An Approach for Proving Lower Bounds: Solution of Gilbert-Pollak's Conjecture on Steiner RatioDing-Zhu DuFrank K. Hwang76-85conf/focs/FOCS31a1990FOCSdb/conf/focs/focs90.html#DuH90https://doi.org/10.1109/FSCS.1990.89526https://doi.ieeecomputersociety.org/10.1109/FSCS.1990.89526