This paper presents a discussion of techniques for designing optimal linear time-invariant regulator systems, which have inaccessible states, and which are based on a quadratic performance functional for the definition of optimality. A definition is given of a suboptimal control vector

, which has

nonzero, constant elements and

elements defined as zero, and which will provide optimal performance from an

-dimensional linear manifold of initial states. An algorithm is given for determining

for a linear manifold lying between two optimal manifolds and an example illustrates the superiority of these routines for initial-state sets about an optimal manifold.