BEAVER, Brad; SARIO, Leo; WANG, Cecilia. Positive biharmonic functions. Annales Fennici Mathematici, [S. l.], v. 7, n. 2, p. 141–146, 1982. DOI: 10.5186/aasfm.1982.0714. Disponível em: https://afm.journal.fi/article/view/134436. Acesso em: 6 jan. 2026.