Martin E. Dyer Leslie Ann Goldberg Mike Paterson On Counting Homomorphisms to Directed Acyclic Graphs. 38-49 2006 conf/icalp/2006-1 ICALP (1) http://dx.doi.org/10.1007/11786986_5 db/conf/icalp/icalp2006-1.html#DyerGP06