Minimum rank and zero forcing number for butterfly networks

The minimum rank of a simple graph $G$ is the smallest possible rank over all symmetric real matrices $A$ whose nonzero off-diagonal entries correspond to the edges of $G$. Using the zero forcing number, we prove that the minimum rank of the butterfly network is $\frac{1}{9}[(3r+1)2^{r+1}-2(-1{)}^r]$ and that this is equal to the rank of its adjacency matrix.