Cobzaş, Ş. “Antiproximinal Sets in Banach Spaces of \(c_0\)-Type”. Anal. Numér. Théor. Approx., vol. 7, no. 2, Aug. 1978, pp. 141-5, https://ictp.acad.ro/jnaat/journal/article/view/1978-vol7-no2-art3.