Massimo Bartoli Marcello Pelillo Kaleem Siddiqi Steven W. Zucker Attributed Tree Homomorphism Using Association Graphs. 2133-2136 2000 ICPR http://csdl.computer.org/comp/proceedings/icpr/2000/0750/02/07502133abs.htm db/conf/icpr/icpr2000-2.html#BartoliPSZ00