Informal Friday Seminar Speaker: Finn Klein, The University of Sydney Abstract: I will give an introduction to selected aspects of the theory of matroids, a combinatorial theory that unifies aspects of linear algebra, graph theory, polytope theory and many other fields. I will motivate the definition of a matroid from these different viewpoints and discuss in more detail the problem of when a matroid is realisable, i.e. arises from linear algebra. The space of realisations of a given matroid can exhibit arbitrary pathologies, which can be beautifully illustrated using relatively basic constructions in projective geometry. If time allows, I will touch on the case of positroids, whose realisation spaces are much better behaved. The talk is aimed at a broad mathematical audience. Details: The Informal Friday Seminar is a space where members of our research group can explain interesting things to each other in a casual setting. The rules of the seminar are the following: Any question/additional explanation from the audience is allowed. Ego out the window. Examples, examples, examples. If you would like to join the mailing list or contribute a talk, please email finn.klein@sydney.edu.au.