The equivalence of planar 3-trees and chordal maximal planar graphs was stated without proof by Patil (1986). For a proof, see Markenzon, Justel & Paciornik (2006). For a more general characterization of chordal planar graphs, and an efficient recognition algorithm for these graphs, see Kumar & Madhavan (1989). The observation that every chordal polyhedral graph is maximal planar was stated explicitly by Gerlach (2004). Patil, H. P. (1986), "On the structure of k-trees", Journal of Combinatorics, Information and System Sciences, 11 (2–4): 57–64, MR0966069. Markenzon, L.; Justel, C. M.; Paciornik, N. (2006), "Subclasses of k-trees: Characterization and recognition", Discrete Applied Mathematics, 154 (5): 818–825, doi:10.1016/j.dam.2005.05.021, MR2207565. Kumar, P. Sreenivasa; Madhavan, C. E. Veni (1989), "A new class of separators and planarity of chordal graphs", Foundations of Software Technology and Theoretical Computer Science, Ninth Conference, Bangalore, India December 19–21, 1989, Proceedings, Lecture Notes in Computer Science, vol. 405, Springer-Verlag, pp. 30–43, doi:10.1007/3-540-52048-1_30, ISBN978-3-540-52048-1, MR1048636. Gerlach, T. (2004), "Toughness and Hamiltonicity of a class of planar graphs", Discrete Mathematics, 286 (1–2): 61–65, doi:10.1016/j.disc.2003.11.046, MR2084280.
Politof (1983) introduced the Δ-Y reducible planar graphs and characterized them in terms of forbidden homeomorphic subgraphs. The duality between the Δ-Y and Y-Δ reducible graphs, the forbidden minor characterizations of both classes, and the connection to planar partial 3-trees are all from El-Mallah & Colbourn (1990). Politof, T. (1983), A characterization and efficient reliability computation of Δ-Y reducible networks, Ph.D. thesis, University of California, Berkeley. As cited by El-Mallah & Colbourn (1990). El-Mallah, Ehab S.; Colbourn, Charles J. (1990), "On two dual classes of planar graphs", Discrete Mathematics, 80 (1): 21–40, doi:10.1016/0012-365X(90)90293-Q, MR1045921.
For the characterization in terms of the maximum number of triangles in a planar graph, see Hakimi & Schmeichel (1979). Alon & Caro (1984) quote this result and provide the characterizations in terms of the isomorphism classes of blocks and numbers of blocks. The bound on the total number of cliques follows easily from the bounds on triangles and K4 subgraphs, and is also stated explicitly by Wood (2007), who provides an Apollonian network as an example showing that this bound is tight. For generalizations of these bounds to nonplanar surfaces, see Dujmović et al. (2009). Hakimi, S. L.; Schmeichel, E. F. (1979), "On the number of cycles of length k in a maximal planar graph", Journal of Graph Theory, 3 (1): 69–86, doi:10.1002/jgt.3190030108, MR0519175. Alon, N.; Caro, Y. (1984), "On the number of subgraphs of prescribed type of planar graphs with a given number of vertices", in Rosenfeld, M.; Zaks, J. (eds.), Convexity and Graph Theory: proceedings of the Conference on Convexity and Graph Theory, Israel, March 1981, Annals of Discrete Mathematics 20, North-Holland Mathematical Studies 87, Elsevier, pp. 25–36, ISBN978-0-444-86571-7, MR0791009. Wood, David R. (2007), "On the maximum number of cliques in a graph", Graphs and Combinatorics, 23 (3): 337–352, arXiv:math/0602191, doi:10.1007/s00373-007-0738-8, MR2320588, S2CID46700417. Dujmović, Vida; Fijavž, Gašper; Joret, Gwenaël; Wood, David R. (2009), "The maximum number of cliques in a graph embedded in a surface", European Journal of Combinatorics, 32 (8): 1244–1252, arXiv:0906.4142, Bibcode:2009arXiv0906.4142D, doi:10.1016/j.ejc.2011.04.001, S2CID1733300.
For subdividing a triangle with rational side lengths so that the smaller triangles also have rational side lengths, see Almering (1963). For progress on the general problem of finding planar drawings with rational edge lengths, see Geelen, Guo & McKinnon (2008). Almering, J. H. J. (1963), "Rational quadrilaterals", Indagationes Mathematicae, 25: 192–199, doi:10.1016/S1385-7258(63)50020-1, MR0147447. Geelen, Jim; Guo, Anjie; McKinnon, David (2008), "Straight line embeddings of cubic planar graphs with integer edge lengths", Journal of Graph Theory, 58 (3): 270–274, doi:10.1002/jgt.20304, MR2419522.
For the characterization in terms of the maximum number of triangles in a planar graph, see Hakimi & Schmeichel (1979). Alon & Caro (1984) quote this result and provide the characterizations in terms of the isomorphism classes of blocks and numbers of blocks. The bound on the total number of cliques follows easily from the bounds on triangles and K4 subgraphs, and is also stated explicitly by Wood (2007), who provides an Apollonian network as an example showing that this bound is tight. For generalizations of these bounds to nonplanar surfaces, see Dujmović et al. (2009). Hakimi, S. L.; Schmeichel, E. F. (1979), "On the number of cycles of length k in a maximal planar graph", Journal of Graph Theory, 3 (1): 69–86, doi:10.1002/jgt.3190030108, MR0519175. Alon, N.; Caro, Y. (1984), "On the number of subgraphs of prescribed type of planar graphs with a given number of vertices", in Rosenfeld, M.; Zaks, J. (eds.), Convexity and Graph Theory: proceedings of the Conference on Convexity and Graph Theory, Israel, March 1981, Annals of Discrete Mathematics 20, North-Holland Mathematical Studies 87, Elsevier, pp. 25–36, ISBN978-0-444-86571-7, MR0791009. Wood, David R. (2007), "On the maximum number of cliques in a graph", Graphs and Combinatorics, 23 (3): 337–352, arXiv:math/0602191, doi:10.1007/s00373-007-0738-8, MR2320588, S2CID46700417. Dujmović, Vida; Fijavž, Gašper; Joret, Gwenaël; Wood, David R. (2009), "The maximum number of cliques in a graph embedded in a surface", European Journal of Combinatorics, 32 (8): 1244–1252, arXiv:0906.4142, Bibcode:2009arXiv0906.4142D, doi:10.1016/j.ejc.2011.04.001, S2CID1733300.
The equivalence of planar 3-trees and chordal maximal planar graphs was stated without proof by Patil (1986). For a proof, see Markenzon, Justel & Paciornik (2006). For a more general characterization of chordal planar graphs, and an efficient recognition algorithm for these graphs, see Kumar & Madhavan (1989). The observation that every chordal polyhedral graph is maximal planar was stated explicitly by Gerlach (2004). Patil, H. P. (1986), "On the structure of k-trees", Journal of Combinatorics, Information and System Sciences, 11 (2–4): 57–64, MR0966069. Markenzon, L.; Justel, C. M.; Paciornik, N. (2006), "Subclasses of k-trees: Characterization and recognition", Discrete Applied Mathematics, 154 (5): 818–825, doi:10.1016/j.dam.2005.05.021, MR2207565. Kumar, P. Sreenivasa; Madhavan, C. E. Veni (1989), "A new class of separators and planarity of chordal graphs", Foundations of Software Technology and Theoretical Computer Science, Ninth Conference, Bangalore, India December 19–21, 1989, Proceedings, Lecture Notes in Computer Science, vol. 405, Springer-Verlag, pp. 30–43, doi:10.1007/3-540-52048-1_30, ISBN978-3-540-52048-1, MR1048636. Gerlach, T. (2004), "Toughness and Hamiltonicity of a class of planar graphs", Discrete Mathematics, 286 (1–2): 61–65, doi:10.1016/j.disc.2003.11.046, MR2084280.
The fact that Apollonian networks minimize the number of colorings with more than four of colors was shown in a dual form for colorings of maps by Birkhoff (1930). Birkhoff, George D. (1930), "On the number of ways of colouring a map", Proceedings of the Edinburgh Mathematical Society, (2), 2 (2): 83–91, doi:10.1017/S0013091500007598.
Politof (1983) introduced the Δ-Y reducible planar graphs and characterized them in terms of forbidden homeomorphic subgraphs. The duality between the Δ-Y and Y-Δ reducible graphs, the forbidden minor characterizations of both classes, and the connection to planar partial 3-trees are all from El-Mallah & Colbourn (1990). Politof, T. (1983), A characterization and efficient reliability computation of Δ-Y reducible networks, Ph.D. thesis, University of California, Berkeley. As cited by El-Mallah & Colbourn (1990). El-Mallah, Ehab S.; Colbourn, Charles J. (1990), "On two dual classes of planar graphs", Discrete Mathematics, 80 (1): 21–40, doi:10.1016/0012-365X(90)90293-Q, MR1045921.
For the characterization in terms of the maximum number of triangles in a planar graph, see Hakimi & Schmeichel (1979). Alon & Caro (1984) quote this result and provide the characterizations in terms of the isomorphism classes of blocks and numbers of blocks. The bound on the total number of cliques follows easily from the bounds on triangles and K4 subgraphs, and is also stated explicitly by Wood (2007), who provides an Apollonian network as an example showing that this bound is tight. For generalizations of these bounds to nonplanar surfaces, see Dujmović et al. (2009). Hakimi, S. L.; Schmeichel, E. F. (1979), "On the number of cycles of length k in a maximal planar graph", Journal of Graph Theory, 3 (1): 69–86, doi:10.1002/jgt.3190030108, MR0519175. Alon, N.; Caro, Y. (1984), "On the number of subgraphs of prescribed type of planar graphs with a given number of vertices", in Rosenfeld, M.; Zaks, J. (eds.), Convexity and Graph Theory: proceedings of the Conference on Convexity and Graph Theory, Israel, March 1981, Annals of Discrete Mathematics 20, North-Holland Mathematical Studies 87, Elsevier, pp. 25–36, ISBN978-0-444-86571-7, MR0791009. Wood, David R. (2007), "On the maximum number of cliques in a graph", Graphs and Combinatorics, 23 (3): 337–352, arXiv:math/0602191, doi:10.1007/s00373-007-0738-8, MR2320588, S2CID46700417. Dujmović, Vida; Fijavž, Gašper; Joret, Gwenaël; Wood, David R. (2009), "The maximum number of cliques in a graph embedded in a surface", European Journal of Combinatorics, 32 (8): 1244–1252, arXiv:0906.4142, Bibcode:2009arXiv0906.4142D, doi:10.1016/j.ejc.2011.04.001, S2CID1733300.
For subdividing a triangle with rational side lengths so that the smaller triangles also have rational side lengths, see Almering (1963). For progress on the general problem of finding planar drawings with rational edge lengths, see Geelen, Guo & McKinnon (2008). Almering, J. H. J. (1963), "Rational quadrilaterals", Indagationes Mathematicae, 25: 192–199, doi:10.1016/S1385-7258(63)50020-1, MR0147447. Geelen, Jim; Guo, Anjie; McKinnon, David (2008), "Straight line embeddings of cubic planar graphs with integer edge lengths", Journal of Graph Theory, 58 (3): 270–274, doi:10.1002/jgt.20304, MR2419522.
For the characterization in terms of the maximum number of triangles in a planar graph, see Hakimi & Schmeichel (1979). Alon & Caro (1984) quote this result and provide the characterizations in terms of the isomorphism classes of blocks and numbers of blocks. The bound on the total number of cliques follows easily from the bounds on triangles and K4 subgraphs, and is also stated explicitly by Wood (2007), who provides an Apollonian network as an example showing that this bound is tight. For generalizations of these bounds to nonplanar surfaces, see Dujmović et al. (2009). Hakimi, S. L.; Schmeichel, E. F. (1979), "On the number of cycles of length k in a maximal planar graph", Journal of Graph Theory, 3 (1): 69–86, doi:10.1002/jgt.3190030108, MR0519175. Alon, N.; Caro, Y. (1984), "On the number of subgraphs of prescribed type of planar graphs with a given number of vertices", in Rosenfeld, M.; Zaks, J. (eds.), Convexity and Graph Theory: proceedings of the Conference on Convexity and Graph Theory, Israel, March 1981, Annals of Discrete Mathematics 20, North-Holland Mathematical Studies 87, Elsevier, pp. 25–36, ISBN978-0-444-86571-7, MR0791009. Wood, David R. (2007), "On the maximum number of cliques in a graph", Graphs and Combinatorics, 23 (3): 337–352, arXiv:math/0602191, doi:10.1007/s00373-007-0738-8, MR2320588, S2CID46700417. Dujmović, Vida; Fijavž, Gašper; Joret, Gwenaël; Wood, David R. (2009), "The maximum number of cliques in a graph embedded in a surface", European Journal of Combinatorics, 32 (8): 1244–1252, arXiv:0906.4142, Bibcode:2009arXiv0906.4142D, doi:10.1016/j.ejc.2011.04.001, S2CID1733300.
This graph is named after the work of Goldner & Harary (1975); however, it appears earlier in the literature, for instance in Grünbaum (1967), p. 357. Goldner, A.; Harary, F. (1975), "Note on a smallest nonhamiltonian maximal planar graph", Bull. Malaysian Math. Soc., 6 (1): 41–42. See also the same journal 6(2):33 (1975) and 8:104-106 (1977). Reference from listing of Harary's publications. Grünbaum, Branko (1967), Convex Polytopes, Wiley Interscience.
For the characterization in terms of the maximum number of triangles in a planar graph, see Hakimi & Schmeichel (1979). Alon & Caro (1984) quote this result and provide the characterizations in terms of the isomorphism classes of blocks and numbers of blocks. The bound on the total number of cliques follows easily from the bounds on triangles and K4 subgraphs, and is also stated explicitly by Wood (2007), who provides an Apollonian network as an example showing that this bound is tight. For generalizations of these bounds to nonplanar surfaces, see Dujmović et al. (2009). Hakimi, S. L.; Schmeichel, E. F. (1979), "On the number of cycles of length k in a maximal planar graph", Journal of Graph Theory, 3 (1): 69–86, doi:10.1002/jgt.3190030108, MR0519175. Alon, N.; Caro, Y. (1984), "On the number of subgraphs of prescribed type of planar graphs with a given number of vertices", in Rosenfeld, M.; Zaks, J. (eds.), Convexity and Graph Theory: proceedings of the Conference on Convexity and Graph Theory, Israel, March 1981, Annals of Discrete Mathematics 20, North-Holland Mathematical Studies 87, Elsevier, pp. 25–36, ISBN978-0-444-86571-7, MR0791009. Wood, David R. (2007), "On the maximum number of cliques in a graph", Graphs and Combinatorics, 23 (3): 337–352, arXiv:math/0602191, doi:10.1007/s00373-007-0738-8, MR2320588, S2CID46700417. Dujmović, Vida; Fijavž, Gašper; Joret, Gwenaël; Wood, David R. (2009), "The maximum number of cliques in a graph embedded in a surface", European Journal of Combinatorics, 32 (8): 1244–1252, arXiv:0906.4142, Bibcode:2009arXiv0906.4142D, doi:10.1016/j.ejc.2011.04.001, S2CID1733300.