Garey, M.R. and Johnsonm D.S. (1979) Computers and Intractability. W. H. Freeman and Company, San Francis-co.
 Papadimitriou, C.H. and Steiglitz, K. (1982) Combinatorial Optimization: Algorithms and Complexity. Prentice-Hall Inc., Upper Saddle River, NY.
 Crescenzi, P. and Kann,V. (1997) Approximation on the Web: A Compendium of NP Optimization Problems. Proceedings of International Workshop RANDOM’97, Bologna, 11-12 July 1997, 111-118.
 Plotnikov, A.D. (2013) On the Structure of the Class NP. Computer Communication & Collaboration, 1, 19-25.