Solutions of “double soliton” type for the multidimensional equation $\Box u=F(u)$

Let $V$ be an even function, the Taylor series of which takes the form $V(u)\sim\frac{u^2}{2!}-\frac{u^4}{4!} + au^6 + \dots$ . It is shown that there exists the unique nontrivial series $u=\sum\limits_{k\geq 0} u_k (\xi,\eta)\mu^{2k}$, $\xi=\mu x$, $\eta=\omega^{-1}\mu\cos \omega t$, $\mu=\sqrt{1-\omega^2}$ ($\omega, \omega^2<1$ – is arbitrary parameter), which satisfies the equation $\Box u=-V'(u)$ and the coefficients of which are exponentially decreasing functions.