A pole of the causally continued collective kinetic response, usually a zero of its scalar response denominator or response-matrix determinant. With time dependence , negative imaginary part gives damping, positive imaginary part growth, and zero imaginary part an undamped oscillation. Damped poles need not be regular discrete eigenmodes of the original collisionless operator: their contribution is obtained by continuing the initial-value response. The least damped excited pole can set the waiting time for a stationary forced response.
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