The transfer function of a linear time-invariant system describes the input output behaviorrnof the system when all initial conditions are zero. Hence, it should be independentrnof the state variables used in a state space description of the system. A state space descriptionrnof an input-output system can be regarded as a time-domain description. Forrnlinear time-invariant systems, transfer functions provide another way of describing inputoutputrnsystems; this description is sometimes called a frequency domain description. Torndiscuss transfer functions of continuous-time systems, we need the Laplace transform