Resonant structure determined with the semianalytical model in the (a₂, e₂) plane surrounding the middle planet (top) and in the (a₃, e₃) plane (bottom). Two-planet commensurabilities between planets m₁ and m₂ are shown in red, and those between the middle and outer planets are presented in blue. In both plots the nominal positions of the planets are identified by filled black circles, and the collision curves are depicted in gray.