Comments on "A discrete optimal control problem for descriptor systems" [with reply]

In the above-mentioned work (see ibid., vol.34, p.177-81 (1989)), necessary and sufficient optimality conditions are derived for a discrete-time optimal problem, as well as other specific cases of implicit and explicit dynamic systems. The commenter corrects a mistake and demonstrates that there is not an ´if and only if´ correspondence between stationarity conditions and minimization of the Hamiltonian. In replying, Lin and Yang acknowledge this and also give a version of theorem two in the context of their response to achieve a balance.<>