Gong, Guobin, and Xi'an Jiaotong. “A Revisit to Higher Variations of a Functional”. Journal of Advance Research in Mathematics And Statistics (ISSN 2208-2409) 6, no. 2 (February 28, 2019): 01–09. Accessed May 11, 2024. https://nnpub.org/index.php/MS/article/view/523.