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Example : undamped Duffing equation Differential equation and energy conservation As an example for the method of multiple-scale analysis , consider the undamped and unforced Duffing equation : which is a second-order ordinary differential equation describing a nonlinear oscillator .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
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