One important problem in system analysis is to determine, see Figure 1, the output (or
response) y(t) or y[k] of a system stimulated by a known input signal u(t) or u[k], where x(t)
or x[k] denotes continuous-time or discrete-time signals and systems, respectively. If the
system is memoryless, the relationship between the input and output can generally be
described by an algebraic function where the solution can be determined directly though
algebraic manipulations: addition, subtraction, multiplication, and division.
Whereas memory linear time-invariant lumped (LTIL) systems have the relationship between the
inputs and outputs described in terms of convolutions, difference or differential equations.
These equations involve integrations and/or differentiations and cannot be studied directly using
only algebraic manipulation. However, they can be changed into algebraic forms after applying
some transformations, e.g. Laplace or Z-transformations. These transformations turn the
problems defined in time domain to algebraic equations in S or Z complex domain and this can
normally be performed on a paper. The limitation of this approach is that only analytic signals or functions, rather than arbitrary ones, are possible to find the transformations. Therefore, this
approach is not practical in the cases of general signals and systems.