Existence of Minimal Projections

Rend. Sem. Mat. Univ. Pol. Torino Vol. 60, 3 2002

R. Khalil MINIMAL PROJECTIONS IN TENSOR PRODUCT SPACES

Abstract. It is the object of this paper to study the existence and the form of minimal projections in some spaces of tensor products of Banach spaces. We answer a question of Franchetti and Cheney for finitely co- dimensional subspaces in CK [1]. Introduction The problem of the existence of minimal projections and the computation of the norm of the minimal projection is an important one [1], [5], [6]. The object of this paper is to present new results on the existence of minimal pro- jections in some tensor products of Banach spaces, and to give the form of the minimal projection in certain Banach spaces. It should be remarked that not much is known on the form of minimal projections. For Banach spaces X and Y , X ∧ ⊗ Y X ∨ ⊗ Y denotes the completed projec- tiveinjectivetensor product of X and Y [5]. For 1 ≤ p ∞, we set L p I, X denote the space of all p−Bochner integrable functions classes on the unit interval I with values in the Banach space X . In case X is the set of reals we write L p I . For f ∈ L 2 I, X , k f k p denotes the usual p−norm of f [5]. For p = ∞, L ∞ I, X denotes the essentially bounded functions from I to X , with the usual norm k·k ∞ . The spaces ℓ 2 X , and ℓ ∞ X are the corresponding sequence spaces. If X is a Banach space, X ∗ denotes the dual of X , and LX, Y the space of all bounded linear operators from X to the Banach space Y . Throughout this paper, 5X, Y denotes the set of all projections from X into Y .

1. Existence of Minimal Projections

Let X be Banach space and Y be a closed subspace of X . If Y is finite dimensional then, a minimal projection from X onto Y exists [2]. The problem of existence of minimal projections in tensor product spaces was discussed in [1], [2] and [4]. In this section, we present some general facts on the existence of minimal projections which are not stated explicitly in the literature. We include the proof for completeness. P ROPOSITION 1. Let X be a dual space. Then for any w ∗ − closed complemented subspace Y of X there exists a minimal projection onto Y . 167 168 R. Khalil Proof. Let K = {P ∈ LX, X : P x = x for x ∈ Y }. Since X is a dual space, and Y is w ∗ - closed in X, it follows that K is w ∗ -closed. Consequently inf{kPk : P ∈ K } is attained at some P, and such P is a projection. This ends the proof. As a consequence we get C OROLLARY 1. Let G and H be finite dimensional subspaces of L p I . Then there exists a minimal projection from L p I × I onto L p I, H + L p I, G, for 1 p ∞. Proof. From theory of tensor products, it is known [5], that L p I × I = L p I ⊗ p L p I , L p I, H = L p I ⊗ p H and L p I, G = G ⊗ p L p I , where X ⊗ p Y is the p-nuclear tensor product of X and Y . Since L p I × I is a dual separable space, and H is finite dimensional, it follows from Proposition 1 above and Proposition 11.2 in [5] that L p I, H + L p I, G is a closed complemented subspace of a reflexive space, and so it is w ∗ - closed. The result now follows. C OROLLARY 2. If G and H are finite dimensional subspaces of ℓ 1 , then there is a minimal projection of ℓ 1 ∧ ⊗ ℓ 1 onto ℓ 1 ∧ ⊗ H + G ∧ ⊗ ℓ 1 . Proof. The proof follows from the above proposition and the fact ℓ 1 ∧ ⊗ H = ℓ 1 H = c G, where G ∗ = H. Another similar result is P ROPOSITION 2. Let X be a Banach space with separable dual .Then for every complemented weakly sequentially complete subspace W in X , there exists a minimal projection from X onto W. Proof. Since W is complemented, i n f {kQk : Q ∈ 5X, W } = r , is finite. So there exists a sequence P n ∈ 5 X, W such that kP n k −→ r . Thus P ∗ n is a bounded sequence in LX ∗ , X ∗ = X ∗ ∧ ⊗ X ∗ ∗ where T ∗ is the adjoint of the operator T . Since X ∗ is separable, X is separable, and so X ∗ ∧ ⊗ X is separable. Hence, using Helly’s selection theorem, we can assume that P ∗ n converges in the weak operator topology. Define Q : X −→ W by Qx , x ∗ = li m x , P ∗ n x ∗ . Since Y is weakly sequentially complete then Q is a projection on W , and kQk ≤ li m P ∗ n = r . So Q is a minimal projections. This ends the proof. Minimal projections 169 P ROPOSITION 3. Let X be a Banach space and W be a complemented subspace of X . If there is a contractive projection from W ∗∗ onto W , then there is a minimal projection from X onto W . Proof. Let r = i n f {kPk : P ∈ 5X, W }. Let P t be a net of projections onto W such that li m t kP t k = r . Now P ∗ t ∈ 5 X ∗ , W ∗ , so P ∗ t ∈ X ∗ ∧ ⊗ X ∗ . So there is a subnet that converges in the weak operator topology. We can assume that P ∗ t −→ U in the operator topology. So U is an element of 5X ∗ , W ∗ , and U x , x = li m P ∗ t x ∗ , x = li m P t x , x ∗ , for all x ∈ X and x ∗ ∈ X ∗ . Now, U ∗ ∈ 5 X ∗∗ , W ∗∗ , and U ∗ x = x for all x ∈ W . But U ∗ x need not be in W for all x ∈ X . For this, we define P : X −→ W P x = J U ∗ x , for all x ∈ X , where J is the contractive projection from W ∗∗ to W . Then P is a projection and k Pk ≤ k J k U ∗ ≤ U ∗ ≤ li m P ∗ t = li m kP t k = r. Hence P is minimal. This ends the proof.

2. Existence of minimal projections in some function spaces