1. P. Berman, V. Ramaiyer. Improved approximations for the Steiner tree problem. In Proc. of 3d ACM-SIAM Symp. on Discrete Algorithms, 325–334, 1992.
2. P. Berman, U. Fößmeier, M. Karpinski, M. Kaufmann, A. Zelikovsky. Approaching the 5/4-Approximations for Rectilinear Steiner Trees. Techn. Rep. WSI-94-6, Tübingen.
3. D. Du, Y. Zhang, Q. Feng. On better heuristic for Euclidean Steiner minimum trees. 32nd IEEE Symp. on Found, of Comp. Science, 431–439, 1991.
4. D. Eppstein, et al. Maintaining of a Minimum Spanning Forest in a Dynamic Planar Graph. Proceedings, 1st ACM-SIAM Symp. on Discrete Algorithms, 1–11, 1990.
5. U. Fößmeier, M. Kaufmann, A. Zelikovsky. Faster Approximation Algorithms for the Rectilinear Steiner Tree Problem. LNCS 762, 533–542, 1993.