Riedel, Erwin and Christoph Janiak. "Frontmatter" Übungsbuch. Berlin, Boston: DE GRUYTER, 2011. I-IV. Web. Retrieved 24 May. 2013, from http://www.degruyter.com/view/books/9783110229653/9783110229653.fm/9783110229653.fm.xml

Riedel, E. & Janiak, C. (2011). Frontmatter. Übungsbuch (pp. I-IV). Berlin, Boston: DE GRUYTER. Retrieved 24 May. 2013, from http://www.degruyter.com/view/books/9783110229653/9783110229653.fm/9783110229653.fm.xml

Riedel, Erwin and Christoph Janiak. 2011. Frontmatter. In Übungsbuch, I-IV. Berlin, Boston: DE GRUYTER. Retrieved 24 May. 2013, from http://www.degruyter.com/view/books/9783110229653/9783110229653.fm/9783110229653.fm.xml