On the stability of the fixed point property inl p spaces

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1 Revista Colombiana de Matematicas Volumen 28 (1994), piigs. 1-6 On the stability of the fixed point property inl p spaces MOHAMED A. KHAMSI The University of Texas at El Paso ABSTRACT. We prove in this work that for any p > 1 there exists an improved constant Cp, such that if d( X, I p ) < c p then X has the fixed point property, Key words and phrases. Fixed point property, nonexpansive mapping, normal structure 1991 Mathematics Subject Classification. Primary 47HIO, Secondary 46B Introduction Let X be a Banach space and let K be a nonempty weakly compact convex subset of X. We will say that K has the fixed point property (J.p.p. for short) if every nonexpansive T : K -t K (i.e., IITx-Tyll ~ Ilx-yli for every x, y E K) has a fixed point, i.e., there exists x E K such that T(x) = x. We will say that X has the j.p.p. if every weakly compact convex subset of X has the j.p.p. The fixed point property, as stated above, originated in four papers which appeared in They mainly assert that the presence of a geometric property, called "normal structure", implies the f.p.p., [10]. A number of abstract results were brought to light, along with important discoveries related both to the structure of the fixed point sets and to techniques for approximating fixed points. The first negative result to the existence part of the theory goes to ALSPACH, d. [2], who gave an example of a weakly compact convex subset K of L1 and an isometry T : K -t K which fails to have a fixed point. This example 1

2 2 MOHAMED A. KHAMSI showed that further assumptions are needed in addition to weak compactness, and at the same time it set the stage for MAUREY'S surprising discovery, [12] (see also [8], [11]). For more on j.p.p., one can consult [1], [5]. 2. Notations, definitions and basic facts. Let K be a nonempty weakly compact convex subset of a Banach space X. Suppose that T : K -+ K is nonexpansive. By Zorn's lemma, K contains a closed nonempty convex subset Ko which is minimal for T. This means that T(Ko) C Ko and that no strictly smaller closed nonempty convex subset of Ko is invariant under T. A classical argument shows that any closed nonempty convex subset of K which is invariant under T contains an approximate fixed point sequence (a.f.p.s.) (xn), i.e., lim Ilxn - T(xn)llx = O. n-oo The following lemma, d. [4], [7], proved to be fundamental for the study of the j.p.p. Lemma 1. Suppose Ko is a minimal weakly compact convex set for T and (x n ) is an a.f.p.s. for T. Then for all x E Ko we have lim n-+noo Ilxn - xii = diam(ko). Since we will be using MAUREY'S technique in proving our main result, we recall some basic definitions and facts. Definition 1. Letj( be a Banach space and let U be a free ultrafilter over N. The ultraproduct X of X is the quotient space of by loo(x) = {(xn) I Xn E X for all n E Nand II(xn)lloo = sup Ilxnll < oo}, n For (xn) E loo(x), we will denote (xn) + N by (xn)u E X. Clearly we have It is also clear that X is isometric to a subspace of X by the mapping x -+ (x, x,... ). Hence, we will se.: X as a subspace of X and we will write X, fj, Z for the general elements of X and x, y, z for those of X. - - Let K and T be as described above. Define K and T by K = {x E X I there exists a representative (x n ) of X with Xn E K for n ~ 1}

3 ON THE STABILITYOF THE FIXED POINT PROPERTYIN lp SPACES 3 and T(x) = (T(xn))u for any x E K. Then K is a bounded closed convex subset of X and T(K) c K. We remark that K is not minimal for T. Indeed, if (xn) C K is an a.f.p.s. then T(x) = ii ; where x = (xn)u. On the other hand, if x = (xn)u with Xn E K and T(x) = x, then there exists a subsequence (x~) of (x n ) which is an a.f.p.s. for T. Finally we recall that if Ko is a minimal set for T and x is a fixed point for T in Ko, then for any x E Ko we have, from Lemma 1, that = diam(ko). IIx - xlix The next lemma was proved by MAUREYin [12]. Lemma 2. Suppose x and fj are two fixed points oft in K. Then for every r E (0,1), there exists a fixed point i oft so that Ilx - ill = rllx - fjll and IIfj - ill = (1 - r)llx - fjll Let p E (1,00) 3. Main result and consider the function defined on [0,1] by l+(l-x)p i.pp(x) = xp + (1 - x)p Then sup i.pp(x) = i.pp(xp) where xp is the only root of xe[o,l] in [0,1]. It can be easily proved that Also one can check that x p < P-I Now recall that the Banach-Mazur distance between two isomorphic Banach spaces X and Y, denoted d(x, Y), is infimum of IIUIIIIU taken over all bicontinuous linear operators U from X onto Y. We now state and prove the main result of this work. Main Theorem. Let X be a Banach space such that

4 4 MOHAMED A. KHAMSI for some p > 1. Then X has the j.p.p. Proof. It is enough to prove that X = (i p, I. I) has the j.p.p., where I. I is an equivalent norm to II. lip satisfying with d < cpo Assume on the contrary that X fails to have the j.p.p. Then there exist K, a nonempty weakly compact convex subset, and T : K ---.K, a nonexpansive map, with no fixed point. Without any loss of generality, we can assume that K is minimal for T and that diam(k) = 1. Classical arguments imply that K contains an a.f.p.s. (x n ) which can be assumed to be weakly convergent to O. Passing to subsequences, if needed, we may suppose that there exist coordinate projections FF n on X (with respect to the canonical Schauder basis of Ip) such that (1) (2) (3) F n n F m = 0 for n::f. m, lim IXn - FF n (xn)1 = 0, n-oo The subsets (F n ) can be chosen to be successive intervals, and (3) follows from Lemma 1. Put Un = FF N (x n ) for all n E N. Then for z E ip we have Let X be an ultraproduct of X and K, Set x = (xn)u and :ii = (xn+du. Then f be as defined in the previous section. Relation (*) translates into X as liill~+ Iii - x - yll~ = Iii- xll~ + IIi- yll~ (**) for every i E X. Let T E (0,1), and let i be a fixed point of i: as given by Lemma 2, such that Then, Ii - xl = T and Ii - :iii= 1 - T.

5 Hence, ON THE STABILITY OF THE FIAED POINT PROPERTY IN lp SPACES 5 Then and since Iii = 1, we get 'Pp(r) ~ d P Since r was arbitrary in (0,1), we deduce that sup 'Pp(r) = 'Pp(xp) ~ d P, re(o,i) which contradicts our assumption on d. The proof of the main theorem is therefore complete. 0 Remarks (1) It is known [3], that if d(x, lp) < 2~ then X has the normal structure property and therefore, via a Theorem of KIRK [10], has the f.p.p. If 1 d(x, lp) = 2;; then BYNUM [3]has proved that X has the f.p.p. He also gave an example of a situation where X fails to have normal structure. For In(2) p > In ( 01-3 )' one has to use the result of LIN [11], to prove that if d(x, lp) < 01-3 then X has the I.v». It is worth mentioning that _ 1 _(3+v'5\2 cp ~ C2 - fi ) I for every p > 1 and C2 > Therefore we get through the main theorem an improvement to all the well known results. (2) It is a surprising fact that the constants (cp) do no decrease as p goes to 00. On the contrary, for p ~ 2 the constants c p increase to 2, which by itself project new light on the stability of the fixed point property (for the lp spaces). (3) For p = 2 the main theorem reduces to the main result of [6].

6 6 MOHAMED A. KHAMSI References 1. A.G. AKSOY AND M.A. KHAMSI, Nonstandard Methods in Fixed point theory, Springer- Verlag, Heidelberg, New York, D.E. ALSPACH, A fixed point free nonexpansive map, Proc. Amer. Math. Soc. 82 (1981), W. L. BYNUM, Normal structure coefficients for Banach spaces, Pac. J. Math. 86 (1980), K. GOEBEL, On the structure of minimal invariant sets for nonexpansive mappings, Ann. Univ. Mariae Curie-Sklodowska 29 (1975), K. GOEBEL AND W.A. KIRK, Topics in Metric Fixed Point Theory, to appear in Cambidge University Press. 6. A.M. JIMENEZ AND E.F. LLORENS, On the stability of the fixed point property for nonexpansive mappings, preprint. 7. L.A. KARLOVITZ, Existence of a fixed point for a nonexpansive map in a space without normal structure, Pac. J. Math. 66 (1966), M.A. KHAMSI, La prop rite du point fixe dans les espaces de Banach avec base inconditionelle, Math. Annalen 211 (1987), M.A. KHAMSI, Normal structure for Banach spaces with Schauder decomposition, Canad. Math. Bull. 32(3) (1989), W.A. KIRK, A fixed point theorem for mappings which do not increase distance, Amer. Math. Monthly 12 (1965), P.K. LIN, Unconditional bases and fixed points of nonexpansive mapping, Pac. J. Math. 116 (1985), B. MAUREY, in Seminaire d' Analyse Fonctionnelle (1980), Point fixes des contractions sur un convex fermi de L1, Ecole Poly technique. (Recibido en julio de 1993) of MOHAMED A. KHAMSI DEPARTMENT OF MATHEMATICAL SCIENCES THE UNIVERSITY OF TEXAS AT EL PASO EL PASO, TEXAS , USA

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