On the homotopy type of the Acyclic MacPhersonian Ricardo Strausz
[email protected] September 24, 2019 In 2003 Daniel K. Biss published, in the Annals of Mathematics [1], what he thought was a solution of a long standing problem, culminating a discovery due to Gel'fand and MacPher- son [4]; namely, he claimed that the MacPhersonian |a.k.a. the matroid Grassmannian| has the same homotoy type of the Grassmannian. Six years later he was encouraged to publish an \erratum" to his proof [2], observed by Nikolai Mn¨ev;up to now, the homotopy type of the MacPhersonian remains a mystery... d The Acyclic MacPhersonian AMn is to the MacPhersonian, as it is the class of all acyclic oriented matroids, of a given dimension d and order n, to the class of all oriented matroids, of the same dimension and order. The aim of this note is to study the homotopy type d of AMn in comparison with the Grassmannian G(n − d − 1; n − 1), which consists of all n − d − 1 linear subspaces of IRn−1, with the usual topology | recall that, due to the duality given by the \orthogonal complement", G(n − d − 1; n − 1) and G(d; n − 1) are homeomorphic. For, we will define a series of spaces, and maps between them, which we summarise in the following diagram: d { d gGn / AMgn π F S η { { Ö m:c:f: ~ d t d π { Gn o Sn τ arXiv:1504.06592v2 [math.AT] 21 Sep 2019 & d { d Gcn / AMn In this diagram, the maps labeled { represents inclusions and those labeled π are natural projections; the compositions of these will prove homotopical equivalences.