ABSTRACT
This chapter examines the basic properties of linear time invariant (LTI) systems based on the physical postulates such systems satisfy in the time domain. The treatment is from an elementary point of view; nevertheless, in order to lend some generality to the discussion an operator formalism is used. Qualitatively speaking, a linear system is one in which the law of superposition holds. That is if two individual signals are superposed, then the respective responses to these signals are similarly superposed to give the total response. Otherwise stated, the response is proportional to the stimulus. If a time invariant system starts in a completely deenergized state when excited, its response will always be the same measured from the time of initiation of the input signal. These properties can be precisely phrased in terms of the operators which reflect the physical properties of the system.
