Nikos Ntarmos Peter Triantafillou Gerhard Weikum Distributed hash sketches: Scalable, efficient, and accurate cardinality estimation for distributed multisets. 2009 27 ACM Trans. Comput. Syst. 1 http://doi.acm.org/10.1145/1482619.1482621 db/journals/tocs/tocs27.html#NtarmosTW09