Measure and Helly's Intersection Theorem for Convex Sets

• Autorzy: N. Stavrakas
• Języki publikacji: EN

Abstrakty

EN

Let $ℱ = {F_α}$ be a uniformly bounded collection of compact convex sets in ℝ ⁿ. Katchalski extended Helly's theorem by proving for finite ℱ that dim (⋂ ℱ) ≥ d, 0 ≤ d ≤ n, if and only if the intersection of any f(n,d) elements has dimension at least d where f(n,0) = n+1 = f(n,n) and f(n,d) = max{n+1,2n-2d+2} for 1 ≤ d ≤ n-1. An equivalent statement of Katchalski's result for finite ℱ is that there exists δ > 0 such that the intersection of any f(n,d) elements of ℱ contains a d-dimensional ball of measure δ where f(n,0) = n+1 = f(n,n) and f(n,d) = max{n+1,2n-2d+2} for 1 ≤ d ≤ n-1. It is proven that this result holds if the word finite is omitted and extends a result of Breen in which f(n,0) = n+1 = f(n,n) and f(n,d) = 2n for 1 ≤ d ≤ n-1. This is applied to give necessary and sufficient conditions for the concepts of "visibility" and "clear visibility" to coincide for continua in ℝ ⁿ without any local connectivity conditions.

• Wydawca: Institute of Mathematics Polish Academy of Sciences
• Czasopismo: Bulletin of the Polish Academy of Sciences. Mathematics
• Rocznik: 2008
• Tom: 56
• Numer: 1
• Strony: 59-65

Daty

wydano

2008

Twórcy

autor

N. Stavrakas

• Department of Mathematics, University of North Carolina, Charlotte, NC 28223, U.S.A.

Identyfikatory

DOI

10.4064/ba56-1-7