History, generalizations and unified treatment of two Ostrowski's inequalities. Varošanec, Sanja (2004) JIPAM. Journal of Inequalities in Pure & Applied Mathematics [electronic only]
Hyers-Ulam constants of Hilbert spaces Taneli Huuskonen, Jussi Väısälä (2002) Studia Mathematica The best constant in the Hyers-Ulam theorem on isometric approximation in Hilbert spaces is equal to the Jung constant of the space.