Cobzaş, Ştefan, and C. Mustăţa. “Best Approximation in Spaces With Asymmetric Norm”. Rev. Anal. Numér. Théor. Approx., vol. 35, no. 1, Feb. 2006, pp. 17-31, https://ictp.acad.ro/jnaat/journal/article/view/2006-vol35-no1-art4.