# Congurations of periodic orbits for equations with delayed positive feedback

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1 Congurations of periodic orbits for equations with delayed positive feedback Dedicated to Professor Tibor Krisztin on the occasion of his 60th birthday Gabriella Vas 1 MTA-SZTE Analysis and Stochastics Research Group, Bolyai Institute, University of Szeged, 1 Aradi v. tere, Szeged, Hungary Abstract We consider scalar delay dierential equations of the form _x (t) = x (t) + f (x (t 1)) ; where > 0 and f is a nondecreasing C 1 -function. If is a xed point of f : R 3 u 7! f (u) = 2 R with f 0 () > 1, then [ 1; 0] 3 s 7! 2 R is an unstable equilibrium. A periodic solution is said to have large amplitude if it oscillates about at least two xed points < + of f with f 0 ( ) > 1 and f 0 ( + ) > 1. We investigate what type of large-amplitude periodic solutions may exist at the same time when the number of such xed points (and hence the number of unstable equilibria) is an arbitrary integer N 2. It is shown that the number of dierent congurations equals the number of ways in which N symbols can be parenthesized. The location of the Floquet multipliers of the corresponding periodic orbits is also discussed. Keywords: Delay dierential equation, Positive feedback, Large-amplitude periodic solution, Slow oscillation, Floquet multiplier 2000 MSC: [2010] 34K13, 37D05 1. Introduction We study the delay dierential equation _x (t) = x (t) + f (x (t 1)) (1.1) under the hypotheses (H0) > 0, 1 Preprint submitted to Journal of Dierential Equations November 11, 2016

2 (H1) feedback function f 2 C 1 (R; R) is nondecreasing. If 2 R is a xed point of f : R 3 u 7! f (u) = 2 R; then ^ 2 C = C ([ 1; 0] ; R), dened by ^ (s) = for all s 2 [ 1; 0], is an equilibrium of the semiow. In this paper we assume that (H2) if is a xed point of f, then f 0 () 6= 1. This hypothesis guarantees that all equilibria are hyperbolic. It is well known that if is an unstable xed point of f (that is, if f 0 () > 1), then ^ is an unstable equilibrium. If is a stable xed point of f (that is, if f 0 () < 1), then ^ is also stable (exponentially stable). The stable and unstable equilibria alternate in pointwise ordering. Mallet-Paret and Sell have veried a Poincare{Bendixson type result for (1.1) in the case when f 0 (u) > 0 for all u 2 R [18]. Krisztin, Walther and Wu obtained further detailed results on the structure of the solutions (see e.g. [8, 9, 10, 13, 14, 15]). They have characterized the geometrical and topological properties of the closure of the unstable set of an unstable equilibrium, the so called Krisztin{Walther{Wu attractor. If there is only one unstable equilibrium, sucient conditions can be given for the closure of the unstable set to be the global attractor. The chief motivation for the present work comes from the paper [18] of Mallet-Paret and Sell. They have shown that if f 0 (u) > 0 for all u 2 R, then 2 : C 3 ' 7! (' (0) ; ' ( 1)) 2 R maps dierent (nonconstant and constant) periodic orbits of (1.1) onto disjoint sets in R 2, and the images of nonconstant periodic orbits are simple closed curves in R 2. They have also shown that a nonconstant periodic solution p : R! R of (1.1) oscillates about a xed point of f if and only if 2 ^ = (; ) is in the interior of 2 fp t : t 2 Rg. See Figure 1.1. These results give a strong restriction on what type of periodic solutions the equation may have for the same feedback function f: Suppose that p 1 : R! R and p 2 : R! R are periodic solutions of equation (1.1). For both i 2 f1; 2g, let E i be the set of those xed points of f about which p i oscillates. Then either E 1 E 2 or E 2 E 1 or E 1 \ E 2 = ;: We can easily extend these assertions to the case when f 0 (u) 0 for all u 2 R, see Proposition 3.4 in Section 3. Figure 1.1: Three examples excluded by Mallet-Paret and Sell. Here we show the images of periodic orbits and equilibria under 2. This paper considers large-amplitude periodic solutions: periodic solutions oscillating about at least two unstable xed points of f. Figure 1.2 lists all 2

3 congurations of large-amplitude periodic solutions allowed by the previously cited results of Mallet-Paret and Sell in case there are three and four unstable equilibria, respectively. It is a natural question whether all of them indeed exist for some nonlinearities f. Allowing any number of unstable equilibria, we conrm the existence of all possible congurations of large-amplitude periodic solutions by constructing the suitable feedback functions and periodic solutions explicitly. The oscillation frequency of these periodic solutions is the lowest possible. The corresponding periodic orbits are hyperbolic, unstable, and they have exactly one Floquet multiplier outside the unit circle. We do not state uniqueness; there may exist more periodic solutions that cannot be obtained from each other by translation of time and oscillate about the same xed points of f. Proving the nonexistence of periodic solutions is a challenging problem in general, see for example the papers [2, 13, 19] for some well-known results. We can verify that unrequired large-amplitude periodic solutions do not appear for the feedback functions constructed in the paper. So for any conguration in Figure 1.2, there is a nonlinearity f such that equation (1.1) admits the marked large-amplitude periodic solutions (maybe even more of the same type), but it has none of those that are not indicated. In the negative feedback case, i.e., when f is nonincreasing, there is at most one equilibrium. Still, it is possible to prove the coexistence of an arbitrary number of slowly oscillatory periodic orbits, see paper [22] for an explicit construction. If f is continuously dierentiable, then these periodic orbits hyperbolic and stable. Figure 1.2: Possible congurations for three or four unstable equilibria: The images of the large-amplitude periodic orbits and the unstable equilibria under The main result Before formulating the main result precisely, we give an introduction to the theoretical background and to the notation used in the paper. Consider equation 3

4 (1.1) under (H0) (H2). The phase space for (1.1) is the Banach space C = C ([ 1; 0] ; R) with the maximum norm. If J is an interval, u : J! R is continuous and [t 1; t] J, then the segment u t 2 C is dened by u t (s) = u (t + s), 1 s 0. A solution of equation (1.1) is either a continuous function x: [t 0 1; 1)! R, t 0 2 R, that is dierentiable for t > t 0 and satises equation (1.1) on (t 0 ; 1), or a continuously dierentiable function x: R! R satisfying the equation for all t 2 R. To all ' 2 C, there corresponds a unique solution x ' : [ 1; 1)! R with x ' = '. 0 Let : [0; 1) C 3 (t; ') 7! x ' t 2 C denote the solution semiow. The global attractor A, if exists, is a nonempty, compact set in C with the following two properties: A is invariant in the sense that (t; A) = A for all t 0. A attracts bounded sets in the sense that for every bounded set B C and for every open set U A, there exists t 0 with ([t; 1) B) U. Global attractors are uniquely determined [4]. In this paper the number of unstable equilibria is an arbitrary integer N 2. We use the notation 1 < 2 < : : : < N for those xed points of f that give the unstable equilibria. Typically we will consider feedback functions for which f admits N +1 further xed points j, j 2 f0; 1; : : : ; Ng, inducing stable equilibria. Then 0 < 1 < 1 < 2 < 2 < : : : < N < N : See Figure 2.1 for an example. v v=f(u) v= u ζ 0 ξ 1 ζ 1 ξ 2 ζ 2... ξ N ζ N u Figure 2.1: A nonlinearity f giving N unstable and N + 1 stable equilibria As usual, an arbitrary solution x is called oscillatory about a xed point of f if the set x 1 () R is not bounded from above. A solution x is slowly oscillatory if for any xed point in x (R) and for any t 2 R such that [t 1; t] is in the domain of x, the function [t 1; t] 3 s 7! x (s) 2 R has one or two sign changes. As it has been mentioned before, we say that a periodic solution has large amplitude if it oscillates about at least two elements of f 1 ; 2 ; : : : ; N g. This denition is the straightforward generalization of the one used in [11]. By an [i; j] periodic solution with 1 i < j N, we mean a large-amplitude periodic solution that oscillates about the elements of f i ; i+1 ; : : : ; j g but not about the elements of f 1 ; 2 ; : : : ; i 1 g [ f j+1 ; : : : ; N g, see Figure 2.2. If p : R! R is a periodic solution with minimal period! > 1, one can consider the period map (!; ) and its derivative M = D 2 (!; p 0 ). M is 4

5 j+1 j... i+1 i i-1 t Figure 2.2: An [i; j] periodic function called the monodromy operator. It is a compact operator, and 0 belongs to its spectrum = (M). Eigenvalues of nite multiplicity { the so called Floquet multipliers of the periodic orbit O p = fp t : t 2 [0;!)g { form (M) n f0g. It is known that 1 is a Floquet multiplier with eigenfunction _p 0. The periodic orbit O p is said to be hyperbolic if the generalized eigenspace of M corresponding to the eigenvalue 1 is one-dimensional, furthermore there are no Floquet multipliers on the unit circle besides 1. We know a lot about the dynamics from previous works of Krisztin, Walther and Wu in the case when f 0 (u) > 0 for all u 2 R. With the notation introduced above, consider the subset C i = f' 2 C : i 1 ' (s) i for all s 2 [ 1; 0]g ; i 2 f1; : : : ; Ng ; (2.1) of the phase space C. Clearly, the equilibria ^ i 1 ; ^ i ; ^ i belong to C i. The monotonicity of f implies that the set (2.1) is positively invariant under the solution semiow, see Proposition 3.1 of this paper. Krisztin, Walther and Wu have characterized the closure of the unstable set n o ' 2 C : x ' exists on R and x ' t! ^ i as t! 1 : It has a so-called spindle-like structure: it contains ^ i 1 ; ^ i ; ^ i, periodic orbits oscillating about i, and heteroclinic connections among them. In the simplest situation the periodic orbit is unique, and it oscillates slowly [13, 14]. In other cases, the closure of the unstable set has a more complicated structure. For example, more periodic orbits appear via a series of Hopf-bifurcations in a small neighborhood of ^ i as f 0 ( i ) increases, see [15]. Under certain technical conditions, the closure of the unstable set of ^ i is the global attractor of the restriction j [0;1) Ci [9, 13]. For further details, see the paper [8], and the references therein. The monograph [14] of Krisztin, Walther and Wu raised originally the question, whether the global attractor is the union of the global attractors A i of the restrictions j [0;1) Ci, i 2 f1; : : : ; Ng. We already know from the previous paper [11] of Krisztin and Vas that this is not necessarily the case. In the N = 2 case there exists a strictly increasing feedback function f such that equation (1.1) has exactly two periodic orbits outside A 1 [ A 2, and the unstable sets of them constitute the global attractor besides A 1 [ A 2. These two periodic 5

6 solutions have large amplitude; they oscillate slowly about 1 and 2. See paper [12] of Krisztin and Vas for the geometrical description of the unstable sets of these large-amplitude periodic orbits. The purpose of this paper is to develop the result of [11] by investigating what type of large-amplitude periodic solutions may exist for the same nonlinearity f if the number of unstable equilibria is an arbitrary integer greater than 1. Our main result can be formulated using parenthetical expressions. A pair of parentheses consists of a left parenthesis "(" and a right parenthesis ")", furthermore, "(" precedes ")" if read from left to right. A parenthetical expression of N numbers consists of the integers 1; 2; : : : ; N and a nite (possibly zero) number of pairs of parentheses such that the integers 1; 2; : : : ; N are used exactly once in increasing order, a pair of parenthesis encloses at least two numbers out of 1; 2; : : : ; N, e.g., the expressions (1) 23 or 1()23 are not allowed, multiple enclosing of the same sublist of numbers is not allowed, e.g., ((12))3 is not allowed, 145 for any two pairs of parentheses, if the left parenthesis "(" of the rst pair precedes the left parenthesis "(" of the second one, then the right parenthesis ")" of the second pair precedes the right parenthesis ")" of the rst one. For example, the parenthetical expressions of 3 numbers are 123; (12)3; 1(23); (123); ((12)3); (1(23)): (2.2) We emphasize that parentheses appear in pairs in a correct parenthetical expression, and it is denite which right parenthesis ")" belongs to a given left parenthesis "(". By the result of Mallet-Paret and Sell, if the derivative of f is positive, p 1 : R! R and p 2 : R! R are periodic solutions of (1.1), and E i is the set of xed points of f about which p i oscillates for both i 2 f1; 2g, then either E 1 E 2 or E 2 E 1 or E 1 \ E 2 = ;: This assertion is already true under hypotheses (H0) (H1). See Proposition 3.4 in Section 3 for a proof in the = 1 case. This property guarantees that we can assign a correct parenthetical expression of N numbers to each and f satisfying (H0) and (H1) if we use the following rule: for all i < j, the numbers i; i + 1; : : : ; j are enclosed by a pair of parentheses (not containing further numbers) if and only if (1.1) with this parameter and nonlinearity f admits at least one [i; j] periodic solution. The monotonicity of f is important here. In general we cannot guarantee that we can assign a correct parenthetical expression in the above explained way to each > 0 and f 2 C 1 (R; R). For example, in case of four unstable equilibria, we cannot exclude that the equation has [1; 3] and [2; 4] periodic solutions 6

7 for the same nonmonotone f 2 C 1 (R; R). Then we would get the incorrect expression ( 1 1( 2 23) 1 4) 2, where ( 1 123) 1 corresponds to the [1; 3] periodic solution, and ( 2 234) 2 corresponds to the [2; 4] periodic solution. Tibor Krisztin has conjectured that the converse statement is true, that is, we can assign a conguration of large-amplitude periodic solutions to each parenthetical expression. The main result of the paper is the following Fix a parenthetical expression of N numbers, where N 2. Then there exists and f satisfying (H0){(H2) such that the following assertions hold. (i) For this and f, there exist exactly N unstable equilibria ^ 1 ; ^ 2 ; : : : ; ^ N with 1 < 2 < : : : < N : For all i; j 2 f1; : : : ; Ng with i < j, the equation (1.1) has an [i; j] periodic solution if and only if there exists a pair of parentheses in the expression that contains only the numbers i; i + 1; : : : ; j. (ii) For any i; j 2 f1; : : : ; Ng such that the numbers i; i+1; : : : ; j are enclosed by a pair of parentheses (not containing further integers), at least one of the [i; j] periodic solutions is slowly oscillatory. The corresponding periodic orbit is hyperbolic, with exactly one Floquet multiplier outside the unit circle, which is real, greater than 1 and simple. Figure 2.3 shows the congurations corresponding to ((1 (23)) (45)) 6 and ((((12) 3) 4) 5) 6: the images of the large-amplitude periodic orbits and unstable equilibria under the projection 2 : C 3 ' 7! (' (0) ; ' ( 1)) 2 R 2 : Figure 2.3: Congurations corresponding to the expressions ((1 (23)) (45)) 6 and ((((12) 3) 4) 5) In the proof of assertion (i) of Theorem 2.1, we explicitly construct a nondecreasing C 1 -function f. This nonlinearity is close to a step function in the sense that it is constant on certain subintervals of the real line. Roughly speaking, we can control whether certain types of large-amplitude periodic orbits appear or not by setting the heights of the steps properly. In general, determining the Floquet multipliers is an innite dimensional problem. Our construction allows us to reduce this problem to a nite dimensional one. This is why we can prove Theorem 2.1.(ii). The hyperbolicity of the periodic orbits guarantees that Theorem 2.1 remains true for nondecreasing perturbations of the feedback function, see Theorem 8.2. In consequence, we can require f in Theorem 2.1 to be even strictly increasing. This correspondence between the congurations of large-amplitude periodic solutions and the parenthetical expressions implies the following under hypothe- 7

8 ses (H0){(H2). If we ignore the exact number of large-amplitude periodic solutions oscillating about the same given subsets of f 1 ; 2 ; : : : ; N g, the number of possible congurations for N unstable equilibria equals the number C N of ways in which N numbers can be correctly parenthesized. One can check that C N, N 2, are the so called large Schroder numbers [1]. Applying a well-known combinatorial tool, generating functions, it can be calculated that C N = 1 2 NX i=0 1 2 i 1 2 N i 3 2 p 2 i p 2 N i ; N 2: (2.3) By this formula, C 2 = 2, C 3 = 6, C 4 = 22, C 5 = 90, C 6 = 394 and C 7 = 1806: Numerical simulation shows that C N grows geometrically. It is an interesting problem to show the existence of unstable periodic orbits for delay equations by computer assisted proofs. Using a technique from [21], Szczelina has recently found numerical approximations of apparently unstable orbits in [20] for an equation of the form (1.1). Lessard and Kiss, applying a dierent approach developed in [16], have rigorously proven the coexistence of three periodic orbits for Wright's equation with two delays in [7], and at least one of them is presumed to be unstable. Although the method of Lessard and Kiss can be applied to determine both stable and unstable periodic solutions, it is not suitable for the stability analysis of the obtained solutions. The paper is organized as follows. For the sake of notational simplicity, we x to be 1. In Section 3 we prove some simple results. The proof of Theorem 2.1.(i) is found in Sections 4{6. In Section 4 we consider feedback functions f for which f (u) = Ksgn (u) if juj 1 and f(u) 2 [ K; K] if u 2 ( 1; 1). We explicitly construct periodic solutions for such nonlinearities. Then we use these feedback functions as building blocks in Sections 5 and 6 to determine a nonlinearity satisfying assertion (i) of Theorem 2.1. For a rst reading one may skip Section 4, only read Corollary 4.10 without proof, and then look at the construction in Sections 5{6. We give a brief introduction to Floquet theory and then verify Theorem 2.1.(ii) in Section 7. The proof of Theorem 2.1.(ii) cannot be read without knowing the details of Section 4. In Section 8 we explain why the statements of Theorem 2.1 remain true for small perturbations of the nonlinearity. We close the paper with discussing open questions in Section Preliminaries We x to be 1 in the rest of the paper and consider the equation _x (t) = x (t) + f (x (t 1)) : (3.1) 225 The results of the paper can be easily modied for other choices of as well. It is natural to use the pointwise ordering on C. For '; 2 C, we say that ' if '(s) (s) for all s 2 [ 1; 0], ' if ' and '(0) < (0). 8

9 230 Relations \" and \" are dened analogously. The semiow induced by equation (3.1) is monotone if f is nondecreasing Assume (H1). Let ' and be elements of C with ' (' ). Then x ' (t) x (t) x ' (t) < x (t) for all t 0. Proof. If x ' : [ 1; 1)! R is a solution of equation (3.1) with x ' 0 = ', then x' can computed recursively on [0; 1) using the variation-of-constants formula: t x ' (t) = x ' (n) e (t n) + n e (t s) f (x ' (s 1)) ds for all nonnegative integers n and t 2 [n; n + 1]. The proposition follows from this formula. The next two propositions have appeared in the paper [18] of Mallet-Paret and Sell for the case f 0 (u) > 0, u 2 R Assume that (H1) holds, and p : R! R is a periodic solution of (3.1) with minimal period! > 0. Fix t 0 < t 1 < t 0 +! so that p (t 0 ) = min t2r p(t) and p (t 1 ) = max t2r p(t). Then (i) p is of monotone type in the sense that p is nondecreasing on [t 0 ; t 1 ] and nonincreasing on [t 1 ; t 0 +!]; (ii) if p oscillates about a xed point of f, then p (t 0 ) < < p (t 1 ) : Proof. Statement (i) is proven in [18] only if f 0 > 0. For the proof of statement (i) under hypothesis (H1), see Proposition 5.1 in [11]. The proof of statement (ii) under (H1). Note that as = 1, ^ is an equilibrium. It is clear that p (t 0 ) p (t 1 ). If p (t 0 ) =, then with ' = ^ and = p t1 we have ', and = x ' (t) < x (t) = p (t + t 1 ) for all t by Proposition 3.1. This is impossible as p oscillates about : Similarly, p (t 1 ) > It follows immediately that if (H1) holds, and p: R! R is a periodic solution of (3.1) with minimal period! 2 (1; 2), then p is slowly oscillatory: On the one hand, Proposition 3.1 easily gives that for all xed points of f in p (R), the map t 7! p (t) has at least one sign change on each interval of length 1. On the other hand, Proposition 3.2 implies that t 7! p (t) has at most two sign changes on each interval of length!, hence also on each interval of length 1. For a simple closed curve c : [a; b]! R 2, let int (c [a; b]) denote the interior, i.e., the bounded component of R 2 n c ([a; b]) Assume (H1). (i) 2 : C 3 ' 7! (' (0) ; ' ( 1)) 2 R 2 maps nonconstant periodic orbits and equilibria of (3.1) into simple closed curves and points in R 2, respectively. The 9

10 images of dierent (nonconstant and constant) periodic orbits are disjoint in R 2. (ii) A periodic solution p : R! R of (3.1) with minimal period! > 0 oscillates about a xed point of f if and only if 2 ^ 2 int ( 2 O p ), where O p = fp t : t 2 [0;!]g. (iii) In consequence, if p 1 : R! R and p 2 : R! R are periodic solutions of equation (3.1), and E i is the set of xed points of f about which p i oscillates for both i 2 f1; 2g, then either E 1 E 2 and p 1 (R) p 2 (R) ; or or E 1 \ E 2 = ;: E 2 E 1 and p 2 (R) p 1 (R) ; Proof. The paper [18] veries (i) in the case f 0 > 0, while [11] gives a proof in the slightly more general case f 0 0. See Proposition 2.4 of [11]. In order to prove (ii), rst assume that p oscillates about a xed point of f. Let! denote the minimal period of p. Set points t 0 < t 1 < t 0 +! such that p (t 0 ) = min t2r p(t) and p (t 1 ) = max t2r p(t). Then p (t 0 ) < < p (t 1 ) by Proposition 3.2.(ii). According to Proposition 3.2.(i), the set of zeros of t 7! p (t) in (t 0 ; t 1 ) is an interval: ft 2 (t 0 ; t 1 ) : p(t) = g = [z 0; ; z 1 ] with t 0 < z 0 z 1 < t 1. One may also set z 2 and z 3 so that [z 2 ; z 3 ] (t 1 ; t 0 +!), p(t) = for t 2 [z 2; ; z 3 ] and p(t) 6= for t 2 (t 1 ; t 0 +!) n [z 2; ; z 3 ]. Of course, z 0 = z 1 or z 2 = z 3 is possible. Consider the curve : [t 0 ; t 0 +!] 3 t 7! 2 p t 2 R 2. By property (i), is a simple closed curve, and (t) 6= 2 ^ = (; ) for t 2 [t 0 ; t 0 +!]. For t 2 (z 1 ; t 1 ], p(t) >, _p(t) 0, hence f (p (t 1)) = _p(t) + p(t) > and necessarily p(t 1) >. We claim that p (t 1) > holds also for t 2 [z 0; ; z 1 ]. If not, then there exists z 2 [z 0; ; z 1 ] so that p (z 1) =, which contradicts (z ) 6= 2 ^. Therefore (t) 2 (u; v) 2 R 2 : u ; v > for t 2 [z 0 ; t 1 ] : It can be veried in a similar manner that p(t 1) < holds for t 2 [z 2 ; t 0 +!] and thus (t) 2 (u; v) 2 R 2 : u ; v < for t 2 [z 2 ; t 0 +!] : 295 Since is a simple closed curve and there exists no t 2 [t 0 ; t 0 +!] n ([z 0 ; z 1 ] [ [z 2 ; z 3 ]) such that (t) is in (; v) 2 R 2 : v 2 R, we obtain that 2 ^ = (; ) 2 int ( [t 0 ; t 0 +!]). The reverse statement is easy. If p does not oscillate about a xed point of f, then p (t) > or p (t) < for all t 2 R, and (t) 2 (u; v) 2 R 2 : u > ; v > for all t 2 R 10

11 300 or (t) 2 (u; v) 2 R 2 : u < ; v < for all t 2 R; respectively. This means that (; ) =2 int ( [t 0 ; t 0 +!]). Statement (iii) follows at once from (i) and (ii). 4. Construction of a single periodic solution Let K > 1. We dene F (K) as the class of functions f 2 C 1 (R; R) with f(u) 2 [ K; K] for u 2 ( 1; 1), f (u) = Ksgn(u) for juj The elements of F (K) are not required to satisfy (H1) or (H2) There exists a threshold number K 0 > 1 such that for all K > K 0 and f 2 F (K), the equation _x (t) = x (t) + f (x (t 1)) (3.1) has a periodic solution p : R! R with the following properties: The minimal period of p is in (1; 2), max t2r p (t) 2 (1; K) and min t2r p (t) 2 ( K; 1) We prove Proposition 4.1 by determining a suitable periodic solution explicitly. The paper [11] has already described two signicantly dierent periodic solutions in the special case when f 2 F (K) and f (x) = 0 for all x 2 [ 1 + "; 1 "] with some small " > 0. Section 3.1 of [11] has determined the rst periodic solution that we now denote by p 1. Section 3.2 of [11] has given the second one p 2. The construction below is a generalization of the one that has been published for p 2 in Section 3.2. In paper [11], the initial functions of p 1 and p 2 were determined as xed points of three-dimensional maps. Here we not only generalize but also simplify the calculations regarding p 2 because now we obtain the initial function of the periodic solution as the xed point of a one-dimensional map. The construction of p 1 is indeed three-dimensional, and at this point we cannot extend it to all f 2 F (K). In the following we assume that f 2 F (K), where K > 1. Step 0. Preliminary observations For both i 2 f K; Kg, consider the map i : R R 3 (s; x ) 7! i + (x i) e s 2 R: If t 0 < t 1, and x is a solution of equation (3.1) on [t 0 1; 1) with x (t 1) 1 for all t 2 (t 0 ; t 1 ), then equation (3.1) reduces to the ordinary dierential equation _x (t) = x (t) + K 11

12 on the interval (t 0 ; t 1 ), and thus x(t) = K (t t 0 ; x (t 0 )) for all t 2 [t 0 ; t 1 ] : (4.1) 330 Similarly, if t 0 < t 1, x is a solution of equation (3.1) on [t 0 1; 1), and x (t 1) 1 for all t 2 (t 0 ; t 1 ), then x(t) = K (t t 0 ; x (t 0 )) for all t 2 [t 0 ; t 1 ] : (4.2) 335 We say that a function x : [t 0 ; t 1 ]! R is of type (K) (or ( K)) on [t 0 ; t 1 ], if (4.1) (or (4.2)) holds. If x : [t 0 1; 1)! R is a solution of equation (3.1), and x is type of (i) on [t 0 1; t 1 1] with some i 2 f K; Kg, then the equality x (t) = x (t 0 ) e t0 t + e t t t 0 e s f ( i (s t 0 ; j)) ds (4.3) 340 holds for all t 2 [t 0 ; t 1 ] with j = x (t 0 1). This observation motivates the next denition. A function x : [t 0 ; t 1 ]! R is of type (i; j) on [t 0 ; t 1 ] with i 2 f K; Kg and j 2 R if (4.3) holds for all t 2 [t 0 ; t 1 ]. Let T 1 denote the time needed by a function of type ( K) to decrease from 1 to 1. As K > 1, T 1 is well-dened, and T 1 = ln K + 1 K 1 : Then T 1 is the time needed by a function of type (K) to increase from 1 to 1. Set T 2 to be the time needed by a function of type (K) to increase from 1 to 0: T 2 = ln K + 1 K : As the reader will see from the rest of the section, we search for a periodic solution p that is of type (K) when it increases from 1 to 1, and of type ( K) when it decreases from 1 to 1. Hence, if J is a subinterval of R mapped by p onto [ 1; 1], then the length of J is T 1, furthermore p is of type (K; 1) or of type ( K; 1) on J + 1 = ft + 1 : t 2 Jg. Step 1. A C 1 -submanifold of initial functions We introduce a one-dimensional C 1 -submanifold of the phase space C. This manifold will contain the initial segment of the periodic solution. If K is large enough, then U 1 = (0; 1 T 1 T 2 ) is a nontrivial open interval. For given a 2 U 1, set s i = s i (a), i 2 f0; 1; 2g, and s 3 as s 0 = 1; s 1 = s 0 + a = 1 + a; s 2 = s 1 + T 1 = 1 + a + T 1 ; s 3 = T 2 : 12

13 The denitions of U 1, T 1 and T 2 imply that 1 = s 0 < s 1 < s 2 < s 3 < 0: 355 For all a 2 U 1, dene the function h (a) 2 C 1 (R; R) by h (a) (t) = 8 >< >: K; if t < s 1 ; f ( K (t s 1 ; 1)) ; if s 1 t < s 2 ; K; if s 2 t < s 3 ; f ( K (t s 3 ; 1)) if s 3 t: See Figure 4.1 for the plot of h (a). Then dene the map : U 1! C by (a) (t) = e t t 1 e s h (a) (s) ds for all 1 t 0: (4.4) It is clear that is continuous on U 1 because U 1 3 a 7! h (a) 2 C (R; R) is continuous. Notice that (a) is the unique solution of the initial value problem ( _y (t) = y (t) + h (a) (t) ; 1 t 0; (4.5) y ( 1) = 0: K s 1 s 2 s 30 -K Figure 4.1: The plot of h (a) The next characterization of U 1 reveals the idea behind the above denitions. See also Figure 4.2 for the plot of a typical element of U A function ' 2 C belongs to U 1 if and only if there exists s 1 2 ( 1; T 1 T 2 ) so that with s 2 = s 1 + T 1 and s 3 = T 2, (i) '( 1) = 0, (ii) ' is of type (K) on [ 1; s 1 ], (iii) ' is of type ( K; 1) on [s 1 ; s 2 ], (iv) ' is of type ( K) on [s 2 ; s 3 ], (v) ' is of type (K; 1) on [s 3 ; 0]. 13

14 x t 3 s 3-1 t 1 s 2 0 τ s 1 t Figure 4.2: The plot of an element of U 1 and of the corresponding solution. 370 We need to examine the smoothness of : For each xed a 2 U 1, the map R 3 t 7! h (a) (t) 2 R is C 1 -smooth with derivative h 0 (a). Fix t 2 (s 2 ; s 3 ). If a 2 U 1 and jj is small enough, then ( h (a) (t ) if t t ; h (a + ) (t) = h (a) (t) if t > t : It follows h (a) (t) = h 0 (a) (t) if t 2 [ 1; t ] ; 0 if t 2 (t ; 0] : Dene the nontrivial element = (a) 2 C by t (t) = e t e h (a) (s) ds for all t 2 [ 1; 0] : The map U 1 3 a 7! (a) 2 C is C 1 -smooth with D (a) 1 = for all a 2 U 1. Proof. (a) is the unique solution of the initial value problem (4.5). Hence the proposition follows from the dierentiability of the solutions of ordinary dierential equations with respect to the parameters. It follows that U 1 is a one-dimensional C 1 -submanifold of C. We look for a periodic solution with initial segment in U 1. We are going to need the exact values of (a) at s i = s i (a), i 2 f1; 2; 3g, and at 0 for all a 2 U 1. Let T1 Note that c 1 deduce that c 1 = 0 e u f ( K (u; 1)) du: is independent of a. Then using the denitions of and h, we (a) (s 1 ) = e s1 s1 1 Ke s ds = K 1 e a ; (4.6) 14

15 385 s2 (a) (s 2 ) = e s2 (a) (s 3 ) = e s3 1 e s h (a) (s) ds = e s1 s2 (a) (s 1 ) + e s2 = e T1 ( (a) (s 1 ) + c 1 ) = K 1 s3 s2 s 1 K + 1 K 1 e a + c 1 ; 1 e s h (a) (s) ds = e s2 s3 (a) (s 2 ) + e s3 s3 s 2 e s f ( K (s s 1 ; 1)) ds ( K) e s ds = e 1+a+T1+T2 ( (a) (s 2 ) + K) K = e 1+a (K + 1) 1 + c 1 K + K + 1 K 1 e 1 (K + 1) K (4.7) (4.8) and 0 0 (a) (0) = e s h (a) (s) ds = e s3 (a) (s 3 ) + e s f ( K (s s 3 ; 1)) ds: 1 s 3 (4.9) We see that (a) (s i ), i 2 f1; 2; 3g, and (a) (0) are continuously dierentiable functions of a 2 U 1. Step 2. Construction of a one-dimensional return map Let U 2 = a 2 U 1 : (a) (s) > 1 for s 2 [s 1 ; s 2 ] and (a) (s) < 1 for s 2 [s 3 ; 0] : 390 It is easy to see from Proposition 4.3 that U 2 is an open subset of U 1. Later we shall see that U 2 is nonempty if K is large enough. For a 2 U 2, there exist 1 < t 1 < s 1 < s 2 < t 2 < t 3 < s 3 such that (a) (t 1 ) = (a) (t 2 ) = 1 and (a) (t 3 ) = 1; 395 see Figure 4.2. As (a) is of type (K) on [ 1; s 1 ] and of type ( K) on [s 2 ; s 3 ], it is strictly monotone on these intervals. Hence t 1 ; t 2 and t 3 are unique. For t 1 we have t1 e t1 Ke s K ds = 1; and thus t 1 = 1 + ln K 1 : (4.10) Similarly, t2 1 t2 1 = e t2 1 e s h (a) (s) ds = e s2 t2 (a) (s 2 ) Ke t2 15 s 2 e s ds

16 and 400 from which 1 = e t3 t3 t 2 = s 2 + ln K + (a) (s 2) K e s h (a) (s) ds = e s2 t3 (a) (s 2 ) Ke t3 t3 s 2 e s ds; and t 3 = s 2 + ln K + (a) (s 2) K 1 follows. Note that t 3 t 2 = T 1 and t 2 ; t 3 are C 1 -smooth functions of a. Let us introduce the notation c 2 = T1 0 e u f ( K (u; 1)) du: (4.11) For a 2 U 2, consider the solution x = x (a) : [ 1; 1)! R of equation (3.1). We need the following result before dening a further open subset of U (i) The maps U 2 3 a 7! x (a) (t 1 + 1) = e T1 (a) (s 3 ) + e T1 c 2 2 R 405 and U 2 3 a 7! x (a) (t 2 + 1) = K + K x (a) (t 1 + 1) K e a 2 R K + (a) (s 2 ) are continuously dierentiable. (ii) The map is continuous. U 2 3 a 7! x (a) j [0;t1+1] 2 C ([0; t 1 + 1] ; R) Proof. Statement (i). As T 1 and c 2 are independent of a, K + (a) (s 2 ) > 0 and (a) (s 2 ) and (a) (s 3 ) are C 1 -smooth functions on U 2, one has to show only that the stated equalities indeed hold. As (a) ( 1) = 0 and (a) is of type (K) on [ 1; t 1 ] (see Remark 4.2), x = x (a) is of type (K; 0) on [0; t 1 + 1]. By (4.3) and (4.9), 410 t x (t) =x (0) e t + e t e s f ( K (s; 0)) ds =e s3 t (a) (s 3 ) + e t 0 0 t e s f ( K (s s 3 ; 1)) ds + e t s 3 0 e s f ( K (s; 0)) ds for all t 2 [0; t 1 + 1]. It follows immediately from the denition of K and from s 3 = T 2 = ln (K= (K + 1)) that K (s s 3 ; 1) = K (s; 0) for all s 2 R: 16

17 Therefore t x (t) = e s3 t (a) (s 3 ) + e t e s f ( K (s s 3 ; 1)) ds s 3 t s3 = e s3 t (a) (s 3 ) + e s3 t e u f ( K (u; 1)) du; t 2 [0; t 1 + 1] : We see from the denition of s 3 and (4.10) that 0 (4.12) 415 t s 3 = ln K K 1 ln K K + 1 = T 1: Hence (4.12) with t = t gives the formula for x (t 1 + 1). By Remark 4.2 and the denition of U 2, (a) strictly increases on [ 1; s 1 ], (a) (t) > 1 for all t 2 [s 1 ; s 2 ], and (a) strictly decreases on [s 2 ; s 3 ]. It follows that (a) (t) > 1 for all t 2 (t 1 ; t 2 ), hence x is of type (K) on the interval [t 1 + 1; t 2 + 1], and thus x (t 2 + 1) = K + (x (t 1 + 1) K) e t1 t2 : By (4.10) and (4.11) and the denition of s 2 ; t 1 t 2 = ln K K + (a) (s 2 ) We obtain that the formula for x (t 2 + 1) indeed holds. Statement (ii). We see from (4.12) that for all a 1 2 U 2 and a 2 2 U 2, max t2[0;t 1+1] x (a1) (t) x (a2) (t) = a: max e s3 t j (a 1 ) (s 3 ) (a 2 ) (s 3 )j t2[0;t 1+1] e s3 j (a 1 ) (s 3 ) (a 2 ) (s 3 )j : Statement (ii) hence follows from the continuity of U 2 3 a 7! (a) (s 3 ) 2 R Now let n o U 3 = a 2 U 2 : x (a) (t) < 1 for all t 2 [0; t 1 + 1] and x (a) (t 2 + 1) > 0 : From Proposition 4.4 it is clear that U 3 is an open subset of R. Later we shall see that U 3 is nonempty. Figure 4.2 shows an element of U Observe that the elements of U 3 can be characterized as follows. A function ' 2 C belongs to U 3 if and only if there exists s 1 2 ( 1; T 1 T 2 ) so that with s 2 = s 1 + T 1 and s 3 = T 2, properties (i)-(v) of Remark 4.2 hold, furthermore (vi) ' (t) > 1 for all t 2 [s 1 ; s 2 ], (vii) if 1 < t 1 < s 1 with ' (t 1 ) = 1, then x ' (t) < 1 for all t 2 [s 3 ; t 1 + 1], (viii) if s 2 < t 2 < s 3 with ' (t 2 ) = 1, then x ' (t 2 + 1) > 0. 17

18 435 For a 2 U 3, x = x (a) is of type (K) on [t 1 + 1; t 2 + 1], hence it is strictly increasing on [t 1 + 1; t 2 + 1]. So there exists unique t 4 and with t < t 4 < < t such that x (t 4 ) = 1 and x () = 0, see Figure 4.2. As (a) strictly decreases on [t 3 ; s 3 ], x (t) < 1 for all t 2 [s 3 ; t 1 + 1] by Remark 4.5, and x strictly increases on [t 1 + 1; t 2 + 1], we deduce that x (t) < 1 for t 2 (t 3 ; t 4 ) and x (t) 2 ( 1; 0) for t 2 (t 4 ; ) : (4.13) 4.6. The map U 3 3 a 7! = ln K x (t 1 + 1) 2 (0; 1) K 1 (4.14) is continuously dierentiable. Proof. As x is of type (K) on the interval [t 1 + 1; t 2 + 1], we have 0 = x () = K + (x (t 1 + 1) K) e t1+1 ; from which the formula easily follows with the aid of (4.10). It is clear that 2 (0; 1) because 2 (t 1 + 1; t 2 + 1) (0; 1). The smoothness of is a consequence of the smoothness of x (t 1 + 1). Similarly, and (4.10) together yield that t1+1 t4 1 = x (t 4 ) = K + (x (t 1 + 1) K) e t 4 = ln K (K x (t 1 + 1)) : (4.15) K 2 1 As the next result shows, solutions with initial functions in U 3 return to U Suppose a 2 U 3 and dene t 2 and as above. Then x +1 2 U 1 and x +1 = (t ) : Proof. It is clear from the above construction (to be more precise, from the denitions of ; t 2 ; t 3 ; t 4, the fact that x is of type (K) on [t 1 + 1; t 2 + 1], property (iv) of Remark 4.2 and the observation (4.13)) that (i) x () = 0, (ii) x is of type (K) on [; t 2 + 1], (iii) x is of type ( K; 1) on [t 2 + 1; t 3 + 1], (iv) x is of type ( K) on [t 3 + 1; t 4 + 1], (v) and x is of type (K; 1) on [t 4 + 1; + 1]. So by Remark 4.2, it suces to show that (a) ^s 1 := (t 2 + 1) ( + 1) = t 2 is in ( 1; T 1 T 2 ), (b) ^s 2 := (t 3 + 1) ( + 1) = t 3 equals ^s 1 + T 1, (c) ^s 3 := (t 4 + 1) ( + 1) = t 4 equals T 2. Property (c) comes from (4.14) and (4.15). By the denition of ^s 1 ; property (b) is equivalent to t 3 = t 2 + T 1, which follows from (4.11). It is clear that ^s 1 > 1. Hence (a) comes from ^s 1 = ^s 2 T 1 < ^s 3 T 1 = T 2 T 1. 18

19 The above results motivate us to dene the map F : U 3! R by F (a) = t : 465 The next proposition is an immediate consequence of the smoothness of t 2 and as functions of a F is C 1 -smooth. Note that if a 2 U 3 and F (a) = a, then x (a) = (a), and +1 x (a) is a periodic solution of equation (3.1) with minimal period + 1. Step 3. The map F has a unique xed point A trivial upper bound for the absolute values of c 1 and c 2 is the following: T jc 1 j; jc 2 j K 0 e u du = 2K K 1 : (4.16) If K > 1 is xed, then c 1 and c 2 are uniformly bounded for all f 2 F (K). We will use a further technical result which holds for more general feedback functions Suppose that f : R! R is continuous, K 1 2 R, K 2 2 R, f (u) 2 [K 1 ; K 2 ] for all u 2 R, t 0 2 R, and x : [t 0 1; 1)! R is a solution of (3.1) with x (t 0 ) 2 (K 1 ; K 2 ). Then x (t) 2 (K 1 ; K 2 ) for all t t 0. Proof. We prove the upper bound for x. Let y : R! R be the solution of the initial value problem ( _y(t) = y(t) + K2; t 2 R; y (t 0 ) = x (t 0 ) : Then y (t) = K 2 + (x (t 0 ) K 2 ) e t0 t < K 2 for t 2 R. We know that _x (t) x(t) + K 2 for all t 2 R. Theorem 6.1 of Chapter I.6 in [5] hence implies that for t t 0, x (t) y (t) < K 2. The lower bound can be veried analogously. Proof. [Proof of Proposition 4.1]We show that if K > 1 is large enough and f 2 F (K), then the map F has a unique xed point in U 3, namely there exists a unique a 2 U 3 such that t = a: (4.17) Substituting (4.11), (4.14) and then the denitions of s 2 and T 1 into equation (4.17), we obtain that (4.17) is equivalent to (a) (s 2 ) = x (t 1 + 1). Then using (4.7), (4.8), the formula for x (t 1 + 1) in Proposition 4.4 and again the denition of T 1, we see that (a) (s 2 ) = x (t 1 + 1) is an equation of second order in e a : it can be written in the form z 2 + z + = 0; (4.18) 19

20 where z = e a, the coecients ; ; are independent of a, and they are dened as 2K = e 1 K 1 + c 1 ; K = c 1 + c 2 K + 1 K = K + 1 : 490 Observe p that > 0 for all K > 1 because of (4.16). As < 0, it is clear that 2 4 > jj. This means that e 1 ; z = p is a negative solution of (4.18). We conclude that for all K > 1 and f 2 F (K), the map F has at most one xed point a in U 3, and it is given by a = ln p : (4.19) 2 It remains to show that if K is chosen suciently large, then a determined by (4.19) is indeed in U 3 for all f 2 F (K), that is, with the notation used before, (i) a 2 (0; 1 T 1 T 2 ), (ii) (a ) (t) > 1 for t 2 [s 1 ; s 2 ], (iii) x (a ) (t) < 1 for all t 2 [s 3 ; t 1 + 1], (iv) x (a ) (t 2 + 1) > 0. Property (i). Applying the bound (4.16) for jc 1 j and jc 1 j, we see that lim sup K!1 f2f(k) and thus 2e 1 = 0; lim sup K!1 lim sup K!1 f2f(k) a f2f(k) + e 1 = 0; ln 1 + p 1 + 8e 4 lim sup j + 1j = 0; K!1 f2f(k) = 0: (4.20) As lim K!1 (1 T 1 T 2 ) = 1, property (i) immediately follows for all large K and for all f 2 F (K). Property (ii). By the denition of and formula (4.6), t (a ) (t) = e t e s h (a) (s) ds 1 t = e s1 t (a ) (s 1 ) + e t = e s1 t K 1 e a + e s1 t t s1 s 1 e s f ( K (s s 1 ; 1)) ds 0 e u f ( K (u; 1)) du 20

21 for all t 2 [s 1 ; s 2 ]. Hence (a ) (t) e s1 s2 K 1 e a e s1 s1 jc 1 j = K 1 K + 1 K 1 e a jc 1 j for all t 2 [s 1 ; s 2 ] : Here we used that s 2 s 1 = T 1. As jc 1 j is bounded for K > 2, and 1 e a has a positive limit as K! 1, we see that (ii) is satised for all f 2 F (K) if K is large enough. Property (iii). The denition of gives that for t 2 [s 3 ; 0], t s3 (a ) (t) = e s3 t (a ) (s 3 ) + e s3 t e u f ( K (u; 1)) du: (4.21) We see from (4.12) that (4.21) actually holds for all t 2 [s 3 ; t 1 + 1]. Regarding the value (a ) (s 3 ), observe that (4.8), the limit of a in (4.20), and the bound for jc 1 j together yield that 0 lim K!1 (a ) (s 3 ) p1+8e 1 2e 1 K = uniformly for f 2 F (K). As the denominator in the above fraction is negative, (a ) (s 3 ) < 0 if K is large enough, and it tends to 1 as K! 1. By using formula (4.21), (a ) (s 3 ) < 0 and t s 3 = T 1, we now obtain the upper bound (a ) (t) K 1 K + 1 (a ) (s 3 ) + jc 2 j for all t 2 [s 3 ; t 1 + 1] : As (a ) (s 3 ) tends to 1, and c 2 is bounded if K > 2, property (iii) also holds for all F (K) if K is chosen suciently large. Property (iv). Recall the formula given by Proposition 4.4 for x (a ) (t 2 + 1) : With the equality (a ) (s 2 ) = x (a ) (t 1 + 1) conrmed at the beginning of this proof, we derive that x (a ) (t 2 + 1) = K Ke a = (a ) (s 1 ) ; and hence (iv) follows from (ii). Dene p : R! R as the ( + 1)-periodic extension of x (a ) j [ 1; ] to R. Then it is clear from the construction that p is a solution of (3.1), the minimal period of p is (1; 2), max t2r p (t) > 1 and min t2r p (t) < 1. It follows from Proposition 4.9 that p (t) 2 ( K; K) for all real t. Extension of the result An analogous result holds for a wider class of feedback functions. Consider any C 1 -nonlinearity dened on a nite closed subinterval of the real line. Next we prove that we can extend this function to the real line such that a new periodic solution appears. The range of this periodic solution contains the original nite interval. 21

22 4.10. Let > 0, A 1 ; A 2 ; B 1 ; B 2 2 R with A 1 < A 2, B 1 < B 2 and 2 < A 2 A 1. Assume that ^f 2 C 1 ([A 1 + ; A 2 ] ; R) is given, and B 1 ^f (u) B 2 for all A 1 + u A 2 : Consider the threshold number K 0 > 1 from Proposition 4.1. Let K 1 < min (A2 A 1 ) K 0 + A 1 + A 2 ; A 1 ; B 1 ; A 1 + A 2 B 2 2 (4.22) 535 and K 2 = A 1 + A 2 K 1 : Let f be a C 1 -extension of ^f to the real line with f (u) = K 1 for u A 1 ; f (u) = K 2 for u A 2 ; 540 and f (u) 2 [K 1 ; K 2 ] for u 2 [A 1 ; A 2 ] : Then equation (3.1) with nonlinearity f has a periodic solution p : R! R such that (i) the minimal period of p is in (1; 2), (ii) max t2r p (t) 2 (A 2 ; K 2 ) and min t2r p (t) 2 (K 1 ; A 1 ). See Figure 4.3 for a plot of f in the corollary. B 1 Figure 4.3: The plot of f in Corollary Proof. First note that if (4.22) holds, then K 2 = A 1 + A 2 K 1 > max fa 2 ; B 2 g : This observation with (4.22) means that the intervals (K 1 ; A 1 ) and (A 2 ; K 2 ) in assertion (ii) are indeed both nontrivial, furthermore, K 1 < B 1 < B 2 < K 2, so 22

23 545 it is possible to choose C 1 -extensions f of ^f such that f (u) 2 [K 1 ; K 2 ] for all u 2 [A 1 ; A 2 ]. Assume that f is a C 1 -function given as in the proposition. Consider the linear transformation L of R that maps A 1 to 1 and A 2 to 1: L (u) = 2u A 1 A 2 A 2 A 1 for u 2 R: 550 Let L 1 denote the inverse linear transformation. Dene g : R! R by g (u) = Lf L 1 u for u 2 R: Then g 2 C 1 (R; R). As the above introduced linear transformations are order preserving, we calculate that g (u) = 2K 1 A 1 A 2 A 2 A 1 for all u L (A 1 ) = 1; and g (u) = 2K 2 A 1 A 2 A 2 A 1 = 2K 1 A 1 A 2 A 2 A 1 for all u L (A 2 ) = 1; 2K 1 A 1 A 2 A 2 A 1 g (u) 2K 2 A 1 A 2 A 2 A 1 for all u 2 ( 1; 1) : We conclude that g 2 F (K) with K = A 1 + A 2 2K 1 A 2 A 1 : (4.23) 555 The assumption (4.22) guarantees that K > K 0. By Proposition 4.1, the equation _y (t) = y (t) + g (y (t 1)) has a periodic solution q : R! R. The minimal period of q is in (1; 2), furthermore, max t2r q (t) 2 (1; K) and min q (t) 2 ( K; 1) : t2r 560 Dene the periodic function p: R! R by p (t) = L 1 q (t) for all t 2 R: Substituting p into equation (3.1), one can see that p is a solution of (3.1) with the above chosen nonlinearity f. It is clear that p has the desired properties. Note that the bounds B 1 and B 2 in the previous corollary are not necessarily strict bounds for ^f: 5. Further auxiliary results Two technical results Set = 1 as before, and consider equation (3.1). The rst proposition in this section studies the ranges of the large-amplitude periodic solutions. 23

24 5.1. Suppose that (H1) holds, and f has exactly 2N + 1 xed points 0 < 1 < 1 < 2 < 2 : : : < N < N 570 with N 2, f 0 ( i ) < 1 for all i 2 f0; 1; : : : ; Ng and f 0 ( i ) > 1 for all i 2 f1; : : : ; Ng. Assume that p: R! R is an [i; j] periodic solution of equation (3.1) with some integers 1 i < j N, namely p oscillates about the elements of f i ; i+1 ; : : : ; j g but not about the elements of f 1 ; 2 ; : : : ; i 1 g[f j+1 ; : : : ; N g. Then i 1 < p (t) < j for all t 2 R: Proof. 1. Set t min 2 R and t max 2 R such that p (t min ) = min t2r p (t) and p (t max ) = max t2r p (t). The proof is based on the observation that f (p (t min )) < p (t min ) and f (p (t max )) > p (t max ) : (5.1) 575 The weaker inequalities f (p (t min )) p (t min ) and f (p (t max )) p (t max ) can be seen from 0 = _p (t min ) = p (t min ) + f (p (t min 1)) p (t min ) + f (p (t min )) and 0 = _p (t max ) = p (t max ) + f (p (t max 1)) p (t max ) + f (p (t max )) : Proposition 3.2.(ii) in addition implies that there exist no equilibria ^ 2 C such that 2 fp (t min ) ; p (t max )g, i.e, f (p (t min )) 6= p (t min ) and f (p (t max )) 6= p (t max ) : We show that p (t) > i 1 for all real t. First suppose that i = 1 and i 1 = 0. Note that by the assumptions of the proposition, f (u) u for all u 2 ( 1; 0 ], and thus (5.1) excludes the possibility that p (t min ) 0. Now assume that i > 1. Then p (t) > i 1 for all real t, otherwise p oscillates about i 1. As f (u) > u for u 2 ( i 1 ; i 1 ) and f ( i 1 ) = i 1, (5.1) shows that it is impossible that p (t min ) 2 ( i 1 ; i 1 ]. Thus p (t) > i 1 for all real t in any case i 1. It is similar to verify that p (t) < j for all t 2 R. The following simple result will be used to exclude the existence of the unwanted large-amplitude periodic solutions Suppose that f : R! R is continuous, and ; ; + ; + 2 R are xed points of f with < < + < +. Suppose that equation (3.1) admits a periodic solution p such that ( ; + ) p (R) ( ; + ). Then log + 1 and log : (5.2) 24

25 Proof. Let y : R! R be the solution of the initial value problem ( _y(t) = y(t) + + ; t 2 R; y (0) = : Then y (t) = + + ( + ) e t for t 2 R. It is a straightforward calculation to show that the unique solution of y (T ) = + is T = log ; that is, y needs T time to increase from to +. We may assume (by considering a time shift of p if necessary) that p (0) =. It is clear that _p (t) p(t) + f ( + ) = p(t) + + for all t 2 R. In consequence, Theorem 6.1 of Chapter I.6 in [5] implies that for t 0, p (t) y (t). Let t > 0 be minimal with p (t ) = +. Necessarily t 1, otherwise p 1 ^ + and thus p (t) < + for all t 1 by Proposition 3.1. On the other hand, the inequality p (t) y (t) for t 0 yields that t T. Summing up, T t 1, i.e., the rst estimate in (5.2) is true. The second estimate can be veried in an analogous manner. The stability of the equilibria given by the xed points < < + < + is irrelevant in the above proposition. However, in accordance with our previously introduced conventions in notation, and + will always denote stable xed points of f in the forthcoming applications, while and + will always denote unstable xed points. We will also need the next technical condition for continuously dierentiable functions dened on R or on a subinterval of R. Let f 0 and f 0 + denote the left hand and right hand derivatives of f, respectively. (C) If and + are the smallest and largest xed points of f, respectively, then f 0 + ( ) = f 0 ( + ) = 0. In addition, f has at least one unstable xed point in both intervals ; and ; + : Nonlinearities generating the simplest congurations of large-amplitude periodic orbits. 620 Let : [0; 2]! R be a nondecreasing C 1 -function with xed points 0; 1; 2 such that (u) < u for u 2 (0; 1), (u) > u for u 2 (1; 2), 0 (0) = 0 (2) = 0 and 0 (1) > 1. The function (u) = sin 2 (u 1)

26 625 is a suitable choice. For all M 1, dene 8 >< >: 0 if u < 0; f M : R 3 u 7! (u 2k) + 2k if u 2 [2k; 2k + 2) and k 2 f0; 1; : : : ; M 1g ; 2M if u 2M: (5.3) Then f M satises (H1){(H2). It has exactly M unstable xed points k = 2k 1; k 2 f1; 2; : : : ; Mg ; and it has M +1 stable xed points k = 2k, k 2 f0; 1; : : : ; Mg, with f 0 M ( k) = 0 for all k 2 f0; 1; : : : ; Mg. See Figure 5.1 for the plot of f v v=u v=f3(u) Figure 5.1: The plot of f 3. u Let M 1. Equation (3.1) with nonlinearity f = f M admits no largeamplitude periodic solutions. Proof. It is clear that if f = f 1, then equation (3.1) cannot have large-amplitude periodic solutions. Suppose for contradiction that M > 1, 1 i < j M, and equation (3.1) with nonlinearity f = f M has an [i; j] periodic solution p. Then it follows from Proposition 5.1 and the location of the equilibria that p (t) 2 (2i 2; 2j) for all real t. We can apply Proposition 5.2 with = i = 2i 1, + = j = 2j 1, = 2i 2 and + = 2j. The rst inequality in (5.2) already gives that 1 log + = log (2 (j i) + 1) log 3; + + j i1 which is impossible as log 3 > 1. Observe that if 2 (0; 1), then f M (u) < u for all u 2 (0; ) and f M (u) > u for all u 2 (2M ; 2M). Let us introduce a new nonlinearity f M for all M 2 by modifying f M on ( 1; ) [ (2M ; 1). So let 2 (0; 1), and choose K 1 ; K 2 2 R with K 1 < M (1 K 0 ) < 0 and K 2 = 2M K 1 > 2M; (5.4) 26

27 where K 0 > 1 is the threshold number from Proposition 4.1. Set f M : R 3 u 7! 8 >< >: K 1 if u 0; 1 (f M (u)) if u 2 (0; ) ; f M (u) if u 2 [; 2M ] ; 2 (f M (u)) if u 2 (2M ; 2M) ; K 2 if u 2M; (5.5) where 1 and 2 are dened so that f M : R! R fullls (H1), furthermore 1 (f M (u)) < f M (u) < u for all u 2 (0; ) (5.6) 645 and 2 (f M (u)) > f M (u) > u for all u 2 (2M ; 2M) : (5.7) See Figure 5.2 for the plot of f 3. One can easily check that the second-order polynomials 1 : (0; f M ()) 3 v 7! K 1 2K v + K fm R ()v2 f M () and K 1 2 : (f M (2M ) ; 2M) 3 v 7! (2M f M (2M )) 2 v2 are suitable choices.! K 1f M (2M ) v (2M f M (2M )) 2 K 1 fm 2 (2M ) (2M f M (2M )) 2 2 R 650 Two remarks regarding the above denition: We need condition (5.4) because we intend to apply Corollary Conditions (5.6) and (5.7) will be used to guarantee that f M has no xed points in (0; ) [ (2M ; 2M) For all M 2, f M satises (H1) ; (H2) and (C). The unstable xed points of f M are k = 2k 1; k 2 f1; 2; : : : ; Mg : 655 In addition, f M has M + 1 stable xed points. The smallest stable xed point of f M is K 1 < 0, the largest one is K 2 > 2M, and the others are k = 2k; k 2 f1; 2; : : : ; M 1g : The derivative of f M vanishes at its stable xed points. 27

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