1. Alsina C., Sikorska J., Santos Tomás M., Norm Derivatives and Characterizations of Inner Product Spaces, World Scientific, Hackensack, New Jersay, 2009.
2. Chmieliński J., Wójcik P., On a ρ-orthogonality, Aequationes Math. 80 (2010), 45–55.
3. Chmieliński J., Wójcik P., ρ-orthogonality and its preservation – revisited, in: Recent Developments in Functional Equation and Inequalities, Banach Center Publ., Polish Acad. Sci. Inst. Math., Warsaw, 2013, pp. 17–30.
4. Dragomir S.S., Semi-inner products and applications, Nova Science Publishers, Inc., Hauppauge, New York, 2004.
5. Day M.M., Normed linear spaces, Ergeb. Math. Grenzgeb. 21, Springer, New York–Heidelberg, 1973.
6. Giles J.R., Classes of semi-inner-product spaces, Trans. Amer. Math. Soc. 129 (1967), 436–446.
7. Lumer G., Semi-inner-product spaces, Trans. Amer. Math. Soc. 100 (1961), 29–43.
8. Wójcik P., Characterizations of smooth spaces by approximate orthogonalities, Aequationes Math. 89 (2015), 1189–1194.
Google Scholar