- Author: Schrenk Michael
- Language: English
- Year: 2007
- Author: Google Inc., Schulz Thomas
- Language: German
- Year: 2015
- Author: Langville Amy N., Meyer Carl D
- Language: English
- Year: 2012
- Author: Schulz Thomas
- Language: German
- Year: 2015
- Author: Burns Christa, Sauers Michael P
- Language: English
- Year: 2014