Lovász (2006) is inconsistent about whether to allow self-loops and multiple adjacencies: he writes on p. 76 that "parallel edges and loops are allowed" but on p. 77 he states that "a graph is a forest if and only if it does not contain the triangle K3 as a minor", true only for simple graphs. Lovász, László (2006), "Graph minor theory", Bulletin of the American Mathematical Society, 43 (1): 75–86, doi:10.1090/S0273-0979-05-01088-8.
Robertson & Seymour (1993); Demaine, Hajiaghayi & Thilikos (2002). Robertson, Neil; Seymour, Paul D. (1993), "Excluding a graph with one crossing", in Robertson, Neil; Seymour, Paul (eds.), Graph Structure Theory: Proc. AMS–IMS–SIAM Joint Summer Research Conference on Graph Minors, Contemporary Mathematics, vol. 147, American Mathematical Society, pp. 669–675. Demaine, Erik D.; Hajiaghayi, MohammadTaghi; Thilikos, Dimitrios M. (2002), "1.5-Approximation for treewidth of graphs excluding a graph with one crossing as a minor", Proc. 5th International Workshop on Approximation Algorithms for Combinatorial Optimization (APPROX 2002), Lecture Notes in Computer Science, vol. 2462, Springer-Verlag, pp. 67–80, doi:10.1007/3-540-45753-4_8, ISBN978-3-540-44186-1