Şerb, Ioan. “On the Multi-Valued Metric Projection in Normed Vectors Spaces II”. Anal. Numér. Théor. Approx., vol. 11, no. 1, Aug. 1982, pp. 155-66, https://ictp.acad.ro/jnaat/journal/article/view/1982-vol11-nos1-2-art19.