (2.4) for all t,t1 ,t2 ∈ R∗+ , and such that the map t → φt (x) is continuous on R∗+ for all x ∈ Ᏹ. Let φt (x0 ) = x0 for all t > 0 and for some point x0 in the bounded domain D ⊂ Ᏹ. If σ(dφt0 (x0 )) ⊂ ∆ for some t0 > 0, Theorem 8.1 applied to the function f = φt0 , implies that, as n → +∞, the sequence {φnt0 : n = 1,2,...} converges to the constant map x → x0 for the topology of local uniform convergence. Let r > 0 be such that BkD x0 ,r D. (8.25) Since the distances and kD are equivalent on BkD (x0 ,r), for any > 0, there is n0 ≥ 1 such that φn0 t0 BkD x0 ,r ⊂ BkD x0 , , (8.26) whenever n ≥ n0 . For all t > n0 t0 , φt BkD x0 ,r = φt−n0 t0 +n0 t0 BkD x0 ,r = φt−n0 t0 φn0 t0 BkD x0 ,r ⊂ φt−n0 t0 BkD x0 , ⊂ BkD x0 , because holomorphic maps contract the Kobayashi distance. Thus the following theorem holds. (8.27) Edoardo Vesentini 251 Theorem 8.5. If φ : R∗+ × D → D fixes a point x0 ∈ D of the bounded domain D, and if σ((dφt0 )(x0 )) ⊂ ∆ for some t0 > 0, then, as t → +∞, φt converges to the constant map x → x0 for the topology of local uniform convergence. 9. Fixed points and idempotents As at the beginning of Section 8, let D be a bounded domain in Ᏹ and let f : D → D be a holomorphic map fixing a point x0 ∈ D. If f is an idempotent of the semigroup Hol(D), a direct inspection of the power series expansion of f at x0 shows that df (x0 ) is an idempotent of ᏸ(Ᏹ). In this section, we show that, if the geometry of D satisfies suitable conditions, the fact that df (x0 ) is an idempotent of ᏸ(Ᏹ) implies that the iterates of f converge for the topology of local uniform convergence to an idempotent of Hol(D). As before, let D be a bounded, open, connected neighbourhood of 0, and let f (0) = 0. Let f be expressed in B(0,r) by the power series (8.2) (and r is given by (8.3)). Let A = df (0) be an idempotent of ᏸ(Ᏹ). Since A2 = A, (8.12) reads, for n ≥ 2, C2(n) (x,x) = AA2 (x,x) + A2 (Ax,Ax) + (n − 2)AA2 (Ax,Ax) (9.1) for all x ∈ Ᏹ. If AA2 (Ax,Ax) ≡ 0, there are y ∈ Ᏹ and λ ∈ Ᏹ (the topological dual of Ᏹ) such that AA2 (Ay,Ay),λ = 0. (9.2) The Cauchy inequalities (8.18) yield, for N = 2 and n = 1,2,..., AA2 (y, y) + A2 (Ay,Ay) + (n − 2)AA2 (Ay,Ay),λ ≤ R y 2 λ 2 r (9.3) for all n = 2,3,..., contradicting (9.2). Hence, AA2 (Ax,Ax) = 0 for all x ∈ Ᏹ, and therefore C2(n) (x,x) = AA2 (x,x) + A2 (Ax,Ax) (9.4) for all n = 2,3,..., and all x ∈ Ᏹ. Thus, C2n (x,x) does not depend on n ≥ 2. Proceeding by induction on N, we show that CN(n) (x,...,x) is independent of n ≥ N for all N. Assuming this fact to hold for C2 ,...,CN , then f N (x) = Ax + C2 (x,x) + · · · + CN (x,...,x) + FN+1 (x,...,x) + · · · (9.5) 252 Periodicity of holomorphic maps for all x ∈ B(0,r), where FN+1 is a homogeneous, continuous polynomial of degree N + 1 from Ᏹ to Ᏹ. Then, setting A1 = A, N f N+1 (x) = Ax + Cq (x,...,x) + AAN+1 (x,...,x) q =2 N Cq A p1 (x,...,x),...,A pq (x,...,x) + q=2 (q,N) + FN+1 (Ax,...,Ax) + · · · , N f N+2 (x) = Ax + Cq (x,...,x) + AAN+1 (x,...,x) q =2 N Cq A p1 (x,...,x),...,A pq (x,...,x) + q=2 (q,N) + FN+1 (Ax,...,Ax) + AAN+1 (Ax,...,Ax) N Cq A p1 (Ax,...,Ax),...,A pq (Ax,...,Ax) + · · · , + q=2 (q,N) .. . N f N+ (x) = Ax + Cq (x,...,x) + AAN+1 (x,...,x) q =2 N Cq A p1 (x,...,x),...,A pq (x,...,x) + FN+1 (Ax,...,Ax) + q=2 (q,N) + ( − 1) AAN+1 (Ax,...,Ax) N Cq A p1 (Ax,...,Ax),...,A pq (Ax,...,Ax) + q=2 (q,N) + ··· (9.6) for all x ∈ B(0,r) and all = 2,3,.... A similar argument to that devised for C2 implies that N Cq A p1 (Ax,...,Ax),...,A pq (Ax,...,Ax) = 0 AAN+1 (Ax,...,Ax) + q=2 (q,N) (9.7) for all x ∈ Ᏹ. Edoardo Vesentini 253 The inductive argument is now complete, showing that f n (x) = Ax + C2 (x,x) + · · · + CN (x,...,x) + O xN+1 (9.8) for all x ∈ B(0,r) and all n ≥ N = 1,2,..., with CN+1 (x,...,x) = AAN+1 (x,...,x) N Cq A p1 (x,...,x),...,A pq (x,...,x) + FN+1 (Ax,...,Ax). + q=2 (q,N) (9.9) Since, by the Cauchy inequalities, N n d f (0) ≤ R N! N (9.10) r for all N ≥ 0, n > 0, and therefore 1 limsup N 1/N dN f n (0) n! ≤ 1 , r (9.11) the CauchyHadamard formula implies that the power series +∞ BN (x,...,x) Ax + (9.12) N =2 converges uniformly on B(0,s) whenever 0 < s < r. Let g be the holomorphic function on B(0,r) represented by this power series. By the Cauchy inequalities, if x ≤ s < r, g(x) − f n (x) ≤ +∞ CN (x,...,x) − C (n) (x,...,x) N N =n+1 +∞ CN (x,...,x) + C (n) (x,...,x) ≤ N N =n+1 +∞ (n) CN + C xN ≤ N N =n+1 +∞ ≤ 2R N =n+1 +∞ ≤ 2R N =n+1 x r s r N+1 = 2R s r N N 1 . 1 − s/r (9.13) 254 Periodicity of holomorphic maps Hence, the sequence { f n } converges to g uniformly on B(0,s). By Vitali’s theorem [8, Theorem 3.18.1], the sequence { f n (x)} converges for all x ∈ D, and the limit is a holomorphic map h : D → Ᏹ. Clearly, hB(0,r) = g. The convergence being uniform on B(0,s), the sequence { f n } tends to h for the topology of local uniform convergence. In conclusion, the following theorem has been established. Theorem 9.1. Let f be a holomorphic map of a bounded domain D into itself. If f fixes a point x0 ∈ D, and if df (x0 ) is an idempotent of ᏸ(Ᏹ), the sequence { f n } converges for the topology of local uniform convergence to a holomorphic map h : D → Ᏹ. Obviously, h(D) ⊂ D, h(x0 ) = x0 , dh x0 = df x0 , (9.14) f ◦ h = h, (9.15) and h ◦ f = h. Furthermore, and therefore Fix f = h(D), provided that h(D) ⊂ D. This latter condition is fulfilled if D satisfies the following principle. Maximum principle. Whenever a holomorphic function h : D → Ᏹ is such that h(D) ⊂ D and h(D) ∩ ∂D = ∅, then h(D) ⊂ ∂D. Example 9.2. If the bounded domain D is convex, its support function is plurisubharmonic [14]. Thus, D satisfies the maximum principle. Summing up, the following proposition holds. Proposition 9.3. Under the hypotheses of Theorem 9.1, and if moreover D satisfies the maximum principle, h is an idempotent of the semigroup of all holomorphic maps of D into D which commute with f and is such that h(D) = Fix f . If df (x0 ) is an idempotent of ᏸ(Ᏹ), then = pσ df x0 ⊂ {0,1}, σ df x0 = {0} =⇒ df x0 = 0, σ df x0 = {1} =⇒ df x0 = I. σ df x0 (9.16) (9.17) (9.18) Since D is bounded, by Cartan’s identity theorem, (9.18) holds if, and only if, f = id. Theorem 8.1 and (9.17) yield the following proposition. Edoardo Vesentini 255 Proposition 9.4. If D is bounded, if f (x0 ) = x0 , and if df (x0 ) is an idempotent of ᏸ(Ᏹ) with σ(df (x0 )) = {0}, then the sequence { f n } converges to the constant map x → x0 for the topology of local uniform convergence on D. Theorem 9.5 [16]. Let D be a bounded, open, convex neighbourhood of 0, and let f ∈ Hol(D) be such that f (0) = 0 and df (0) is an idempotent of ᏸ(Ᏹ). If ∂D ∩ Randf (0) consists of complex extreme points of D, then h(D) = D ∩ Randf (0). Proof. Let A = df (0) and Ᏺ = ker(I − A) = RanA. As a consequence of the strong maximum principle [15, Corollary 5.4], if x ∈ Ᏺ ∩ D, f (x) = Ax = x, and with the same notations of (8.2), A2 (x,x) = A3 (x,x,x) = · · · = 0 ∀x ∈ Ᏺ. (9.19) A2 (Ax,Ax) = A3 (Ax,Ax,Ax) = · · · = 0 ∀x ∈ Ᏹ. (9.20) Therefore, Thus, by (9.4), C2 (x,x) = AA2 (x,x) ∀x ∈ Ᏹ. (9.21) Similarly, for any N = 2,3,..., if x ∈ Ᏺ ∩ D, then f N (x) = Ax = x, and C2 (Ax,Ax) = · · · = CN (Ax,...,Ax) = FN+1 (Ax,...,Ax) = 0 ∀x ∈ Ᏹ. (9.22) Assuming that there are continuous polynomials x → C̃2 (x,x),...,x → C̃N (x,...,x) such that C2 = AC̃2 ,...,CN = AC̃N , (9.9) yields CN+1 = AC̃N+1 (9.23) with N C̃N+1 (x,...,x) = AN+1 (x,...,x) + C̃q A p1 (x,...,x),...,A pq (x,...x) . q=2 (q,N) (9.24) This inductive argument shows that h(B(0,r)) ⊂ Ᏺ, and therefore h(D) ⊂ Ᏺ ∩ D. Since, on the other hand, Ᏺ ∩ D ⊂ Fix f = h(D), the conclusion fol lows. 10. Extensions to semiflows In this section, we apply the results of Section 8 to the case in which f is an element of a semiflow. Thus, let x0 ∈ D be a fixed point of a semiflow φ : R+ × D → D acting by holomorphic maps φt on a domain D of Ᏹ. Denoting by dφt (x) ∈ ᏸ(Ᏹ) the Fréchet diﬀerential of φt at x, then dφt1 +t2 x0 = dφt1 x0 dφt2 x0 ∀t1 ,t2 ∈ R+ , dφ0 x0 = I. (10.1) 256 Periodicity of holomorphic maps Lemma 10.1. If the semiflow φ is continuous, the semigroup dφ• (x0 ) : R+ → ᏸ(Ᏹ) is strongly continuous. If the domain D is bounded, the semigroup is uniformly bounded. Proof. Choose r > 0 so small that B(x0 ,r) ⊂ D. If ξ ∈ Ᏹ, choose s > 0 in such a way that φt (x0 + ζξ) ∈ B(x0 ,r) whenever ζ  ≤ s and for any t in a neighbourhood of 0 in R+ . If λ ∈ Ᏹ , the Cauchy integral formula yields 1 dφt x0 ξ,λ = 2πi ∂∆(0,s) φt x0 + ζξ ,λ dζ. ζ2 (10.2) Since, for ζ ∈ ∂∆(0,s), φt x0 + ζξ ,λ r λ ≤ , ζ2 s2 (10.3) the dominated convergence theorem implies that lim dφt x0 ξ − ξ,λ = 0, (10.4) t ↓0 that is, the semigroup dφ• (x0 ) is weakly, hence strongly, continuous. The uniform boundedness of the semigroup follows from the Cauchy in equalities. Let Z : Ᏸ(Z) ⊂ Ᏹ → Ᏹ be the infinitesimal generator of the strongly continuous semigroup dφ• (x0 ) : R+ → ᏸ(Ᏹ). Let D be a bounded domain in Ᏹ, and let φ : R+ × D → D be a continuous semiflow of holomorphic maps of D into D fixing a point x0 ∈ D. If φ2t0 = φt0 , for some t0 > 0, then dφt0 is an idempotent of ᏸ(Ᏹ). If σ(dφt0 (x0 )) = {0}, (9.17) applied to f = φt0 shows that the semigroup dφ• (x0 ) is nilpotent. Theorem 8.5 implies that, as t → +∞, φt converges to the constant map x → x0 for the topology of local uniform convergence. If σ(dφt0 (x0 )) = {1}, (9.18) applied to f = φt0 , coupled with Cartan’s identity theorem, implies that φt0 = id, and therefore φ is the restriction to R+ of a continuous periodic flow with period t0 / p for some positive integer p. How many values of the semigroup dφ• (x0 ) can be idempotent in ᏸ(Ᏹ)? Clearly, if dφt0 (x0 ) is an idempotent of ᏸ(Ᏹ), then dφnt0 (x0 ) is an idempotent of ᏸ(Ᏹ) for n = 1,2,.... If dφt0 (x0 ) is an idempotent of ᏸ(Ᏹ) for some t0 > 0, and if 1 ∈ σ(dφt0 (x0 )), then 2nπi/t0 ∈ pσ(Z) for some n ∈ Z. Letting V := n ∈ Z : 2nπi ∈ pσ(Z) , t0 (10.5) Edoardo Vesentini 257 then V = ∅, 2πi V, t0 2nπi ker I −Z . ker I − dφt0 x0 = t0 n ∈Z σ(Z)\{0} = pσ(Z)\{0} = (10.6) For any t > 0 and n ∈ V e2nπit/t0 ∈ pσ dφt x0 . (10.7) Hence, if dφt1 (x0 ) is an idempotent of ᏸ(Ᏹ) for some t1 > 0, for any n ∈ V , e2nπit1 /t0 = 1, (10.8) 2nπit1 = 2πim, t0 (10.9) nt1 = mt0 . (10.10) that is, there is m ∈ Z such that that is, As a consequence, if t1 /t0 ∈ Q, then n = m = 0. Hence, V = {0}, therefore pσ dφt x0 = { 1 }, Randφt x0 = ker I − dφt x0 = kerZ (10.11) ∀t ∈ R+ . (10.12) Thus, since dφt0 (x0 ) is an idempotent, Ᏹ = ker dφt0 x0 ⊕ kerZ. (10.13) Let Π and Λ = I − Π be the projectors, with ranges kerdφt (x0 ) and kerZ, associated to this direct sum decomposition of Ᏹ. Since, for any x ∈ Ᏹ and any t ≥ t0 , dφt x0 Πx = dφt−t0 x0 dφt0 x0 Πx = 0, (10.14) then, by (10.12), dφt x0 x = dφt x0 Λx = Λx, (10.15) and therefore dφ2t x0 x = dφt x0 Λx = Λx = dφt x0 x. (10.16) 258 Periodicity of holomorphic maps Hence, if dφt0 (x0 ) and dφt1 (x0 ) are idempotents of ᏸ(Ᏹ), and if t1 /t0 ∈ Q, then dφt x0 = dφt0 x0 ∀t ≥ min t0 ,t1 . (10.17) Let 0 < t < t0 . If x ∈ kerdφt0 (x0 ) and dφt (x0 )x = 0, then Λdφt x0 x ∈ kerZ \{0}, (10.18) and therefore 0 = dφt0 +t x0 x = dφt x0 Λdφt x0 x = Λdφt x0 x = 0. (10.19) This contradiction proves that if x ∈ kerdφt0 (x0 ), then x ∈ dφt (x0 ) for all t ∈ (0,t0 ]. Summing up, if 1 ∈ σ(dφt0 (x0 )) and if t1 /t0 ∈ Q, then dφt (x0 ) is an idempotent of ᏸ(Ᏹ) which is independent of t > 0. The strong continuity of the semigroup dφ• (x0 ) implies then that dφt (x0 ) = I for all t ≥ 0. Since D is a bounded domain, Cartan’s identity theorem yields the following theorem. Theorem 10.2. If dφt0 (x0 ) and dφt1 (x0 ), with t1 /t0 ∈ Q, are idempotents of ᏸ(Ᏹ), and if 1 ∈ σ(dφt0 (x0 )), then φt = id for all t ∈ R+ . As in Section 4, and with the same notations, let D be the open unit ball B of the complex Hilbert space , and let φ be the periodic continuous semiflow, with period τ, of holomorphic automorphisms of B, defined by the group T. If 0 ∈ Fixφ, (4.8) shows that φ is (the restriction to B of) a strongly continuous group of linear operators on , φt = dφt (0)B , (10.20) and Z = X11 − iX22 I . If 0 ∈ pσ(Z) and x ∈ kerZ \{0}, then φt (x) = dφt (0)x = x ∀t ∈ R. (10.21) Vice versa, if φt (x) = x for some x ∈ B \{0} and all t ∈ R, Bart’s theorem in [1] implies that 0 ∈ pσ(Z). That proves the following lemma. Lemma 10.3. Let 0 ∈ Fixφ. Then {0} = Fixφ if, and only if, 0 ∈ pσ(Z). References [1] [2] H. Bart, Periodic strongly continuous semigroups, Ann. Mat. Pura Appl. (4) 115 (1977), 311–318. C. J. Earle and R. S. Hamilton, A fixed point theorem for holomorphic mappings, Global Analysis (Proc. Sympos. 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