Left frame: Grid 𝒢₁ marked by black crosses, which is the work domain for the integrable approximation ${\overline{H}}_a$. Red (blue) curve marks the location of resonance center (saddle) at ${\overline{Q}}_1=0\left({\overline{Q}}_1=\pi \right)$. Green and yellow curves are the libration boundaries for centers at ${\overline{Q}}_1=0$. Right frame: Projection of the grid 𝒢₁ as well as the resonance equilibriums and libration boundaries onto the a₁ − β₁ plane.