CHENEY, E. W.; WULBERT, D. E. The Existence and Unicity of Best Approximations. Mathematica Scandinavica, Berkeley, CA, United States, v. 24, p. 113–140, 1969. DOI: 10.7146/math.scand.a-10925. Disponível em: https://journals.msp.org/mscand/article/view/2378. Acesso em: 25 jul. 2026.