Algebraic Theory for Multivariable Linear Systems (Enhanced Edition‪)‬

The early theory of control systems as developed and applied during and
after World War I1 was based almost entirely on the transfer function
technique. The mathematical framework needed for this purpose was
provided by the mathematics of the Laplace and Fourier transforms and
later also by the mathematics of the 2-transform. The transfer function
technique was straightforward to apply and easy to grasp, at least as far
as single input-single output systems were concerned. Its close relationship
with experimental methods based on frequency response measurements
made it particularly attractive to people working with practical control
problems.