CHANG, C. Some new proof of the sum formula and restricted sum formula. Tamkang Journal of Mathematics, [S. l.], v. 44, n. 2, p. 123–129, 2013. DOI: 10.5556/j.tkjm.44.2013.1128. Disponível em: https://journals.math.tku.edu.tw/index.php/TKJM/article/view/1128. Acesso em: 23 jul. 2024.