Guillaume Ducoffe, Ruxandra Marinescu-Ghemeci, Alexandru Popa. On the (di)graphs with (directed) proper connection number two. [Research Report] Université Côte d’Azur, Inria, CNRS, I3S, France; University of Bucharest, Faculty of Mathematics and Computer Science; National Institute for Research and Development in Informatics, Romania; The Research Institute of the University of Bucharest ICUB, Romania. 2017.
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