1. Ageev, A., Ye, Y., Zhan, J.: Improved combinatorial approximation algorithms for the k-Level facility location problem. SIAM J. Discrete Math. 18(1), 207–217 (2004)
2. Lecture Notes in Computer Science;A Bumb,2001
3. Garey, M.R., Johnson, D.S.: Computers and Intractability. Freeman, San Francisco (1979)
4. Mirchandani, P., Francis, R. (eds.): Discrete Location Theory. Wiley, New York (1990)
5. Gimad, E.Kh.: Effective algorithms for multistage location problem on a path. Diskret. Anal. Issled. Oper. Ser. 1 2(4), 13–31 (1995). (in Russian)