LAPPAN, P. The spherical derivative and normal functions. Annales Fennici Mathematici, [S. l.], v. 3, n. 2, p. 301–310, 1976. DOI: 10.5186/aasfm.1977.0312. Disponível em: https://afm.journal.fi/article/view/134325. Acesso em: 26 apr. 2025.