“…However, he concluded that in contrast to the linear case in which the chessboard de#ection pattern is a natural choice, its selection in the non-linear problem must be more carefully assessed [75]. It may be worth noticing here that the question of the choice of the de#ection shape has been rediscussed recently in reference [92]. Using Donnell's shallow-shell theory, Reissner isolated a single half-wave (lobe) of the vibration mode, and analyzed it for simply supported shells, and found that the non-linearity could be either of the hardening or softening type, depending upon the geometry of the lobe.…”