Piotr Berman Marek Karpinski Alexander Zelikovsky 1.25-Approximation Algorithm for Steiner Tree Problem with Distances 1 and 2. 86-97 2009 WADS http://dx.doi.org/10.1007/978-3-642-03367-4_8 conf/wads/2009 db/conf/wads/wads2009.html#BermanKZ09