A characterization of the ternary matroids with no M(K4)-minor
Brylawski identified the class of binary matroids with no minor isomorphic to M(K4) as being the class of series-parallel networks. From this he deduced that, for all such matroids M, the critical exponent c(M; 2) is at most 2. He also conjectured that a similar result is true over all finite fields GF(q). This paper examines the classes of ternary and GF(4)-representable matroids with no M(K4)-minor. The main result characterizes the former class by showing that, with one exception, the only non-trivial 3-connected members of this class are whirls or minors of the Steiner system S(5, 6, 12). This characterization is then used to show that, for all ternary matroids M with no M(K4)-minor, c(M; 3)≤2, thereby verifying Brylawski's conjecture in the case that q = 3. The characterization is also used to give excluded-minor descriptions for the class of ternary gammoids and two other related classes. The first of these results answers a question of Ingleton and verifies another conjecture of Brylawski. © 1987.
Publication Source (Journal or Book title)
Journal of Combinatorial Theory, Series B
Oxley, J. (1987). A characterization of the ternary matroids with no M(K4)-minor. Journal of Combinatorial Theory, Series B, 42 (2), 212-249. https://doi.org/10.1016/0095-8956(87)90041-4