Full paper in PDF:
$%J.-L. Lions and E. Zuazúa, The cost of controlling unstable systems: time irreversible systems,
Rev. Mat. Univ. Complut. Madrid 10 (1997), no. 2, 481–523.%$
                                                      Collège de France 3 rue d’Ulm 75231 Paris Cedex 05 -- France  |                                          Departamento de Matemática Aplicada Universidad Complutense 28040 Madrid -- Spain  | 
     We   discuss   the   cost   of   controlling   parabolic   equations   of   the   form
     
 in  a  bounded  smooth  domain  
     
  of  
     
  with
     homogeneous  Dirichlet  boundary  conditions  where  
     
  is  fixed.  The
     control 
     
 acts on the system through the open and non-empty subset 
     
 of
                                                                      
                                                                      
     
. As 
     
 the system becomes more and more instable. We analyze the
     dependence of the control 
     
 on the parameter 
     
 in the context of various
     different control problems. We show that the norm of the control diverges when
     the trajectory is driven from an initial state 
     
 to the null state at
     any time 
     
. However, we prove that the control converges to zero when
     the null initial state 
     
 is driven into a ball of an arbitrary radius 
     
     around a given terminal state 
     
or when the solution fulfills exactly
     a finite number of constraints at time 
     
.
     
2000 Mathematics Subject Classification: 93B05, 35K05, 35B35.