Before revision:  We shall also discuss how to solve some special two-dimensional Helmholtz equations using DAT developed for one-dimensional problems. As demonstrated in all our numerical experiments, the convergence rates of DAT measured in relative L_2, L_\infty and H^1 energy norms as a result of using our M-th order compact finite difference scheme are of order M .
After revision:  We shall also discuss how to solve some special two-dimensional Helmholtz equations using DAT .