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===Coordinate transform to amplitude/phase variables===
Alternatively, modern approaches derive these sorts of models using coordinate transforms, like in the [[Normal form (dynamical systems)|method of normal forms]],
A solution <math>y\approx r\cos\theta</math> is sought in new coordinates <math>(r,\theta)</math> where the amplitude <math>r(t)</math> varies slowly and the phase <math>\theta(t)</math> varies at an almost constant rate, namely <math>d\theta/dt\approx 1.</math> Straightforward algebra finds the coordinate transform{{citation needed|date=June 2015}}
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==References==
{{
*{{citation
| last1=Kevorkian | first1=J.
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| publisher = Wiley–VCH Verlag | isbn = 978-0-471-39917-9
}}
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==External links==
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