W. N. A. W. Ahmad, S. F. Sufahani, M. A. H. Mohamad, M. S. Rusiman, M. Z. M. Maarof, and M. A. lmran Kamarudin, “NON-CLASSICAL OPTIMAL CONTROL PROBLEM: A CASE STUDY FOR CONTINUOUS APPROXIMATION OF FOUR-STEPWISE FUNCTION”, Adv. Differ. Equ. Control Process., vol. 30, no. 4, pp. 309-321, Sep. 2023.