Co-citation,bibliographic coupling and a characterization of lattice citation networks

In this article we study directed, acyclic graphs. We introduce the head and tail order relations and study some of their properties. Recalling the notions of generalized bibliographic coupling and generalized co-citation, and introducing a new property, called the l - property, we come to a characterization of lattices. As document citation networks are concrete realizations of directed acyclic graphs all our results are directly applicable to citation analysis. This revised version was published online in June 2006 with corrections to the Cover Date.