László Egri, Benoit Larose, Pascal Tesson. Directed st-Connectivity Is Not Expressible in Symmetric Datalog. In Luca Aceto, Ivan Damgård, Leslie Ann Goldberg, Magnús M. Halldórsson, Anna Ingólfsdóttir, Igor Walukiewicz, editors, Automata, Languages and Programming, 35th International Colloquium, ICALP 2008, Reykjavik, Iceland, July 7-11, 2008, Proceedings, Part II - Track B: Logic, Semantics, and Theory of Programming & Track C: Security and Cryptography Foundations. Volume 5126 of Lecture Notes in Computer Science, pages 172-183, Springer, 2008. [doi]
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