Şerb, I. “On the Behaviour of the Tangential Modulus of a Banach Space I”. Rev. Anal. Numér. Théor. Approx., vol. 24, no. 1, Aug. 1995, pp. 241-8, https://ictp.acad.ro/jnaat/journal/article/view/1995-vol24-nos1-2-art27.