On the Weak Maximizing Properties

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On the Weak Maximizing Properties

Luis García-Lirola, Colin Petitjean

To cite this version:

Luis García-Lirola, Colin Petitjean. On the Weak Maximizing Properties. 2020. �hal-02974714�

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LUIS C. GARC´IA-LIROLA AND COLIN PETITJEAN

Abstract. Quite recently, a new property related to norm-attaining operators has been introduced: the weak maximizing property (WMP). In this note, we define a generalised version of it considering other topologies than the weak one (mainly the weak^∗topology). We provide new sufficient conditions, based on the moduli of asymptotic uniform smoothness and convexity, which imply that a pair (X, Y) enjoys a certain maximizing property. This approach not only allows us to (re)obtain as a direct consequence that the pair (`p, `q) has the WMP, but also provides many more natural examples of pairs having a given maximizing property.

1. Introduction

Let X, Y be two real Banach spaces and let T: X → Y be a bounded linear operator (we will writeT ∈ L(X, Y)). A maximizing sequence forT is a sequence (xn)n⊂X withkxnk= 1 for everyn∈Nand such that lim

n→∞kT xnkY =kTk. Next, we say as usual thatT:X →Y attains its norm whenever there exists a vector x∈X of norm 1 such thatkT xk_Y =kTk. Now following [5], a pair of Banach spaces (X, Y) is said to have the weak maximizing property (WMP) if for any bounded linear operator T:X →Y, the existence of a non-weakly null maximizing sequence forT implies thatT attains its norm. For instance, it is proved in [28, Theorem 1]

that the pair (`p, `q) has the WMP whenever 1< p <∞and 1≤q <∞. That result was extended in [5, Proposition 2.2] to the pair (`p(Γ1), `q(Γ2)) where Γ1,Γ2

are arbitrary indices.

Needless to say that the theory of norm attaining operators finds many applications in both pure and applied mathematics. A motivation for the study of the WMP is the following application which was noted in [5] (and extends a former result due to J. Kover in the Hilbert case [22]): if (X, Y) has the weak maximizing property, T ∈ L(X, Y) and K:X →Y is a compact operator such that kTk <kT+Kk, then T+Kis norm attaining. As a consequence, the authors could deduce that a pair (X, Y) has the weak maximizing property for someY 6={0} if and only ifX is reflexive.

In this note, we provide a new (possibly simpler) approach based on a comparison between the modulus of asymptotic uniform convexity of X and the modulus of asymptotic uniform smoothness of Y, and conversely (see Section2for precise definitions). Moreover, we consider the following related and quite natural properties.

Definition 1.1. LetX and Y be Banach spaces. Letτ_X and τ_Y be any topology onX andY respectively. We say that the pair:

• (X^∗, Y) has the weak^∗ maximizing property (W^∗MP) if for any bounded linear operator T:X^∗→Y, the existence of a non-weak^∗ null maximizing sequence forT implies thatT attains its norm.

Date: July, 2020.

2010Mathematics Subject Classification. Primary 46B20; Secondary 46B04, 54E50.

Key words and phrases. Norm attainment, Banach Space, Weak Maximizing Property, Asymp- totic Uniform Smoothness, Asymptotic Uniform Convexity.

1

arXiv:2007.01939v1 [math.FA] 3 Jul 2020

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• (X, Y) has the τ_X-to-τ_Y maximizing property (τ_X-to-τ_YMP) if for any bounded linear operator T: X → Y which is τX-to-τY continuous, the existence of a nonτX-null maximizing sequence forT implies thatT attains its norm.

In this note, τ_X andτ_Y will mainly be the usual weak or weak^∗ topologies. For instance, the WMP corresponds to the weak-to-weakMP. Notice that if the pair (X^∗, Y) has the WMP then it also has the W^∗MP, and consequently the weak^∗- to-weakMP. Also, if X is reflexive, then a pair (X, Y) has the WMP if and only if (X, Y) has the W^∗MP if and only if (X, Y) has the weak^∗-to-weakMP. However, we will prove that these three properties do not coincide in general.

Remark 1.2. The definition of the above properties can be stated in terms of nets instead of sequences. Indeed, given an operatorT:X →Y, the following properties are equivalent:

i) There exists a nonτX-null maximizing sequence for T. ii) There exists a nonτX-null maximizing net forT.

iii) There exists a net which is maximizing forT and does not admit 0 as a τX-cluster point.

Clearly, i)⇒ii). To see that iii)⇒i), let (xα)α⊂SX be a net such that 0 is not aτX- cluster point and limαkT xαk=kTk. Pick inductivelyαnsuch thatkT xα_nk ≥1−_n¹ and αn ≥αm ifn ≥m. Then the sequence (xα_n)n is a subnet of (xα)α, so it is not τX-null, and it is maximizing forT. Finally, assume that ii) holds and let’s prove iii). Let (xα)αbe a nonτX-null net maximizing forT. Then 0 is not the only τX-cluster point of (xα)α, otherwise, all the subnets of (xα)α would converge to 0, a contradiction (see e.g. [4]). Thus, there is a subnet (yβ)β of (xα)αconvergent to y6= 0, and (yβ)β is also maximizing forT.

Remark 1.3. Replacing the WMP by the weak^∗-to-weak^∗MP, one can follow the lines of [5, Proposition 2.4] to obtain the following result:

Suppose that (X^∗, Y^∗) has the weak^∗-to-weak^∗MP. LetT, K:X^∗→Y^∗ be weak^∗- to-weak^∗ continuous linear operators such thatK is compact. IfkTk<kT+Kk then T+Kis norm attaining.

We now describe the main findings of this paper. Throughout the paper,X and Y will denote real Banach spaces whileX^∗denotes as usual the topological dual of X. After recalling the definition of the modulusδ^∗_X∗(t) of weak^∗ asymptotic uniform convexity and of the modulusρ_Y(t) of asymptotic uniform smoothness in Section2, we prove our first theorem in Section3.

Theorem 3.1. LetX, Y be Banach spaces. Assume that for everyt >0,δ^∗_X∗(t)≥ ρ_Y(t)and, for allt≥1, eitherδ^∗_X∗(t)> ρ_Y(t)or ρ_Y(t)> t−1. Then,

(1) the pair(X^∗, Y)has the weak^∗-to-weakMP.

(2) If moreoverY ≡Z^∗ is a dual space, then the pair (X^∗, Z^∗)has the weak^∗- to-weak^∗MP.

(3) Ifδ^∗_X∗(t) =t, then(X^∗, Y)has the W^∗MP for any Banach spaceY. In particular, ifX is reflexive then the pair(X^∗, Y)has the WMP.

Since for any infinite sets Γ1 and Γ2and for 1< p < q <∞, it is well known that δ^∗_`_p_(Γ₁₎(t) = (1 +t^p)^1/p−1>(1 +t^q)^1/q−1 =ρ_`_q_(Γ₂₎, our last theorem provides a new proof of the fact that the pair (`p(Γ1), `q(Γ2)) has the WMP. Note that the casep=qalso follows from the last theorem. In both papers [5, 28], the proof for the case 1≤q < p <∞follows from Pitt’s theorem asserting that any bounded operatorT:`p→`q is compact and thus attains its norm. In fact, Pitt’s theorem can also be generalised using the modulusρ_X(t) of asymptotic uniform smoothness

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ofX and the modulusδ_Y(t) of asymptotic uniform convexity ofY. Namely, if there existst >0 such thatρ_X(t)< δY(t), then every bounded linear operator fromX to Y is compact (see [17, Proposition 2.3], where this is stated only for 0< t <1, but the proof works for anyt >0). We take advantage of this fact to deduce the next result.

Theorem 3.8. LetX, Y be Banach spaces and| · | be an equivalent norm on X^∗. If there exists t >0 such thatρ_|·|(t)< δY(t), then

(2) If moreover Y ≡ Z^∗ is a dual spaces, then (X^∗, Z^∗) has the weak^∗-to- weak^∗MP.

In particular, ifX is reflexive then(X^∗, Y)has the WMP.

In Section4, we discuss the case of some classical Banach spaces and the connec- tions between several weak maximizing properties. Notably, we show in Proposi- tion 4.2that ifX is reflexive,Y has the Dunford–Pettis property and (X, Y) has the WMP then (Y^∗, X^∗) has the weak^∗-to-weak^∗MP. Next, we prove that Schur spaces such as `1 are the best range spaces for the WMP: the pair (X, Y) has the WMP for any reflexive space X and any Schur spaceY (Proposition4.7). It follows from Theorem 3.1 (3) (andδ^∗_`

1(t) =t) that`₁ is also a very good domain for the W^∗MP (see Corollary4.8) while this is not the case of every Schur space.

Indeed, in Example4.9(Example4.10respectively) we provide a dual Banach space Z =X^∗ with the Schur property and such that the pair (X^∗,R) fails the W^∗MP (the pair (X^∗, `1) fails the weak^∗-to-weak^∗MP respectively). We end the paper by dealing with the classical James sequence spaces, Orlicz spaces and we also add some comments about the pair (Lp([0,1]), Lq([0,1])).

2. Preliminaries: Asymptotic uniform smoothness and convexity Consider a real Banach space (X,k·k) and letS_X be its unit sphere. Fort >0, x∈S_X we consider

δX(t, x) = sup

dim(X/Y)<∞

y∈SinfY

kx+tyk −1;

ρ_X(t, x) = inf

dim(X/Y)<∞ sup

y∈SY

kx+tyk −1. Themodulus of asymptotic uniform convexity ofX is given by

δ_X(t) = inf

x∈SX

δ_X(t, x),

and modulus of asymptotic uniform smoothness ofX is given by ρ_X(t) = sup

x∈SX

ρ_X(t, x).

The space (X,k·k) is said to beasymptotically uniformly convex (AUC for short) if δX(t)>0 for eacht >0 and it is said to beasymptotically uniformly smooth (AUS for short) if limt→0t⁻¹ρ_X(t) = 0. IfX is a dual space and we considered only weak^∗ closed subspaces ofX then the corresponding modulus is denoted byδ^∗_X(t). The space X is said to beweak^∗ asymptotically uniformly convex (AUC*) ifδ^∗_X(t)>0 for each t >0. We refer the reader to [17] and the references therein for a detailed study of these properties.

Let p, q ∈ [1,∞). We say that X is weak^∗ p-asymptotic uniformly convex (abbreviated byp-AUC^∗) if there exists C >0 so thatδ^∗_X(t)≥Ct^pfor all t∈[0,1].

Similarly, we say thatX isq-asymptotic uniformly smooth (abbreviated byq-AUS) if there exists C >0 so thatρ_X(t)≤Ct^q for all t∈[0,1]. Let us highlight that the following is proved in [12, Corollary 2.4].

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Proposition 2.1. LetX be a Banach space.

(i) Thenk kX is AUS if and and only if k kX^∗ is AUC^∗.

(ii) Ifp∈(1,∞]and q∈[1,∞)are conjugate exponents, thenk kX is p-AUS if and and only ifk kX^∗ isq-AUC^∗.

It is worth mentioning that ifX ≡Z^∗and the dual norm k · kZ^∗is AUS, thenX must be reflexive (see for instance Proposition 2.6 in [9]). The following proposition is elementary.

Proposition 2.2. LetX be a Banach space. For anyt >0, any weakly null net (x_α)_α in X and any x∈X\ {0}we have:

lim sup

α

kx+xαk ≤ kxk

1 +ρ_X

lim sup_αkxαk kxk

.

For any weak^∗-null net(x^∗_α)α⊂X^∗ and for any x^∗∈X^∗\ {0} we have lim inf

α kx^∗+x^∗_αk ≥ kx^∗k

1 +δ^∗_X

lim inf_αkx^∗_αk kx^∗k

.

Assume now thatϕ: [0,∞)→[0,∞) is an Orlicz function which is 1-Lipschitz and such that lim_t→∞ϕ(t)/t= 1. Consider for (s, t)∈R²,

N₂^ϕ(s, t) =

(|s|+|s|ϕ(|t|/|s|) ifs6= 0,

|t| ifs= 0.

The following is proved in [18] (see Lemma 4.3 and its preparation).

Lemma 2.3. The function N₂^ϕ is an absolute (or lattice) norm on R², meaning that N₂^ϕ(s1, s2)≤N₂^ϕ(t1, t2), whenever|si| ≤ |ti| for alli≤2.

When X is a Banach space, it is easy to see that ρ_X is a 1-Lipschitz Orlicz function such that lim_t→∞ρ_X(t)/t= 1. Thus, one can consider the normN₂^ρ^X. We will use in the sequel the following reformulation of Proposition2.2in terms of the norm N₂^ρ^X.

Lemma 2.4. Let X be a Banach space. If(x_α)_α⊂X is weakly-null and x∈X, then

lim sup

α

kx+xαk ≤N₂^ρ^X(kxk,lim sup

α

kxαk).

Proof. Ifx= 0 there is nothing to do, so we may assume thatx6= 0. By application of Proposition 2.2we see that

lim sup

α

kx+x_αk ≤ kxk

1 +ρ_X

lim sup_αkx_αk kxk

=N₂^ρ^X(kxk,lim sup

α

kx_αk)

Example 2.5. Let us recall the asymptotic moduli for some classical spaces, mostly taken from [26].

• Let 1 ≤p <∞. IfX = (P∞

n=1E_n)_p, where dim(E_n)<∞, then δ_X(t) = ρ_X(t) = (1 +t^p)^1/p−1 for allt >0.

• For any infinite set Γ and 1≤p <∞,δ_`_p_(Γ)(t) =ρ_`

p(Γ)(t) = (1 +t^p)^1/p−1 for allt >0.

• For any infinite set Γ,δc₀(Γ)(t) =ρ_c₀_(Γ)(t) = max{0, t−1}for allt >0.

• LetJ be the James space. ThenδJ(t) =ρ_J(t) = (1 +t²)^1/2−1 for allt >0.

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• If X has a Schauder basis (e_n)^∞_n=1 which satisfies a lower (resp. upper)

`p-estimate with constant one, thenδX(t)≥(1 +t^p)^1/p−1 (resp. ρ_X(t)≤ (1 +t^p)^1/p−1). For instance, thep-convexified Tsirelson spaceT^p (see [8]) and the Lorentz sequence space d(w, p) satisfy an upper`p-estimate with constant 1 [24, p. 177]. Moreover,d(w, p) contains almost isometric copies of`_p soρ_d(w,p)(t) = (1 +t^p)^1/p−1 for all t >0.

3. Proof of the main results Let us begin with the proof of our main theorem.

Theorem 3.1. LetX, Y be Banach spaces. Assume that for everyt >0,δ^∗_X∗(t)≥ ρ_Y(t)and, for allt≥1, eitherδ^∗_X∗(t)> ρ_Y(t)or ρ_Y(t)> t−1. Then,

(2) If moreoverY ≡Z^∗ is a dual space, then the pair (X^∗, Z^∗)has the weak^∗- to-weak^∗MP.

(3) Ifδ^∗_X∗(t) =t then(X^∗, Y)has the W^∗MP for any Banach space Y. In particular, ifX is reflexive then the pair(X^∗, Y)has the WMP.

Proof. We will only prove the first assertion since the proof of the second and third one are very similar (replacing the weak topology inY by the weak^∗ topology inY).

Let T:X^∗ → Y be a bounded operator which is weak^∗-to-weak continuous.

Without loss of generality, we may assume that T has norm 1. Let (xn) be a normalized maximizing sequence in X^∗ which is not weak^∗-null. Let (xα) be a subnet which is weak^∗ convergent tox6= 0. By extracting a subnet again, we may assume that limαkxα−xkand limαkT x−T xαkexists. SinceT is weak^∗-to-weak continuous, we have thatT x_α−→^w T x. Using Lemma2.4, we thus have the following estimates:

1 = kTk= lim

α kT xαk= lim

α kT x+T xα−T xk

≤ N₂^ρ^Y(kT xk,lim

α kT x_α−T xk).

Now notice that (we use Lemma 2.3) N₂^ρ^Y(kT xk,lim

α kT xα−T xk)≤N₂^ρ^Y(kxk,lim

α kT xα−T xk)

≤N₂^ρ^Y(kxk,lim

α kxα−xk), but also

N₂^ρ^Y(kT xk,lim

α kT x_α−T xk)≤N₂^ρ^Y(kT xk,lim

α kx_α−xk)

≤N₂^ρ^Y(kxk,lim

α kxα−xk).

Sincex6= 0, we deduce from the definition ofN₂^ρ^Y the following estimates:

N₂^ρ^Y(kxk,lim

α kxα−xk) ≤ kxk+kxkρ_Y

limαkxα−xk kxk

≤ kxk+kxkδ^∗_X∗

lim_αkxα−xk kxk

≤ lim

α kx+xα−xk

= 1.

This implies that all the previous inequalities are in fact equalities, in particular N₂^ρ^Y(kT xk,lim

α kx_α−xk) =N₂^ρ^Y(kxk,lim

α kx_α−xk) = 1 =N₂^ρ^Y(0,1).

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This means that the points (kT xk,lim_αkx_α−xk), (kxk,lim_αkx_α−xk) and (0,1) are aligned inR². IfkT xk=kxkor limαkx−xαk= 0, thenT attains its norm at xand we are done. Otherwise, it follows that limαkxα−xk= 1 and

1 =N₂^ρ^Y(kxk,1) =kxk+kxkρ_Y(1/kxk) =kxk+kxkδ^∗_X∗(1/kxk).

That is,δX^∗(_kxk¹ ) =ρ_Y(_kxk¹ ) =_kxk¹ −1, which contradicts our assumptions.

Remark 3.2. TakeX =R⊕_∞`2andY =c0. ThenδX(t) = max{0, t−1}=ρ_Y(t) for allt >0. However, the operator T:X →Y given by T((0, en)) = _n+1ⁿ en and T((1,0)) = 0 does not attain the norm and admits the non-weakly null maximizing sequence (xn)^∞_n=1 given byxn = (1, en). Thus, the pair (X, Y) fails the WMP.

Remark 3.3. Since ρ_Y is 1-Lipschitz and ρ_Y(t) ≤ max{0, t−1}, the condition ρ_Y(1) = 0 is equivalent toρ_Y(t) = max{0, t−1} for all t >0. These spaces are called metric weak^∗ Kadec-Klee spaces. In the separable case, they are precisely those spaces which are (1 +ε)-isomorphic to a subspace ofc0, for everyε >0 [15].

As a direct consequence we obtain that for a special class of Banach spaces, namely the dual ones X^∗ with Sz(X) =ω(whereω stands for the first countable ordinal), one can find an equivalent norm | · |on X^∗ and a Banach spaceY such that the pair (X^∗,| · |), Y

has the weak^∗-to-weak^∗MP. By Sz(X) we mean the Szlenk index ofX; we refer to [23] for its definition, basic properties as well as its main applications to the geometry of Banach spaces.

Corollary 3.4. If X is separable withSz(X) =ω, then there exists an equivalent norm| · |_X^∗ onX^∗ and q≥1such that the pair (X^∗,| · |_X^∗), `_q

has the weak^∗-to- weak^∗MP.

In particular if X ≡Z^∗ is a separable reflexive space with Sz(Z)≤ω, then there exists an equivalent norm onX and there existsq≥1such that the pair (X, `q)has the WMP.

Proof. Let us start by recalling the fundamental renorming result for spaces with Szlenk index equal to ω. The result is due to H. Knaust, E. Odell and Th. Sch- lumprecht [21, Corollary 5.3]: ifX is a separable Banach space such thatSz(X) =ω, then there exists p∈(1,∞) such that X^∗ admits an equivalentp-AUC* norm| · |, moreover δ^∗_|·|(t)≥(1 +t^p)¹^p−1. Let us considerq > p. As we already mentioned in Example2.5,ρ_`_q(t) = (1+t^q)¹^q−1. Thus for everyt >0 we have thatδ^∗_|·|(t)> ρ_`_q(t).

The result now readily follows from Theorem3.1.

The next result follows the idea of [28, Theorem 2], where the same is obtained in the caseX^∗=`p,Y =`q,p6=q.

Corollary 3.5. Let X, Y be Banach spaces. Assume that X is separable and δ^∗_X∗(t) > ρ_Y(t) for all t > 0. Let T: X^∗ → Y be a weak^∗-to-weak continuous operator. Then any non-weak^∗ null maximizing sequence for T has a convergent subsequence.

Proof. Let (x_n)^∞_n=1 ⊂S_X^∗ be a non-weak^∗ null maximizing sequence for T. By extracting a subsequence, we may assume that (x_n) is weak^∗ convergent to x6= 0 and the limit lim_nkx_n−xk exists. The proof of Theorem3.1shows thatT attains its norm at x, and moreover

ρ_Y

limnkxn−xk kxk

=δ^∗_X∗

limnkxn−xk kxk

.

Thus limnkxn−xk= 0.

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We will now turn to the case when the modulus of asymptotic uniform smoothness of X is bounded above by the modulus of asymptotic uniform convexity ofY. To this aim, we will need the following two lemmata.

Lemma 3.6 (Proposition 2.3 in [17]). LetX, Y be Banach spaces. If there exists t >0 such thatρ_X(t)< δY(t)then every linear operator fromX toY is compact.

The next one is classical and easy to prove.

Lemma 3.7. LetX, Y be Banach spaces and letτ_Y be any Hausdorff topology on Y which is coarser than the norm topology. If T:X^∗→Y is a compact operator which is weak^∗-to-τ_Y continuous, then T attains its norm.

Proof. Let (x^∗_α) be a net inSX^∗ such that limαkT x^∗_αk=kTk, we may assume that (x^∗_α) is weak^∗-convergent tox^∗∈BX^∗. Then (T x^∗_α) isτY-convergent toT x^∗. Since the norm topology and τY agree on the norm-compact setT(BX^∗), we have that

kTk= limαkT x^∗_αk=kT x^∗k.

We are now ready to prove the desired theorem.

Theorem 3.8. LetX, Y be Banach spaces and| · | be an equivalent norm on X^∗. If there exists t >0 such thatρ_|·|(t)< δY(t), then

(2) If moreover Y ≡ Z^∗ is a dual space, then (X^∗, Z^∗) has the weak^∗-to- weak^∗MP.

In particular, ifX is reflexive then(X^∗, Y)has the WMP.

Proof. We only prove the first assertion, the proof of the second one is similar and the last statement is quite obvious. Thanks to Lemma 3.6, we know that every bounded linear operator from (X^∗,| · |) toY is compact. Since| · |is an equivalent norm onX^∗, this readily implies that every bounded linear operator fromX^∗toY is compact. According to Lemma3.7, we deduce that every weak^∗-to-weak continuous

operator fromX^∗to Y attains its norm.

We highlight some consequences of the previous theorems for particular spaces.

In all the cases the proof follows from Theorem3.1or Theorem3.8and Example2.5.

Corollary 3.9. Any of the following conditions ensues that the pair(X, Y)has the WMP:

a) X is a reflexive AUS space (e.g. X =`p,T^p, or d(w, p),1< p <∞), and Y =`1.

b) X is a reflexive AUC space (e.g. X=`p) and Y =c0.

c) X =`p, and either Y =`q,1< p, q <∞, orY =J, the James space.

d) X =`_p and either Y =T^q orY =d(w, q) where1< p≤q <∞.

4. Application to some classical Banach spaces

4.1. Dunford–Pettis spaces. We recall that a Banach space Y has the Dunford–

Pettis property if and only if for every sequence (yn)n inY converging weakly to 0 and every sequence (y^∗_n)n inY^∗ converging weakly to 0, the sequence of scalars (y^∗_n(yn))n converges to 0. In particular, Schur spaces have it. Classical spaces with the Dunford–Pettis property include as c0, C(K) spaces (where K is a compact Hausdorff space) and L1(µ) (whereµis aσ-finite measure), see e.g. [11]. However reflexive spaces never have it.

Lemma 4.1. Assume thatX is reflexive and thatY has the Dunford–Pettis property.

LetT:X →Y be a bounded operator. Then the following are equivalent:

(i) There exists a non-weakly null maximizing sequence forT.

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(ii) There exists a non-weakly null maximizing sequence forT^∗.

Proof. First of all, if (x_n)_n⊂X is a maximizing sequence forT, we define (y^∗_n)_n⊂ S_Y^∗ so thaty_n^∗(T x_n)>kT xnk − _n¹. It is easy to see that

kT^∗y^∗_nk ≥ |y^∗_n(T xn)|>kT xnk − 1 n, and taking the limit n → ∞ we obtain lim

n→∞kT^∗y^∗_nk ≥ kTk. Since kT^∗k = kTk we deduce that (y_n^∗)n is a maximizing sequence for T^∗. We use the exact same argument to build a maximizing sequence forT starting from one forT^∗.

Now letT:X→Y be an operator and let (xn)nbe a non-weakly null maximizing sequence for T. By weak-compactness, we may extract a subsequence (still denoted the same way) which weakly converges to some x. Now asT is weakly continuous, we have that (T x_n) weakly converges toT x. Let (y^∗_n)_n be the sequence constructed above. We claim that (y^∗_n)_n is not weakly null. Indeed, aiming for a contradiction, assume that (y_n^∗)_n is weakly null. AsY has the Dunford–Pettis property, we have that lim

n→∞y^∗_n(T xn) = lim

n→∞y_n^∗(T xn−T x) = 0, which contradicts our definition of (y^∗_n)_n.

The converse is proved in the same way.

Proposition 4.2. LetX be a reflexive space and letY be a Banach space with the Dunford–Pettis property. If(X, Y)has the WMP then any dual operatorT^∗:Y^∗ → X^∗ which admits a non-weakly null maximizing sequence attains its norm. In particular the pair(Y^∗, X^∗)has the weak^∗-to-weak^∗MP.

Proof. let T^∗:Y^∗ →X^∗ be the dual operator of a bounded operatorT:X →Y. Let (y^∗_n)nbe a non-weakly null maximizing sequence forT. According to Lemma4.1, T admits a non-weakly null maximizing sequence. Since (X, Y) has the WMP,T attains its norm at somex. Now using Hahn-Banach theorem, we may picky^∗∈Y^∗ such that y^∗(T x) =kT xk=kTk. It is readily seen that the operatorT^∗attains its

norm at y^∗.

Note that the hypothesis “X is reflexive” in the previous proposition is actually superfluous as the assumption “(X, Y) has the WMP” already implies thatX must be reflexive [5].

We do not know whether the hypothesis on the Dunford–Pettis property is actually needed in the previous result. More precisely:

Question 4.3. Assume that (X, Y) has the WMP. Does it follow that (Y^∗, X^∗) has the weak^∗-to-weak^∗MP?

On the other hand, there are spaces X, Y so that (Y^∗, X^∗) has the WMP but (X, Y) fails the WMP: just takeY =RandX any non-reflexive space. We do not know if it is possible to construct such an example in the reflexive case, that is:

Question 4.4. LetX, Y be reflexive spaces so that (Y^∗, X^∗) has the WMP. Does it follow that (X, Y) has the WMP?

Notice that, under the same conditions as in Proposition 4.2, (X, Y) having the WMP does not imply that (X^∗∗, Y^∗∗) has the weak^∗-to-weak^∗MP (while (Y^∗, X^∗) does so). Indeed, the next example shows that (`2, c0) has the WMP but (`2, `_∞) fails the weak^∗-to-weak^∗MP. It was also suggested to us by R. Aron to consider the following variation of the WMP:

Definition 4.5. We say that a pair (X, Y) has the bidual WMP if for every operator T:X →Y, the existence of a non weakly-null maximizing sequence forT implies that T^∗∗:X^∗∗→Y^∗∗ attains its norm.

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It is obvious from the definitions that if (X^∗∗, Y^∗∗) has the weak^∗-to-weak^∗MP then (X, Y) has the bidual WMP. However, the next example shows in particular that the converse does not hold in general.

Example 4.6. First of all, notice that (`₂, c₀) has the WMP thanks to Corol- lary3.9 b), thus it also has the bidual WMP. Moreover,`2 is reflexive andc0has the Dunford-Pettis property so that Proposition 4.2 implies that (`1, `2) has the weak^∗-to-weak^∗MP (even the W^∗MP, see Corollary4.8). Next, we will prove that (`2, `_∞) fails the weak^∗-to-weak^∗MP (and consequently also fails the WMP and the W^∗MP).

LetT :`2→`_∞defined for everyx= (xn)n∈`2by T x=

x₁, x₁+ 1−1

2

x₂, x₁+ 1−1

3

x₃, . . . , x₁+ 1−1

n

x_n , . . .

. In other words, T e₁=1(the constant equal to 1 sequence) andT e_n= (1−_n¹)e_n. Our claim follows from the items bellow:

• kTk=√

2. Indeed, kT xk_∞= sup

|x1+ (1−1/n)xn|:n≥1

≤sup

(|x1|²+|xn|²)^1/2(1 + (1−1/n)²)^1/2:n≥2

≤√ 2kxk2. Moreover,

T 1

√2(e₁+e_n) →√

2, so kTk = √

2 the sequence ^√¹

2(e1+en)

n is a normalized maximizing sequence which is not weakly^∗ null (it weakly^∗converges to ^√¹

2e1).

• T is weak*-to-weak*-continuous: one can easily chech thatT =S^∗ where S:`1→`2is given bySx= (P∞

n=1xn)e1+P∞

n=1(1−1/n)xnen.

• T does not attain its norm: if kT xk∞ = √

2 for some x with kxk2 = 1, then the above estimation implies that lim sup_n(|x1|²+|xn|²)^1/2= 1. Thus

|x1|= 1, which implies thatx=±e1, a contradiction.

4.2. The case of `1 and Schur spaces. Recall that a Banach spaceY is said to have the Schur property if every weakly convergent sequence is also norm-convergent.

First of all, let us point out that Schur spaces are the best range spaces for the WMP as this is shown by the next result.

Proposition 4.7. IfX is reflexive and Y is Schur, then (X, Y)has the WMP.

Proof. LetT:X →Y be an operator and let (xn)nbe a non-weakly null maximizing sequence for T. By weak-compactness, we may extract a subsequence (still denoted the same way) which weakly converges to somex∈BX. SinceT is weakly continuous, (T xn)n weakly converges toT x. SinceY has the Schur property, (T xn)n actually converges to T x in the norm topology. The fact that lim

n→∞kT xnk=kTk readily

implies that kT xk=kTk.

It is well known that `₁ ≡ c^∗₀ is weak^∗ asymptotically uniformly convex with modulus δ^∗_`₁(t) = t, and the same is true for Z^∗ instead of `1 whenever Z is a subspace ofc0. Consequently, the next result is a direct consequence of Theorem3.1.

Let us mention that if a dual Banach spaceX ≡Z^∗ has modulusδ^∗_X(t) =t, then Z is asymptotically uniformly flat (that is there existst0>0 such that for every

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t < t₀,ρ_Z(t) = 0). This is actually equivalent to the fact thatZ is isomorphic to a subspace of c0 (see [17, Theorem 2.9]).

Corollary 4.8. IfZ is a closed subspace ofc₀, then the pair(Z^∗, Y)has the W^∗MP for every Banach space Y.

In particular(`₁, Y)has the W^∗MP for every Banach space Y.

However, the previous result is not true anymore if one replaces`1 by any dual space with the Schur property as this is shown by the following example. It also shows that the W^∗MP is not an isomorphic property.

Example 4.9. There exists a dual spaceZ =X^∗with the Schur property such that the pair (X^∗,R) fails the W^∗MP. In factZ is going to be the Lipschitz-free space F(M) whereM ={0} ∪ {xn : n∈N} ⊂c0 is the pointed metric space given by x1= 2e1, and xn=e1+ (1 + ¹_n)en forn≥2, where (en) is the canonical basis ofc0. Then it is known [19] thatF(M) has the Schur property asM is uniformly discrete and bounded (it is actually isomorphic to`1) and it is also known [14, Example 5.6]

that F(M)≡X^∗ where

X ={f ∈Lip₀(M) : lim

n→∞f(x_n) = 1 2f(x₁)}.

It is readily seen that the sequence ((1 +_n¹)⁻¹δ(xn))nweak^∗converges to ¹₂δ(x1)6= 0.

Now letf:M →Rbe the Lipschitz map defined byf(0) =f(x1) = 0 andf(xn) = 1 for every n ≥ 2. It is obvious that kfkL = lim

n→∞

f(x_n)−f(0)

d(xn,0) = 1. Now using the linearization property of free spaces, we may consider the bounded operator f:F(M)→Rsuch thatf(δ(xn)) =f(xn). Observe that ((1 +_n¹)⁻¹δ(xn))n is a maximizing sequence which is not weak^∗-null. To finish, we just need to show that f does not attain its norm. Assume thatf(µ) = 1 forµ=P∞

n=1anδ(xn)∈S_F(M). First note that

1 =f(µ) =

∞

X

n=2

a_n.

Let A = {n ≥ 2 : an > 0} and consider the one-Lipschitz function g: M → R given by g(xn) = 1 if n∈ A andg(xn) = 0 otherwise. By evaluating the linear extension of g toF(M) at µwe get P

n∈Aan ≤1. It follows that an ≥0 for all n≥2. Similarly, by evaluating at the one-Lipschitz functionh: M →Rgiven by h(x1) = 2 andh(xn) = 1 we get that −1≤a1≤0. Now, we have that

µ=

∞

X

n=1

anδ(xn) =

∞

X

n=2

an|a1|(δ(xn)−δ(x1)) +

∞

X

n=1

(1− |a1|)anδ(xn).

Finally, consider the one-Lipschitz functionj:M →Rgiven byj(0) =j(x₁) = 0 and j(x_n) = 1 + 1/n. Then,

1 =kµk ≥ hj, µi=

∞

X

n=2

a_n|a₁|

1 + 1 n

+

∞

X

n=2

(1− |a₁|)a_n

1 + 1 n

=

∞

X

n=2

an

1 + 1

n

= 1 +

∞

X

n=2

an

n.

Thus,an= 0 for alln≥2. Thenf(µ) = 0, a contradiction.

Notice that the Banach space F(M) in the previous example is not weak^∗-AUC:

1

2δ(x1) is an extreme point of B_F(M₎[3] but not a preserved extreme point [2], in particular it does not belong to slices of small diameter.

The following example shows that the weak^∗-to-weak^∗MP is not stable by iso- morphisms.

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Example 4.10. There exists a spaceX isomorphic toc₀ such that (X^∗, `₁) fails the weak^∗-to-weak^∗MP. Indeed, let M be the metric space given byM =N∪ {0}

whered(n,0) =d(n,1) = 1 + 1/n andd(n, m) = 2 + 1/n+ 1/mfor alln, m≥2 . The same argument as [14, Example 5.6] gives that F(M)≡X^∗, where

X ={f ∈Lip₀(M) : lim

n→∞f(x_n) = 1 2f(x₁)}.

It is easy to check that the mapT:c0→X given byT x(1) =x1andT x(n) = ¹₂(x1+ xn) forn≥2 satisfies thatkTk ≤1. Moreover,T is an isomorphism with inverse R:X→c0 given byRf =f(1)e1+P

n≥2(2f(n)−f(1))en. Consider the weak^∗-to- weak^∗-continuous operatorT^∗:F(M)→`₁. First note thatT^∗(δ(n)) = ¹₂(e₁+e_n) for n≥2. Thus, kT^∗k = 1 and the sequence ((1 +_n¹)⁻¹δ(n))n is a maximizing sequence for T^∗which weak^∗-converges to ¹₂δ(1)6= 0. To finish, we need to check thatT^∗ does not attain its norm. Assume that there isµ=P∞

n=1anδ(xn)∈S_F(M) is so that kT^∗µk1 = 1. Consider the 1-Lipschitz function f:M → R given by f(0) =f(1) = 0 andf(n) = (1 +¹_n) sign(an) forn≥2. Then

1 =kµk ≥ hf , µi=X

n≥2

|a_n|(1 + 1 n).

Now, consider the 1-Lipschitz functiong: M →Rgiven byg(0) = 0,g(1) = 2 and g(n) = 1 forn≥2. Then

1 =kµk ≥ |hf , µi|=|2a1+X

n≥2

an|.

Note that

T^∗µ=



a₁+1 2

X

n≥2

a_n



e₁+1 2a_ne_n and so

1 =kT^∗µk1=|a1+1 2

X

n≥2

an|+1 2

X

n≥2

|an| ≤1.

It follows thatP

n≥2|an|= 1≥P

n≥2|an|(1 + _n¹), a contradiction.

4.3. The case of the James sequence spaces. Letp∈(1,∞). We now recall the definition and some basic properties of the James space Jp. We refer the reader to [1, Section 3.4] and references therein for more details on the classical casep= 2.

The James spaceJp is the real Banach space of all sequencesx= (x(n))_n∈_Nof real numbers with finite p-variation and satisfying lim_n→∞x(n) = 0. The spaceJp is endowed with the following norm

kxkJp= supn ^k−1X

i=1

|x(pi+1)−x(p_i)|^p^1/p

: 1≤p₁< p₂< . . . < p_ko . This is the historical example, constructed for p= 2 by R. C. James, of a quasi- reflexive Banach space which is isomorphic to its bidual. In factJ_p^∗∗ can be seen as the space of all sequences x= (x(n))_n∈N of real numbers with finitep-variation, which isJp⊕R1, where1denotes the constant sequence equal to 1.

The standard unit vector basis (en)^∞_n=1(en(i) = 1 ifi=nanden(i) = 0 otherwise) is a monotone shrinking basis for Jp. Hence, the sequence (e^∗_n)^∞_n=1 of the associated coordinate functionals is a basis of its dualJ_p^∗. Then the weak topologyσ(Jp,J_p^∗) is

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easy to describe: a sequence (x_n)^∞_n=1inJ_p converges to 0 in theσ(J_p,J_p^∗) topology if and only if it is bounded and limn→∞xn(i) = 0 for everyi∈N.

For x∈ Jp, we define suppx={i∈N : x(i)6= 0}. Forx, y∈ Jp, we denote:

x≺y whenever max suppx <min suppy.

The detailed proof of the following proposition can be found in [27, Corollary 2.4].

This a consequence of the following fact: there existsC≥1 such thatkPn

i=1x_ik^p_J_p≤ CPn

i=1kx_ik^p_J

p, for allx₁≺. . .≺x_n inJ_p.

Proposition 4.11. There exists an equivalent norm| · |on J_p such that it has the following property: for any x, y∈ J_p such thatx≺y, we have that

|x+y|^p≤ |x|^p+|y|^p.

In particular, the modulus of asymptotic uniform smoothness ofJfp := (Jp,| · |) is ρ

Jfp(t)≤(1 +t^p)¹^p−1 for allt≥0.

There is also a natural weak^∗topology onJp. Indeed, the summing basis (s_n)^∞_n=1 (s_n(i) = 1 if i≤nands_n(i) = 0 otherwise) is a monotone and boundedly complete basis for J_p. Thus,J_p is naturally isometric to a dual Banach space: J_p=X^∗ with X being the closed linear span of the biorthogonal functionals (e^∗_n−e^∗_n+1)^∞_n=1 in J_p^∗ associated with (sn)^∞_n=1. Note that X={x^∗∈ J_p^∗, P∞

n=1x^∗(n) = 0}. Thus, a sequence (xn)^∞_n=1 in Jp converges to 0 in theσ(Jp, X) topology if and only if it is bounded and limn→∞ xn(i)−xn(j)

= 0 for everyi6=j∈N. The next lemma is classical, we include its proof for completeness.

Lemma 4.12. Let(x_n)^∞_n=1 be a weak^∗-null sequence inJp. Then, for everyx∈ Jp, we have

lim sup

n→∞

kx+xnk^p≥ kxk^p+ lim sup

n→∞

kxnk^p.

Consequently, the modulus of weak^∗ asymptotic uniform convexity ofJp is given by δ^∗_J_p(t)≥(1 +t^p)¹^p−1.

Proof. Letx= (x(n))_n∈ Jp, (x_n)^∞_n=1 be a weak^∗-null sequence andε >0 be fixed.

Without loss of generality, we may and do assume thatkxk= 1. Let (pi)^N_i=1⊂Nbe an increasing family such that

1−ε≤

N−1

X

i=1

|x(p_i+1)−x(p_i)|^p^1/p .

Then there existsN₁> p_N such that for any increasing finite sequence (q_i)^k_i=1 with q₁≥N₁,

k−1

X

i=1

|x(qi+1)−x(qi)|^p^1/p

≤η,

where 0< η < εwill be precisely chosen at the end of the proof. Now, since (x_n)^∞_n=1 is a weak^∗-null sequence, it is bounded by some M >0 and there exists N₂ ∈N large enough so that for every n≥N₂ and for any increasing finite sequence (r_i)^l_i=1 withr_l< N₁:

l−1

X

i=1

|x_n(r_i+1)−x_n(r_i)|^p≤ η

3 and |x_n(r_l)−x_n(N₁)|^p≤η⁰,

whereη⁰ is chosen so that||x| − |y||^p≥ |x|^p−^η₃ for everyx∈[0, M] andy∈[0, η⁰] (the mapf(x, y) =|x−y|^p is Lipschitz on [0, M]×[0, M], soη⁰= min{_{3 Lip(f)}^η , M}

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does the work). Note also that in such a case one also has

|xn(r)−xn(N1)|^p≥ ||xn(r)−xn(rl)| − |xn(rl)−xn(N1)||^p

≥ |x_n(r)−x_n(r_l)|^p−η 3, for any r≥N1.

Now, letn≥N2. It follows from the above estimates that we may find a fixed increasing sequence (qi)^k_i=1 withq1≥N1 and

k−1

X

i=1

|xn(qi+1)−xn(qi)|^p≥ kxnk^p−η.

Note thatpN < N1≤q1 by construction so that (?) kx+xnk^p≥

N−1

X

i=1

|x(pi+1)−x(pi) +xn(pi+1)−xn(pi)|^p

+

k−1

X

i=1

|x(qi+1)−x(q_i) +x_n(q_i+1)−x_n(q_i)|^p. But

^NX⁻¹

i=1

|x(pi+1)−x(pi) +xn(pi+1)−xn(pi)|^p¹_p

≥^NX⁻¹

i=1

|x(pi+1)−x(pi)|)^p¹_p

−ε

≥1−2ε, which implies thatPN−1

i=1 |x(p_i+1)−x(p_i) +x_n(p_i+1)−x_n(p_i)|^p≥1−2pε.

Similarly, we obtain that

N−1

X

i=1

|x(qi+1)−x(q_i) +x_n(q_i+1)−x_n(q_i)|^p≥ kxnk^p−2pM η.

Finally, if we choseη <min(ε,_M^ε), we obtain from (?) the following last estimate kx+x_nk^p≥1 +kx_nk^p−4pε.

As an application of Theorem3.1and the two previous lemmata, we obtain the following corollary.

Corollary 4.13. If 1< p≤q <∞then(J_p,Jf_q) has the weak^∗-to-weak^∗MP.

We now turn to an application of Lemma3.6and Theorem3.8.

Corollary 4.14. If1< q < p <∞then every bounded operator from Jp to Jq is compact. In particular, the pair (Jp,Jq)has the weak^∗-to-weak^∗MP.

Proof. Since ρ

Jfp(t)≤(1 +t^p)^1/p−1,δ^∗_J

q(t)≥(1 +t^q)^1/q−1 andδ_J_q(t)≥δ^∗_J

q(t), we obtain from Lemma3.6that every bounded operator fromJfp toJq is compact.

SinceJfp andJp are isomorphic, this yields the same conclusion for every bounded operator from Jp toJq. To conclude, Theorem3.8 (2) provides the fact that the

pair (Jp,Jq) has the weak^∗-to-weak^∗MP.

Having in mind the previous corollaries, it is quite natural to wonder the following:

Question 4.15. Does the pair (Jp,Jq) has the weak^∗-to-weak^∗MP for 1< p ≤ q <∞?

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Remark 4.16. Quite surprisingly, the pair (J₂,R) fails the W^∗MP (but has the weak^∗-to-normMP thanks to Corollary4.13). Indeed, considerT:J2→Rbe the linear operator given by

∀x= x(j)∞

j=1∈ J2, T x:= −x(1)

2 +

∞

X

j=2

x(j) j² .

That is, T = (⁻¹₂ ,₂¹₂,₃¹₂, . . .) ∈ J₂^∗. Note that T is not weak^∗ continuous since T(sn) 6→0 while the summing basis (sn) is weak^∗-null. Since the standard unit vector basis (en)^∞_n=1 is monotone and shrinking, it follows that for everyx^∗∗∈ J₂^∗∗, kx^∗∗k_J^∗∗

2 = sup_nk(x^∗∗(e^∗₁), x^∗∗(e^∗₂), . . . , x^∗∗(e^∗_n),0, . . .)k_J₂ (see e.g. Proposition 4.14 in [13]. Thus, it not hard to prove that any norming functionalx^∗∗ forT is of the formx^∗∗= √ ¹

(t2−t1)²+t²₂(t1, t2, t2, . . .). Consequently, in order to maximizeT(x^∗∗) it suffices to maximize the function

g(t₁, t₂) =

−t1

2 +t₂(^π₆² −1) p(t2−t1)²+t²₂ =

−1 2

t1

t₂ + (^π₆²−1) q(1−^t_t¹

2)²+ (^t_t¹

2)² .

We sett= ^t_t¹

2 and we are now looking for the maximum of the following one variable map:

f(t) =

−1

2 t+ (^π₆² −1) p(1−t)²+t².

A basic study of the mapf shows that it attains its maximum attmax= _2π^π²2⁻⁹−15

(and f(tmax) ' 0.66). To conclude, fix t1, t2 ∈ R such that ^t_t¹

2 = tmax and p(t2−t1)²+t²₂ = 1. Now for everyn∈Nlet xn ∈ J2 be such that xn(1) =t1, xn(2) = . . . = xn(n) = t2 and xn(j) = 0 whenever j > n (that is, xn = (t1, t2, . . . , t2,0, . . .)). Then (xn)^∞_n=1 is a normalized maximizing sequence for T which is not weak^∗-null. However, it is easy to see thatT does not attains its norm (as every element inJ2 is a sequence that converges to 0).

4.4. Orlicz spaces. Given an Orlicz functionϕ: [0,+∞)→[0,+∞) (that is,ϕis a continuous convex unbounded function withϕ(0) = 0), the Orlicz sequence space

`ϕis the space of all real sequencesx= (xn)^∞_n=1such thatP∞

n=1ϕ(|xn|/λ)<∞. It is a Banach space when equipped with the Luxemburg norm:

kxkϕ= inf{λ >0 :

∞

X

n=1

ϕ(|xn|/λ)≤1}.

The closed linear span of{en :n∈N}in `ϕ is denotedhϕ. The spacehϕ coincides with`ϕprecisely ifϕsatisfies the ∆2 condition at zero, i.e. lim sup_t→0ϕ(2t)/ϕ(t)<

∞. The space hϕ (or`ϕ) is reflexive if and only if both ϕandϕ^∗ satisfy the ∆2

condition at zero, whereϕ∗(t) = sup{st−ϕ(s)} is the convex conjugate ofϕ. The Boyd indices of an Orlicz function ϕare defined as follows:

αϕ= sup{p >0 : sup

0<u,t≤1

ϕ(tu)

ϕ(u)t^p <∞}, βϕ = inf{p >0 : inf

0<u,t≤1

ϕ(tu) ϕ(u)t^p >0}.

It is known that β_ϕ < ∞ precisely if ϕ satisfies the ∆₂ condition at 0. The asymptotic moduli of the space h_ϕ is linked to the Boyd indices: h_ϕ is AUS (resp.

AUC) if and only if α_ϕ > 1 (resp. β_ϕ <∞). Indeed, h_ϕ is AUS if and only if αϕ>1, andαϕ is the supremum of the numbersαsuch thatρ_h_ϕ has power typeα