Khamsemanan, N. “NIELSEN THEORY AND SOME SELECTED APPLICATIONS TO DIFFERENTIAL AND INTEGRAL EQUATIONS; A BRIEF SURVEY”. Journal of Nonlinear Analysis and Optimization: Theory & Applications 2, no. 1 (December 21, 2011): 19-25. Accessed December 6, 2021. http://www.math.sci.nu.ac.th/ojs302/index.php/jnao/article/view/22.