Cobzaş, Ştefan. “Phelps Type Duality Results in Best Approximation”. Rev. Anal. Numér. Théor. Approx. 31, no. 1 (February 1, 2002): 29–43. Accessed November 21, 2024. https://ictp.acad.ro/jnaat/journal/article/view/2002-vol31-no1-art5.