The energy-frequency diagram of the (1+1)-dimensional $\Phi^4$ oscillon
Two different methods are used to study the existence and stability of the (1+1)-dimensional $\Phi^4$ oscillon. The variational technique approximates it by a periodic function with a set of adiabatically changing parameters. An alternative approach treats oscillons as standing waves in a finite-size box; these are sought as solutions of a boundary-value problem on a two-dimensional domain. The numerical analysis reveals that the standing wave's energy-frequency diagram is fragmented into disjoint segments with $\omega_{n+1} < \omega < \omega_{n}$, where $\omega_n= \omega_0/ (n+1)$, $n=0,1,2, ...$, and $\omega_0$ is the endpoint of the continuous spectrum (mass threshold of the model). The variational approximation involving the first, zeroth and second harmonic components provides an accurate description of the oscillon with the frequency in $(\omega_1, \omega_0)$, but breaks down as $\omega$ falls out of that interval.