I am reading a book on Control Theory by Norman Nise. I don't get his description of forced and natural response based on poles and zeroes. I have posted a screenshot of the page http://www.flickr.com/photos/66943862@N06/6134272368/sizes/l/in/photostream/" [Broken]. I understand upto Eq 4.2. I do not understand how he arrives at Points 1,2,3 & 4 after Eq 4.2 I guess "Pole of the Input function" means when s = 0. How does this generate the form of a forced response? Likewise for points 2, 3 & 4? If anyone can help, it would be great - I spent a lot of time trying to figure this out. No other text books have this kind of description.