CHENEY, E. W.; WULBERT, D. E. The Existence and Unicity of Best Approximations. MATHEMATICA SCANDINAVICA, [S. l.], v. 24, p. 113–140, 1969. DOI: 10.7146/math.scand.a-10925. Disponível em: https://tidsskrift.dk/math/article/view/10925. Acesso em: 14 aug. 2026.