Hindawi Publishing Corporation Journal of Inequalities and Applications Volume 2011, Article ID 875649, 15 pages doi:10.1155/2011/875649 Research Article Nonsquareness and Locally Uniform Nonsquareness in Orlicz-Bochner Function Spaces Endowed with Luxemburg Norm Shaoqiang Shang,1 Yunan Cui,2 and Yongqiang Fu1 1 2 Department of Mathematics, Harbin Institute of Technology, Harbin 150001, China Department of Mathematics, Harbin University of Science and Technology, Harbin 150080, China Correspondence should be addressed to Shaoqiang Shang, sqshang@163.com Received 5 July 2010; Accepted 12 February 2011 Academic Editor: Nikolaos Papageorgiou Copyright q 2011 Shaoqiang Shang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Criteria for nonsquareness and locally uniform nonsquareness of Orlicz-Bochner function spaces equipped with Luxemburg norm are given. We also prove that, in Orlicz-Bochner function spaces generated by locally uniform nonsquare Banach space, nonsquareness and locally uniform nonsquareness are equivalent. 1. Introduction A lot of nonsquareness concepts in Banach spaces are known see 1. Nonsquareness are important notions in geometry of Banach space. One of reasons is that these properties are strongly related to the fixed point property see 2. The criteria for nonsquareness and locally uniform nonsquareness in the classical Orlicz function spaces have been given in 3, 4 already. However, because of the complicated structure of Orlicz-Bochner function spaces equipped with the Luxemburg norm, the criteria for nonsquareness and locally uniform nonsquareness of them have not been found yet. The aim of this paper is to give criteria for nonsquareness and locally uniform nonsquareness of Orlicz-Bochner function spaces equipped with Luxemburg norm. Let X, · be a real Banach space. SX and BX denote the unit sphere and unit ball, respectively. Let us recall some geometrical notions concerning nonsquareness. A Banach space X is said to be nonsquare if for any x, y ∈ SX we have min{xy/2, x−y/2} < 1. A Banach space X is said to be uniformly nonsquare if there exists δ > 0 such that for any x, y ∈ SX, min{x y/2, x − y/2} < 1 − δ. A Banach space X is said to be locally uniformly nonsquare if for any x ∈ SX, there exists δx > 0 such that min{x y/2, x − y/2} < 1 − δx , where y ∈ SX. 2 Journal of Inequalities and Applications Let R be set of real numbers. A function M : R → R is called an N-function if M is convex, even, M0 0, Mu > 0 u / 0 and limu → 0 Mu/u 0, and limu → 0 Mu/u ∞. Let T, Σ, μ be a nonatomic measurable space. p denotes right derivative of M. Moreover, for a given Banach space X, · , we denote by XT the set of all strongly μmeasurable function from T to X, and for each u ∈ XT , we define the modular of u by ρM u Mutdt. 1.1 G Put LM ut ∈ XT : Mλutdt < ∞ for some λ > 0 . 1.2 G The linear set LM endowed with the Luxemburg norm u ≤1 u inf λ > 0 : ρM λ 1.3 is a Banach space. We say that an Orlicz function M satisfies condition Δ2 M ∈ Δ2 if there exist K > 2 and u0 ≥ 0 such that M2u ≤ KMu u ≥ u0 . 1.4 First let us recall a known result that will be used in the further part of the paper. Lemma 1.1 see 3. Suppose M ∈ Δ2 . Then ρM un −→ 0 ⇐⇒ un −→ 0, ρM un −→ 1 ⇐⇒ un −→ 1n −→ ∞. 1.5 2. Main Results Theorem 2.1. LM is nonsquare if and only if a M ∈ Δ2 ; b X is nonsquare. In order to prove the theorem, we give a lemma. Lemma 2.2. If X is nonsquare, then for any x, y / 0, we have x y − min x y, x − y > 0. 2.1 Journal of Inequalities and Applications 3 Proof. Case 1. If x < y, then x y ≤ x x x · y 1 − · y y y < x x y − x x y 2.2 or x − y < x y. 2.3 Case 2. If x ≥ y, then y x x y ≤ · x y − 1 · y x y 2.4 < y y x − y x y or x − y < x y. 2.5 This implies x y − min{x y, x − y} > 0. This completes the proof. Proof of Theorem 2.1. a Necessity. Suppose that M ∈ / Δ2 , then there exist u ∈ SLM and δ > 0 such that ρM u 1 − δ < 1. Pick c > 0 such that E {t ∈ T : ut ≤ c} is not a null set. Since ∞ M∈ / Δ2 , there exist sequence {rn }∞ n1 and disjont subsets {En }n1 of E such that rn > 2nc, M 1 1 1 1 rn > 2n M rn , n 2n Therefore, if we define v ρM lv ≥ ∞ 1 1 rn μEn 2−n δ. 2n 2.6 then for any l > 1, we have ∞ ∞ 1 1 ρM lrn χEn ≥ ρM 2n ρM 1 1 rn χEn > rn χEn n 2n nm nm n1 ∞ ∞ 2n M 1 nm ρM v n1 rn χEn , 2n M ∞ n1 Mrn μEn < ∞ 1 rn μEn 2n · 2−n δ ∞, 2n nm ∞ n1 Mrn cμEn < ∞ 1 M 1 rn μEn δ. 2n n1 2.7 4 Journal of Inequalities and Applications This yields v 1, ρM u ± v ≤ ρM u ∞ Mrn cμEn 1 − δ δ 1. 2.8 n1 Hence, u ± v ≤ 1. But u v u − v ≤ 2u 2, and we deduce that u v u − v 1. Moreover, we have 1/2u v 1/2u − v 1 and 1/2u v − 1/2u − v} 1, a contradiction with nonsquareness of LM . If b is not true, then there exist x, y ∈ SX such that x y 1/2x y 1/2x − y. Pick α > 0 such that T Mαdt 1. Put ut α · x · χT t, vt α · y · χT t. 2.9 Then we have Mαxdt ρM u T ρM v Mαdt 1, T M αy dt T 2.10 Mαdt 1. T It is easy to see u, v ∈ SLM . We know that xy ut vt α· · χT t, 2 2 x−y ut − vt α· · χT t. 2 2 2.11 Hence, we have x y dt M α · Mαdt 1, ρM 2 2 T T x − y u−v dt ρM M α · Mαdt 1. 2 2 T T u v 2.12 It is easy to see 1/2u v, 1/2u − v ∈ SLM , a contradiction! Sufficiency. Suppose that there exists u, v ∈ SLM such that 1 1 u v u v u − v 1. 2 2 We will derive a contradiction for each of the following two cases. 2.13 Journal of Inequalities and Applications 5 Case 1. μ{t ∈ T : ut / 0} ∩ {t ∈ T : vt / 0} 0. Let G {t ∈ T : ut / 0}. Hence, we have 1 1 1 ρM u ρM v 2 2 2 1 2 Mutdt G 1 2 T \G Mvtdt 1 Mut vtdt 2 G T \G Mut vtdt 1 1 > M M ut vt dt ut vt dt 2 2 G T \G 2.14 1 M ut vt dt 2 T ρM 1 u v . 2 Since M ∈ Δ2 , we have ρM u ρM v 1. Hence, ρM u v/2 < 1. This implies u v/2 < 1, a contradiction! Case 2. μ{t ∈ T : ut / 0} ∩ {t ∈ T : vt / 0} > 0. By Lemma 2.2, without loss of generality, we may assume that there exists T1 ⊂ {t ∈ T : ut / 0} ∩ {t ∈ T : vt / 0} such that ut vt > ut vt, t ∈ T1 and μT1 > 0. Therefore, 1 1 1 ρM u ρM v 2 2 2 T Mutdt T 1 1 1 1 M ut vt dt ut vt dt 2 2 2 2 T \T1 1 1 M ut vt dt ut vt dt 2 2 T \T1 T1 M T1 ρM Mvtdt T M > 1 1 Mut Mvtdt 2 2 ≥ 1 2 2.15 u v 2 . Since M ∈ Δ2 , we have ρM u ρM v 1. Hence, ρM u v/2 < 1. This implies u v/2 < 1, a contradiction! Theorem 2.3. LM is locally uniformly nonsquare if and only if a M ∈ Δ2 ; b X is locally uniformly nonsquare. 6 Journal of Inequalities and Applications In order to prove the theorem, we give a lemma. Lemma 2.4. If X is locally uniformly nonsquare, then a For any x / 0, r1 ≥ r2 > 0, we have inf x y − min x y, x − y : x ∈ X, r2 ≤ y ≤ r1 > 0 y/ 0 2.16 b If xn → x, then limn → ∞ δxn δx, where δx inf x y − min x y, x − y : x ∈ X, r2 ≤ y ≤ r1 . y/ 0 2.17 Proof. a Since X is locally uniformly nonsquare, we have ηx > 0 and ηλx ληx , where λ > 0 and ηx inf x y − min x y, x − y : x y > 0 . y 2.18 In fact, since X is locally uniformly nonsquare, we have ηx inf x y − min x y, x − y : x y > 0 y x x y y x · inf 2 − min , − : x y > 0 > 0, y x y x y ηλx inf λx y − min λx y, λx − y : λx y > 0 y 1 1 1 1 λ · inf x y − min x y, x − y : x y > 0 y λ λ λ λ λ · inf x y − min x y, x − y : x y > 0 y λ · ηx . 2.19 Journal of Inequalities and Applications 7 Case 1. If x ≥ y, then x y ≤ 1− ≤ 1− y y xy x x x y y x y x x x 2.20 ≤ x − y y y − ηy/xx ≤ x y − ηr2 /xx or x − y ≤ x y − ηr /xx . 2 2.21 x x x y ≤ · y x 1 − · y y y ≤ x x − ηx y − x x y − ηx ≤ x y − ηx 2.22 x − y ≤ x y − ηx . 2.23 Case 2. If x < y, then or Therefore, we get, the following inequality inf x y − min x y, x − y : x ∈ X ≥ min ηr2 /xx , ηx > 0 y 2.24 holds. b1 Suppose that lim supn → ∞ δxn > δx, where xn → x n → ∞. Then there exist a > 0 and subsequence {n} of {n}, such that δxn − δx ≥ a. By definition of δx, there exist y0 ∈ X such that a x y0 − min x y0 , x − y0 < δx , 8 r1 ≤ y0 ≤ r2 . We will derive a contradiction for each of the following two cases. 2.25 8 Journal of Inequalities and Applications Case 1. x y0 x − y0 . Since xn → x n → ∞, there exists n0 such that xn0 − x < a/8. Therefore, xn0 y0 − xn0 y0 ≤ x xn0 − x y0 − xn0 y0 ≤ x xn0 − x y0 − x y0 − xn0 − x x y0 − x y0 2xn0 − x a 2xn0 − x 8 a a < δx 2 · 8 8 ≤ δx 2.26 3 δx a. 8 This implies δxn0 ≤ δx 3/8a, a contradiction! x y0 . Without loss of generality, we may assume x − y0 > x y0 r, Case 2. x − y0 / where r > 0. Since xn → x n → ∞, there exists n0 such that xn0 −x < min{1/8a, 1/8r}. Therefore, we have xn − y0 x − y0 xn − x 0 0 ≥ x − y0 − xn0 − x 1 ≥ x − y0 − r, 8 xn y0 x y0 xn − x 0 0 ≤ x y0 xn0 − x 2.27 1 ≤ x y0 r. 8 This implies xn − y0 ≥ x − y0 − 1 r ≥ x y0 r − 1 r ≥ x y0 1 r ≥ xn y0 . 0 0 8 8 8 2.28 Similarly, we have 3 xn0 y0 − xn0 y0 ≤ δx a. 8 2.29 3 xn0 y0 − min xn0 y0 , xn0 − y0 ≤ δx a. 8 2.30 Therefore, we have This implies δxn0 ≤ δx 3/8a, a contradiction! Hence, lim supn → ∞ δxn ≤ δx. Journal of Inequalities and Applications 9 b2 Suppose that lim infn → ∞ δxn < δx, where xn → x n → ∞. Then there exist b > 0 and subsequence {n} of {n}, such that δx − δxn ≥ b. Since xn → x n → ∞, then there exist n0 ∈ N such that xn0 − xn < 1/8b, whenever n ≥ n0 . By definition of δxn0 , there exist y0 ∈ X such that b xn0 y0 − min xn0 y0 , xn0 − y0 < δxn0 , 8 r2 ≤ y0 ≤ r1 . 2.31 Therefore, we have xn y0 − min xn y0 , xn − y0 xn0 − xn0 xn y0 − min xn0 − xn0 xn y0 , xn0 − xn0 xn − y0 1 1 ≤ xn0 b y0 − min xn0 y0 , xn0 − y0 b 8 8 1 xn0 y0 − min xn0 y0 , xn0 − y0 b 4 2.32 1 1 < δxn0 b b 8 4 3 < δxn0 b 8 whenever n ≥ n0 . Since xn → x n → ∞, there exists n1 > n0 such that |ηx − ηxn1 | < 1/8b, where ηx x y0 − min x y0 , x − y0 . 2.33 1 1 1 7 ηxn1 > ηx − b ≥ δx − b ≥ δxn0 b − b δxn0 b. 8 8 8 8 2.34 7 xn1 y0 − min xn1 y0 , xn1 − y0 ≥ δxn0 b, 8 2.35 Hence, we have This implies which contradict 2.32. Hence, lim infn → ∞ δxn ≥ δx. Combing b1 with b2, we get limn → ∞ δxn δx. This completes the proof. 10 Journal of Inequalities and Applications Proof of Theorem 2.3. Necessity. By Theorem 2.1, M ∈ Δ2 . If b is not true, then there exist such that 1/2x yn → 1 and 1/2x − yn → 1 as n → ∞. x ∈ SX, {yn }∞ n1 ⊂ SX Pick α > 0 such that T Mαdt 1. Put ut α · x · χT t, vn t α · yn · χT t. 2.36 Then we have Mαxdt ρM u T M αyn dt ρM vn Mαdt 1, T T 2.37 Mαdt 1. T It is easy to see u, vn ∈ SLM . We know that x yn ut vn t α· · χT t, 2 2 x − yn ut − vn t α· · χT t. 2 2 2.38 Moreover, we have Mα·xyn /2 ≤ Mα, Mα·x−yn /2 ≤ Mα. By the dominated convergence theorem, we have x yn x yn dt dt M α· lim M α · Mαdt 1, 2 2 T T n→∞ T x − yn x − yn dt dt M α· lim M α · Mαdt 1. 2 2 T T n→∞ T lim n→∞ lim n→∞ 2.39 It is easy to see ρM 1/2u vn → 1, ρM 1/2u − vn → 1 as n → ∞. By Lemma 1.1, we have 1/2u vn → 1 and 1/2u − vn → 1 as n → ∞, a contradiction with locally uniform nonsquareness of LM . Sufficiency. Suppose that there exist u ∈ SLM , {vn }∞ n1 ⊂ SLM such that 1/2u vn → 1, 1/2u − vn → 1 as n → ∞. We will derive a contradiction for each of the following two cases. Case 1. There exist ε0 > 0, σ0 > 0 such that μGn > ε0 , where Gn {t ∈ T : vn t ≥ σ0 }. Put Hn t ∈ T : σ0 ≤ vn t ≤ M −1 4 ε0 . 2.40 We have 1 Mvn tdt ≥ T Gn \Hn Mvn tdt ≥ 4 · μGn \ Hn . ε0 2.41 Journal of Inequalities and Applications 11 This implies μGn \ Hn ≤ 1/4ε0 . Hence, μHn > 1/2ε0 . We define a function ηt inf ut y − min ut y, ut − y : σ0 ≤ y ≤ M−1 y /0 4 ε0 2.42 on T0 , where T0 {t ∈ T : ut / 0}. By Lemma 2.4, we have ηt > 0 μ-a.e on T0 . Let hn t → ut μ-a.e on T0 , where hn is simple function. Hence, 4 ηn t inf hn t y −min hn ty, hn t−y : σ0 ≤ y ≤ M−1 y/ 0 ε0 2.43 is μ-measurable. By Lemma 2.4, we have ηn t → ηt μ-a.e on T0 . Then ηt is μ-measurable. Using ∞ t ∈ T0 : T⊃ i1 1 1 < ηt ≤ , i1 i 2.44 we get that there exists η0 > 0 such that μH < 1/8ε0 , where H t ∈ T0 : ηt ≤ 2η0 . 2.45 Let En Hn \ H, En1 Hn ∩ {t ∈ T : ut / 0} \ H, En2 Hn ∩ {t ∈ T : ut 0} \ H. It is easy to see En En1 ∪ En2 , En1 ∩ En2 φ and μEn ≥ 3/8ε0 . If t ∈ En1 , by Lemma 2.4, we have ut vn t − min{ut vn t, ut − vn t} ≥ ηt ≥ 2η0 . 2.46 Without loss of generality, we may assume that there exists Fn1 ⊂ En1 such that 1 1 μE , 2 n μFn1 ≥ ut vn t − ut vn t ≥ 2η0 t ∈ Fn1 . 2.47 Moreover, for any u ≥ v > 0, we have 1 1 1 1 Mu − M u − Mv − M v 2 2 2 2 1/2u 1/2v 1 u 1 v ptdt − ptdt − ptdt ptdt 2 0 2 0 0 0 1/2v u 1/2u 1 1 v ptdt − ptdt − ptdt − ptdt 2 0 2 0 0 0 ≥ 1 2 u ptdt − v 1/2u1/2v v 1/2u 1/2v ptdt ptdt − 1/2u 1/2v ptdt ≥ 0. 2.48 12 Journal of Inequalities and Applications Hence, if t ∈ En2 , then σ 1 1 1 0 Mvn t Mvn t ≥ Mσ0 − M > 0. 2 2 2 2 2.49 Let Fn Fn1 ∪ En2 . Then μFn ≥ 1/8ε0 . Therefore, u v 1 1 n ρM u ρM vn − ρM 2 2 2 1 1 ut vn t dt Mutdt Mvn tdt − M 2 T 2 T 2 T 1 1 ut vn t Mut Mvn t − M dt 2 2 T 2 1 1 ut vn t Mut Mvn t − M dt ≥ 2 2 Fn1 2 1 1 ut vn t Mut Mvn t − M dt 2 2 En2 2 1 1 ut vn t dt ≥ M ut vn t − M 2 2 2 Fn1 1 1 ut vn t Mut Mvn t − M dt 2 2 En2 2 ut vn t ut vn t η0 − M dt ≥ M 2 2 Fn1 1 vn t Mvn t − M dt 2 En2 2 σ 1 0 Mσ0 − M dt M η0 dt ≥ 2 Fn1 En2 2 σ 1 0 ≥ dt min M η0 , Mσ0 − M 2 2 Fn σ 1 0 min M η0 , Mσ0 − M · μFn 2 2 σ 1 1 0 · ε0 . ≥ min M η0 , Mσ0 − M 2 2 8 2.50 By Lemma 1.1, we have ρM u ρM vn 1, ρM u vn /2 → 1 as n → ∞. This is in contradiction with 1/2ρM u 1/2ρM vn − ρM u vn /2 ≥ min{Mη0 , 1/2Mσ0 − Mσ0 /2} · 1/8ε0 . Journal of Inequalities and Applications 13 Case 2. For any ε > 0, σ > 0, there exists N such that μ{t ∈ T : vn t ≥ σ} < ε whenever n > N. By the Riesz theorem, without loss of generality, we may assume that vn t → 0 μ− a.e on T . Using ∞ 0} ⊃ {t ∈ T : ut / n1 1 1 < ut ≤ , t∈T : n1 n 2.51 we get that there exist r > 0 such that μT1 < 1/8μT0 , where T1 {t ∈ T : 0 < ut < d}, T0 {t ∈ T : ut / 0}. 2.52 Since M is N-function, we can choose 0 < h < d such that 1/2Md 1/2Mh − Md h/2 > 0. Since vn t → 0μ− a.e on T , by the Egorov theorem, there exists N1 such that vn t < h, t ∈ F whenever n > N1 , where F ⊂ T , μT \ F < 1/8μT0 . Next, we will prove that if u1 ≥ u2 ≥ v2 ≥ v1 > 0, then u v 1 u v 1 1 1 1 1 2 2 Mu1 Mv1 − M ≥ Mu2 Mv2 − M . 2 2 2 2 2 2 2.53 In fact, we have u v 1 u v 1 1 1 1 1 1 2 Mu1 Mv1 − M − Mu1 Mv2 − M 2 2 2 2 2 2 u v 1 u v 1 1 1 1 2 Mv1 − M − Mv2 − M 2 2 2 2 1 2 v1 ptdt − u1 v1 /2 0 0 v u1 v2 /2 1 2 ptdt − ptdt − ptdt 2 0 0 u1 v1 /2 u1 v2 /2 v1 1 v2 1 ptdt − ptdt ptdt − ptdt 2 0 2 0 0 0 1 − 2 ≥ v2 ptdt v1 u1 v2 /2 u1 v1 /2 u1 v2 /2 u1 v1 /2 ptdt − ptdt v2 v1 v2 /2 ptdt ≥ 0. 2.54 14 Journal of Inequalities and Applications Moreover, we have u v 1 u v 1 1 2 2 2 Mu1 − M − Mu2 − M 2 2 2 2 u2 v2 /2 u1 u1 v2 /2 1 1 u2 ptdt − ptdt − ptdt − ptdt 2 0 2 0 0 0 1 2 ≥ u1 ptdt − u1 v2 /2 u2 u1 u2 /2 u2 v2 /2 ptdt − ptdt u1 v2 /2 u2 2.55 u2 v2 /2 ptdt ≥ 0. By 2.54 and 2.55, we have u v 1 u v 1 1 1 1 1 2 2 Mu1 Mv1 − M ≥ Mu2 Mv2 − M . 2 2 2 2 2 2 2.56 This shows that if t ∈ T2 T0 \ T1 ∪ T \ F, then Mut Mvn t − 2M ut vn t 2 ≥ Md Mh − 2M dh . 2 2.57 It is easy to see μT2 ≥ 1/4μT0 . Therefore, u v 1 1 n ρM u ρM vn − ρM 2 2 2 1 1 ut vn t dt Mutdt Mvn tdt − M 2 T 2 T 2 T 1 1 ut vn t Mut Mvn t − M dt 2 2 T 2 1 1 ut vn t Mut Mvn t − M ≥ dt 2 2 T 2 1 1 ut vn t Mut Mvn t − M dt ≥ 2 2 T2 2 1 dh 1 Md Md − M dt ≥ 2 2 T2 2 1 dh 1 1 Md Md − M ≥ · μT0 , 2 2 2 4 2.58 for n large enough. By Lemma 1.1, we have ρM u ρM vn 1, ρM uvn /2 → 1 as n → ∞, which contradicts 1/2ρM u 1/2ρM vn − ρM u vn /2 ≥ 1/2Md 1/2Md − Md h/2 · 1/4μT0 , for n large enough. This completes the proof. Journal of Inequalities and Applications 15 Corollary 2.5. The following statements are equivalent: a LM is locally uniformly nonsquare if and only if LM is nonsquare; b X is locally uniformly nonsquare. Acknowledgments The authors would like to thank the anonymous referee for some suggestions to improve the manuscript. This work was supported by China Natural Science Fund under Grant no. 11061022. References 1 R. C. James, “Uniform nonsquare Banach space,” Annals of Mathematics, vol. 80, no. 3, pp. 542–550, 1964. 2 J. Garcı́a-Falset, E. Llorens-Fuster, and E. M. Mazcuñan-Navarro, “Uniformly nonsquare Banach spaces have the fixed point property for nonexpansive mappings,” Journal of Functional Analysis, vol. 233, no. 2, pp. 494–514, 2006. 3 S. T. Chen, “Geometry of Orlicz spaces,” Dissertationes Math, vol. 356, pp. 1–204, 1996. 4 C. X. Wu, T. F. Wang, S. T. Chen, and Y. W. Wang, Geometry Theory of Orlicz Spaces, H.I.T Print House, Harbin, China, 1986.