CHANG IL KIM, and GILJUN HAN. “ORTHOGONALLY EULER-LAGRANGE TYPE CUBIC FUNCTIONAL EQUATIONS IN ORTHOGONALITY NORMED SPACES”. Journal of Computational Analysis and Applications (JoCAAA), vol. 29, no. 4, Aug. 2021, pp. 765-74, http://eudoxuspress.com/index.php/pub/article/view/190.