Visible Points in Convex Sets and Best Approximation
  The concept of a visible point of a convex set relative to a given point is introduced. A number of basic properties of such visible point sets is developed. In particular, it is shown that this concept is useful in the study of best approximation, and it also seems to have potential value in the study of robotics. 
Frank Deutsch and Hein Hundal and Ludmil Zikatanov
math.FA math.NA