Authors: Federico Ardila ^1,2; Federico Castillo ³; Jose Samper ⁴

• 1 San Francisco State University
• 2 Universidad de los Andes [Bogota]
• 3 University of California [Davis]
• 4 University of Washington [Seattle]

We prove that the external activity complex Act<(M) of a matroid is shellable. In fact, we show that every linear extension of Las Vergnas's external/internal order <ext/int on M provides a shelling of Act<(M). We also show that every linear extension of Las Vergnas's internal order <int on M provides a shelling of the independence complex IN(M). As a corollary, Act<(M) and M have the same h-vector. We prove that, after removing its cone points, the external activity complex is contractible if M contains U3,1 as a minor, and a sphere otherwise.