Koyck Model:

A model with  an infinite  lag  structure was suggested  by  Koyck.  He assumes an explicit form for the lag coefficients.  

Notice that this  structural  formulation  allows us to capture an  infinite lag with just two parameters p  and cp.  It  is assumed that cp  is non-negative and smaller than one in magnitude. The response structure in  this scheme is rigid and very restrictive.  There is both  an  immediate response and a  delayed  or  lagged  response  to  the  external event/stimulus.  The  response  in  subsequent periods declines  in  the  manner  of  a convergent  geometric series,  giving  the  scheme its commonly used moniker,  the geometric lag structure.