A Straightforward Formalization of the Relational Model.

http://doi.acm.org/10.1145/984540.984542 db/journals/sigmod/NiemiJ84.html Timo Niemi Kalervo Järvelin A Straightforward Formalization of the Relational Model. 15-38 sigmodR/14-1/P015.pdf 1984 14 SIGMOD Record 1 db/journals/sigmod/sigmod14.html#NiemiJ84