  It is shown that the interacting Pais-Uhlenbeck oscillator necessarily leads
to a description with a Hamiltonian that contains positive and negative
energies associated with two oscillators. Descriptions with a positive definite
Hamiltonians, considered by some authors, can hold only for a free
Pais-Uhlenbeck oscillator. We demonstrate that the solutions of a
self-interacting Pais-Uhlenbeck oscillator are stable on islands in the
parameter space, as already observed in the literature. If we slightly modify
the system, by considering a sine interaction term, and/or by taking unequal
masses of the two oscillators, then the system is stable on the continents that
extend from zero to infinity in the parameter space. Therefore, the
Pais-Uhlenbeck oscillator is quite acceptable physical system.
