“…From the open literature, analytic solutions for free in-plane vibration of orthotropic rectangular plates with non-Lévy-type boundary conditions, i.e., those without two opposite edges simply supported, are still quite deficient, which motivates the present work. It is noteworthy that a symplectic superposition method (SSM) has been developed recently to deal with plate and shell problems involving out-of-plane deformation, including bending 17,18 , buckling 19,20 , and transverse vibration [21][22][23] . Such an analytic method is developed based on an elegant integration of the superposition technique and the symplectic approach 24,25 ; it is not conducted in the Euclidean space but in the symplectic space where several important mathematical treatments, such as the separation of variables and the symplectic eigen expansion, prove to be valid.…”