We prove the conjecture of A. Postnikov that ($\mathrm{A}$) the number of regions in the inversion hyperplane arrangement associated with a permutation $w \in \mathfrak{S}_n$ is at most the number of elements below $w$ in the Bruhat order. and ($\mathrm{B}$) that equality holds if and only if $w$ avoids the patterns $4231$. $35142$. https://smokeysmountainfurnitureoutlets.shop/product-category/clock/