Cobzaş, Ştefan, and I. Muntean. “Duality Relations and Characterizations of Best Approximation for P-Convex Sets”. Anal. Numér. Théor. Approx., vol. 16, no. 2, Aug. 1987, pp. 95-108, https://ictp.acad.ro/jnaat/journal/article/view/1987-vol16-no2-art2.