1. Management and Social Determinants of Health Research Center, Mashhad University of Medical Sciences, Mashhad 9133913716, Iran;
2. School of Mathematics, Institute for Research in Fundamental Sciences (IPM), P.O. Box 19395-5746, Tehran, Iran e-mail:
3. Department of Applied Mathematics, Ferdowsi University of Mashhad, Mashhad 9177948974, Iran e-mail: