Before revision:  Two model simplification methods are applied to multi-variable models: (1) the phase response curves (PRC) and the circadian phase dynamics of these models are generated by impulse response simulation; (2) the Principal Orthogonal Decomposition (POD) is performed to reduce these models to ones with merely two variables.
After revision:  Two model simplification methods are applied to multi-variable models: (1) the phase response curves (PRC) and the Principal Orthogonal Decomposition (POD) is performed to reduce these models to ones with merely two variables.