Remarks on self-affine fractals with polytope convex hulls

Suppose that the set T = {T₁, T₂,..., T_q} of real n × n matrices has joint spectral radius less than 1. Then for any digit set D = {d₁,..., d_q} ⊂ ℝⁿ, there exists a unique non-empty compact set F = F(T , D) satisfying F = ∪_j=1^q T_j(F+d_j), which is typically a fractal set. We use the infinite digit expansions of the points of F to give simple necessary and sufficient conditions for the convex hull of F to be a polytope. Additionally, we present a technique to determine the vertices of such polytopes. These answer some of the related questions of Strichartz and Wang, and also enable us to approximate the Lebesgue measure of such self-affine sets. To show the use of our results, we also give several examples including the Levy dragon and the Heighway dragon.