2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/194912The development of instability in the dynamics of theories with higher derivatives is traced in detail in the framework of the Pais-Uhlenbeck fourth oder oscillator. For this aim the external friction force is introduced in the model and the relevant solutions to equations of motion are investigated. As a result, the physical implication of the energy unboundness from below in theories under consideration is revealed.9 pages, no figures and no tables, revtex4; a few misprints are correctedHigh Energy Physics - Theorytext