A Characterization of Extremal Sets in Hilbert Spaces
  We give a characterization of extremal sets in Hilbert spaces that generalizes a classical theorem of H. W. E. Jung. We investigate also the behaviour of points near to the circumsphere of such a set with respect to the Kuratowski and Hausdorff measures of non-compactness. 
V. NguyenKhac, K. NguyenVan