.. currentmodule:: control

The classes listed below are used to represent models of input/output systems (both linear time-invariant and nonlinear). They are usually created from factory functions such as :func:`tf` and :func:`ss`, so the user should normally not need to instantiate these directly.

.. autosummary::
   :toctree: generated/
   :template: custom-class-template.rst

   InputOutputSystem
   LTI
   StateSpace
   TransferFunction
   FrequencyResponseData
   NonlinearIOSystem
   InterconnectedSystem
   LinearICSystem

The following figure illustrates the relationship between the classes and some of the functions that can be used to convert objects from one class to another:

.. autosummary::
   :template: custom-class-template.rst
   :nosignatures:

   DescribingFunctionNonlinearity
   DescribingFunctionResponse
   flatsys.BasisFamily
   flatsys.FlatSystem
   flatsys.LinearFlatSystem
   flatsys.PolyFamily
   flatsys.SystemTrajectory
   optimal.OptimalControlProblem
   optimal.OptimalControlResult
   optimal.OptimalEstimationProblem
   optimal.OptimalEstimationResult

The use of these classes is described in more detail in the :ref:`flatsys-module` module and the :ref:`optimal-module` module