“…As is known, saddle-point problems correspond to the Kuhn-Tucker conditions for linearly constrained quadratic programming problems, which typically result from mixed or hybrid finite element approximations of second-order elliptic problems, elasticity problems, or the Stokes equations (see, e.g., Brezzi and Fortin [13]) and from Lagrange multiplier methods (see, e.g., Fortin and Glowinski [17]). A number of structured preconditioners [15,16,25,11,10] and iterative methods [14,22,4,2,9] have been studied in the literature for these problems. See also [27,21,20,26,19,18,6,8] and the references therein.…”