George H. L. Fletcher Marc Gyssens Jan Paredaens Dirk Van Gucht On the Expressive Power of the Relational Algebra on Finite Sets of Relation Pairs. 939-942 2009 21 IEEE Trans. Knowl. Data Eng. 6 http://dx.doi.org/10.1109/TKDE.2008.221 db/journals/tkde/tkde21.html#FletcherGPG09