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If there is a function that satisfies , it must be a solution of the differential equation , which is a third-order autonomous ordinary differential equation.
Its characteristic equation has a real root and two complex conjugate roots, they are , and .
The real root suggests that is a linearly independent solution; the complex conjugate roots suggest that and are two linearly independent solutions of the differential equation.
Therefore, the general solution of the differential equation is .
Using Euler's identity, we can rewrite the last two terms as and .
Regrouping and simplification of the constants gives the final result,
.
Due to , there must be .
Some of my other answers on differential equations: