@article {MR1049357,
    AUTHOR = {Coron, Jean-Michel},
     TITLE = {A necessary condition for feedback stabilization},
   JOURNAL = {Systems Control Lett.},
  FJOURNAL = {Systems \& Control Letters},
    VOLUME = {14},
      YEAR = {1990},
    NUMBER = {3},
     PAGES = {227--232},
      ISSN = {0167-6911},
     CODEN = {SCLEDC},
   MRCLASS = {93D15},
  MRNUMBER = {MR1049357},
}