Descriptor state-space models can include both dynamic and algebraic equations, as is common in electrical circuits or constrained mechanical systems. This makes modeling easier and allows for more general systems. Wolfram Language descriptor state-space models generalize the full suite of modeling, analysis, and design functionality supported by standard state-space models.
StateSpaceModel — model a linear descriptor system
TransferFunctionModel — model an improper transfer function
StateFeedbackGains — pole placement-based feedback gains
LQEstimatorGains — LQ-based state estimator gains
KroneckerModelDecomposition — decouple fast and slow subsystems