GPT答：

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(1) Analyze the common forms of gear faults in compound planetary gear systems and their causes. Establish vibration signal models for single and compound faults in compound planetary gear systems based on the transmission principle of the system, and conduct simulations. Analyze the characteristics of local faults and compound fault models in sun gears and external planet gears, and calculate the characteristic frequencies of major components that experience local faults in compound planetary gear systems.