Growth of a peanut shaped bulge via resonant trapping of stellar orbits   in the vertical Inner Lindblad Resonances
  We present a simple resonant Hamiltonian model for the vertical response of a stellar disk to the growth of a bar perturbation. As a bar perturbation grows stars become trapped in vertical Inner Lindblad resonances and are lifted into higher amplitude orbits. The vertical structure of a boxy and peanut shaped bulge as a function of radius and azimuthal angle in the galaxy plane can be predicted from the strength and speed of the bar perturbation and the derivatives of the gravitational potential. This model predicts that stars on the outer side of the resonance are lifted higher than stars on the inner side, offering an explanation for the sharp outer edge of the boxy/peanut. 
A. C. Quillen