Fig. 3

Period ratios P1/ 2P2 (triangles) and P1/ 3P3 (squares) as a function of the aspect ratio a/L. The symbols represent numerical calculations. The solid lines correspond to the analytical expressions for the ratios given by Eqs. (3) and (5), while the dashed lines represent approximations for a high density contrast given by Eqs. (4) and (6).
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