Amin Coja-Oghlan Sven Oliver Krumke Till Nierhoff A Hard Dial-a-Ride Problem that is Easy on Average. 197-210 2005 8 J. Scheduling 3 http://dx.doi.org/10.1007/s10951-005-6811-3 db/journals/scheduling/scheduling8.html#Coja-OghlanKN05