A note on self-extremal sets in Lp(Ω) spaces

نویسندگان

  • Viet Nguyen-Khac
  • Khiem Nguyen-Van
چکیده

We give a necessary condition for a set in Lp(Ω) spaces (1 < p <∞) to be self-extremal that partially extends our previous results to the case of Lp spaces. Examples of self-extremal sets in Lp(Ω) (1 < p <∞) are also given. In [4, 5], we introduced the notion of (self-) extremal sets of a Banach space (X ,‖ · ‖). For a nonempty bounded subset A of X , we denote by d(A) its diameter and by r(A) the relative Chebyshev radius of A with respect to the closed convex hull coA of A, that is, r(A) := inf y∈coA supx∈A‖x − y‖. The self-Jung constant of X is defined by Js(X) := sup{r(A) : A ⊂ X , with d(A) = 1}. If in this definition we replace r(A) by the relative Chebyshev radius rX(A) of A with respect to the whole X , we get the Jung constant J(X) of X . Recall that a bounded subset A of X consisting of at least two points is said to be extremal (resp., self-extremal) if rX(A) = J(X)d(A) (resp., r(A) = Js(X)d(A)). Throughout the note, unless otherwise mentioned, we will work with the following assumption: (Ω,μ) is a σ-finite measure space such that Lp(Ω) is infinite-dimensional. The Jung and self-Jung constants of Lp(Ω) (1 ≤ p <∞) were determined in [1, 3, 6, 7]:

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عنوان ژورنال:
  • Int. J. Math. Mathematical Sciences

دوره 2005  شماره 

صفحات  -

تاریخ انتشار 2005