@article{ITA_2009__43_2_179_0,
     author = {Leinders, Dirk and Jan Van Den Bussche},
     title = {Repetitions and permutations of columns in the semijoin algebra},
     journal = {RAIRO - Theoretical Informatics and Applications - Informatique Th\'eorique et Applications},
     pages = {179--187},
     publisher = {EDP-Sciences},
     volume = {43},
     number = {2},
     year = {2009},
     doi = {10.1051/ita:2008023},
     mrnumber = {2512253},
     zbl = {1166.68014},
     language = {en},
     url = {http://www.numdam.org/articles/10.1051/ita:2008023/}
}