V. Vianu. Proceedings of the Twentieth ACM SIGACT-SIGMOD-SIGART Symposium on Principles of Database Systems, page 1-15. Santa Barbara, California, ACM Press, (May 2001)
Y. Stavrakas, K. Pristouris, A. Efandis, and T. Sellis. East-European Conference on Advances in Databases and Information Systems (ADBIS 2004), Budapest, Hungary, (September 2004)