1. Ausiello, G., Crescenzi, P., Gambosi, G., Kann, V., Marchetti-Spaccamela, A., Protasi, M.: Complexity and approximation. In: Combinatorial optimization problems and their approximability properties, Springer, Berlin (1999)
2. Garey, M.R., Johnson, D.S.: Computers and intractability. In: A guide to the theory of NP-completeness, W. H. Freeman, San Francisco (1979)
3. Bazgan, C., Escoffier, B., Paschos, V.Th.:Completeness in standard and differential approximation classes: Poly-(D)APX- and (D)PTAS-completeness. Cahier du LAMSADE 217, LAMSADE, Université Paris-Dauphine, Available on (2004),
http://www.lamsade.dauphine.fr/cahiers.html
4. Crescenzi, P., Panconesi, A.: Completeness in approximation classes. Information and Computation 93, 241–262 (1991)
5. Ausiello, G., Crescenzi, P., Protasi, M.: Approximate solutions of NP optimization problems. Theoret. Comput. Sci. 150, 1–55 (1995)