Continuous-Time Singular Linear-Quadratic Control: Necessary and   Sufficient Conditions for the Existence of Regular Solutions
  The purpose of this paper is to close the remaining gaps in the understanding of the role that the constrained generalized continuous algebraic Riccati equation plays in singular linear-quadratic (LQ) optimal control. Indeed, in spite of the vast literature on LQ problems, it is only in a recent paper that a sufficient condition for the existence of a non-impulsive optimal control has for the first time connected this equation with the singular LQ optimal control problem. In this paper, we establish four equivalent conditions providing a complete picture that connects the singular LQ problem with the generalized continuous algebraic Riccati equation and with the geometric properties of the underlying system. 
Augusto Ferrante and Lorenzo Ntogramatzidis
math.OC