David Fernández-Baca Jens Lagergren On the Approximability of the Steiner Tree Problem in Phylogeny. 129-145 1998 88 Discrete Applied Mathematics 1-3 http://dx.doi.org/10.1016/S0166-218X(98)00069-9 db/journals/dam/dam88.html#Fernandez-BacaL98