Pinar Heggernes Pim van 't Hof Benjamin Lévêque Daniel Lokshtanov Christophe Paul Contracting Graphs to Paths and Trees. 55-66 2011 IPEC http://dx.doi.org/10.1007/978-3-642-28050-4_5 conf/iwpec/2011 db/conf/iwpec/ipec2011.html#HeggernesHLLP11