@TECHREPORT{RieBreKru08, author = "Rieck, Konrad and Brefeld, Ulf and Krueger, Tammo", title = "Approximate Kernels for Trees", institution = "Fraunhofer Institute FIRST", year = "2008", number = "FIRST 5/2008", month = "September", folder = "mlsec", pdf = "http://www.user.tu-berlin.de/konrad.rieck/docs/2008-first.pdf" }