Cobzaş, Ştefan. “Best Approximation in Spaces of Bounded Vector-Valued Sequences”. Rev. Anal. Numér. Théor. Approx., vol. 23, no. 1, Aug. 1994, pp. 63-69, https://ictp.acad.ro/jnaat/journal/article/view/1994-vol23-no1-art6.