Dieter Happel, Claus Michael Ringel,
The derived category of a tubular algebra Representation Theory I: Finite dimensional algebras: Proceedings of the Fourth International Conference on Representations of Algebras, held in Ottawa, Canada, August 16-25, ... Part 1. ,vol. 1177, pp. 156- 180 ,(1986) ,
10.1007/BFB0075264