Mustăţa, Costică. “On the Best Approximation in Metric Spaces”. Anal. Numér. Théor. Approx. 4, no. 1 (February 1, 1975): 45–50. Accessed September 3, 2025. https://www.ictp.acad.ro/jnaat/journal/article/view/1975-vol4-no1-art5.