A negative result on positive controllability of delay systems

This paper is concerned with complete controllability of linear retarded systems to the Sobolev space W2 (1) (-h,O;X) under a positivity constraint for the control; here X denotes a "negative", since this positive controllability property can only be achived under a very strong and obviously sufficient condition. For systems governed by ordinary differential equations the situation is - as is well known - entirely different.