An anharmonic oscillator ( in contrast to a simple harmonic oscillator ) is one in which the potential energy is not quadratic in the extension " q " ( the generalized position which measures the deviation of the system from equilibrium ).
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In the paper, Heisenberg tried to explain the energy levels of a one-dimensional anharmonic oscillator, avoiding the concrete but unobservable representations of electron orbits by using observable parameters such as transition probabilities for quantum jumps, which necessitated using two indexes corresponding to the initial and final states.
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Further examples of anharmonic oscillators include the large-angle pendulum; nonequilibrium semiconductors that possess a large hot carrier population, which exhibit nonlinear behaviors of various types related to the effective mass of the carriers; and ionospheric plasmas, which also exhibit nonlinear behavior based on the anharmonicity of the plasma.
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In his work a simple example of an anharmonic oscillator evolving under the force of the form as a superposition of incoherent sinusoidal oscillations with continuous spectrum was used to show that depending on a specific approximation time scale the evolution of the system can be either deterministic, or a stochastic process satisfying Fokker-Planck equation, or even a process which is neither deterministic nor stochastic.
