Petunina , M. Yu. “Average Quadratic Error of the Estimate of the First Order Splines in the Model of Linear Regression”. Ukrains’kyi Matematychnyi Zhurnal, vol. 43, no. 3, Feb. 1991, pp. 429-32, https://umj.imath.kiev.ua/index.php/umj/article/view/9628.