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Di Battista & Frati (2012); Bertolazzi et al. (1994); Frati (2008). Di Battista, Giuseppe; Frati, Fabrizio (2012), "Drawing trees, outerplanar graphs, series–parallel graphs, and planar graphs in small area", Thirty Essays on Geometric Graph Theory, Algorithms and combinatorics, vol. 29, Springer, pp. 121–165, doi:10.1007/978-1-4614-0110-0_9, ISBN9781461401100. Section 5, "Upward Drawings", pp. 149–151. Bertolazzi, Paola; Cohen, Robert F.; Di Battista, Giuseppe; Tamassia, Roberto; Tollis, Ioannis G. (1994), "How to draw a series–parallel digraph", International Journal of Computational Geometry & Applications, 4 (4): 385–402, doi:10.1142/S0218195994000215, MR1310911. Frati, Fabrizio (2008), "On minimum area planar upward drawings of directed trees and other families of directed acyclic graphs", International Journal of Computational Geometry & Applications, 18 (3): 251–271, doi:10.1142/S021819590800260X, MR2424444.