db/journals/jacm/Fagin83.html Degrees of Acyclicity for Hypergraphs and Relational Database Schemes Ronald Fagin 514-550 J. ACM July 1983 30 3 db/journals/jacm/jacm30.html#Fagin83 http://doi.acm.org/10.1145/2402.322390 JACM/30/P0514.pdf