Linear Time Invariant

A Linear Time Invariant System is one that:
  1. Is unaffacted by time. That is, if you perform an experiment on Monday to find the systems response to a sine wave, you will get the same result if you do the experiment again on Wednesday.
  2. Is Linear. Given two input signals (ax, cx) and that they produce two output signals (by, dy), the system is linear if, and only if, the input signal ax + cx produces the output signal by + dy

    3.  

     
     
     

More formally:

 In a Time Invariant system:

 In a Linear System:

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