1. The rectilinear Steiner tree problem is NP-complete;Garey;SIAM J. Appl. Math.,1977
2. On the removal of forbidden graphs by edge-contraction or edge-deletion;Watanabe;Tech. Rep. IECE of Japan,1980
3. Node- and edge-deletion NP-complete problems;Yannakakis;Proc. 10th Ann. ACM Symp. on Theory of Computing,1978