===== Reference Material =====
  - http://ceur-ws.org/Vol-295/paper01.pdf
  - http://www.ifw-members.ifw-kiel.de/publications/the-small-core-of-the-german-corporate-board-network/KWP_1446_Small%20Core.pdf
  - http://www.ritsbagakkai.jp/pdf/454_03.pdf