Ding-Zhu Du E. Y. Yao Frank K. Hwang A Short Proof of a Result of Pollak on Steiner Minimal Trees. 396-400 1982 32 J. Comb. Theory, Ser. A 3 db/journals/jct/jcta32.html#DuYH82 http://dx.doi.org/10.1016/0097-3165(82)90056-5