Applications of the Strong Splitter Theorem: decomposition results

Abstract

We use the Strong Splitter Theorem to decompose the excluded minor class of binary matroids with no $E_4$-minor. Using this theorem we can get the 3-decomposers and the extremal internally 4-connected matroids as well as any other important matroids in the class. The matroid $E_4$ is a self-dual 10-element binary 3-connected matroid that plays a useful role in structural results. It is a single-element coextension of $P_9$, which is a single-element extension of the 4-wheel. We show that the extremal matroids in this class are the binary rank-$r$ spikes $Z_r$, the rank 3 and 4 projective geometries $F_7$ and $PG(3,2)$, respectively, the 17-element internally 4-connected matroid $R_{17}$, and one 12-element rank-6 matroid. All the other 3-connected members have $P_9$ or $P_9^*$ as 3-decomposers. As immediate corollaries we get decomposition results for $EX[P_9^*]$ and $EX[P_9]$ as well as the internally 4-connected members of these classes.