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Subharmonic oscillation is an undesired phenomenon in switching converters. In past studies, its prediction has been mainly tackled by explicitly deriving a discrete-time model and then linearizing it in the vicinity of the operating point. However, the results obtained from such an approach cannot be applied for design purpose except for simple cases such as peak or valley current-mode control (CMC). Alternatively, in this brief, this phenomenon is analyzed by using a unified formal symbolic approach that can be applied for different control strategies. This approach is based on expressing the condition for subharmonic oscillation occurrence using Fourier series and then converting the result into a matrix-form expression that explicitly depends on the system parameters, making the results directly applicable for design purpose. Under certain practical conditions concerning these parameters, the matrix-form expression can be approximated by standard polynomial functions depending on the operating duty cycle. The results presented in this work clearly generalize the well-known stability condition of peak/valley CMC.