“…where F is replaced, for example, by A k in Equation 9, Ã k in Equation 10, Â k in Equations 13 and 14, orǍ k+1 in Equation 16. Typically, the right-hand side is assumed to be nonpositive definite, that is, Y ⩾ 0 and the solution X is required to be nonnegative definite, that is, X ⩾ 0.…”