(Following Landau para 27)
A one-dimensional simple harmonic oscillator, a mass on a spring,
has two parameters, and For some systems, the parameters can be changed externally (an example being the length of a pendulum if at the top end the string goes over a pulley).
We are interested here in the system's response to some externally imposed periodic variation of its parameters, and in particular we'll be looking at resonant response, meaning large response to a small imposed variation.
Note first that imposed variation in the mass term is easily dealt with, by simply redefining the time variable to , meaning
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