VakarchukS. B., and MyskinK. Y. “Some Problems of Simultaneous Approximation of Functions of Two Variables and Their Derivatives by Interpolation Bilinear Splines”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 57, no. 2, Feb. 2005, pp. 147–157, https://umj.imath.kiev.ua/index.php/umj/article/view/3583.