  The study shows that errors exist in the derivation of equilibrium equations
in terms of displacement. It is discovered that when the equilibrium equations
in terms of displacement are derived, the variation of the differential order
of displacement may cause the variation of the stress state in an elastomer.
For plane stress problems, the Lame-Navier equations are not equivalent to the
equilibrium equations described with stress. By submitting the displacement
field of the well-known issue of a rectangular beam purely bent into the
Lame-Navier equations, the conclusion is confirmed. It is also revealed that
the velocity of longitudinal wave in an elastomer is affected by its thickness.
