The Möbius matroids

In three influential papers in the 1980s and early 1990s, Joe Kung laid the foundations for extremal matroid theory which he envisaged as finding the growth rate of certain classes of matroids along with a characterisation of the extremal matroids in each such class. At the time, he was particularly interested in the minor-closed classes of binary matroids obtained by excluding the cycle matroids of the Kuratowski graphs K3,3 and/or K5. While he obtained strong bounds on the growth rate of these classes, it seems difficult to give the exact growth rate without a complete characterisation of the matroids in each class, which at the time seemed hopelessly complicated. Many years later, Mayhew, Royle and Whittle gave a characterisation of the internally 4-connected binary matroids with no M(K3,3)-minor, from which the answers to Kung's questions follow immediately. In this characterisation, two thin families of binary matroids play an unexpectedly important role as the only non-cographic infinite families of internally 4-connected binary matroids with no M(K3,3)-minor. As the matroids are closely related to the cubic and quartic Möbius ladders, they were called the triangular Möbius matroids and the triadic Möbius matroids. Preliminary investigations of the class of binary matroids with no M(K5)-minor suggest that, once again, the triangular Möbius matroids will be the extremal internally 4-connected matroids in this class. Here we undertake a systematic study of these two families of binary matroids collecting in one place fundamental information about them, including their representations, connectivity properties, minor structure, automorphism groups and their chromatic polynomials. Along the way, we highlight the different ways in which these matroids have arisen naturally in a number of results and problems (both open and settled) in structural and extremal matroid theory. © 2019