1. Alonso J., Benítez C.: Orthogonality in normed linear spaces: a survey. Part I: main properties. Extracta Math. 3, 1–15 (1988)
2. Alonso J., Benítez C.: Orthogonality in normed linear spaces: a survey. Part II: relations between main orthogonalities. Extracta Math. 4, 121–131 (1989)
3. Alsina, C., Sikorska, J., Tomás, M.S.: Norm Derivatives and Characterizations of Inner Product Spaces. World Scientific Publishing Co., Hackensack (2010)
4. Alsina C., Tomás M.S.: On some orthogonality relations in real normed spaces and characterizations of inner products. Boll. Unione Mat. Ital. Sez. B Artic. Ric. Mat. 10(8), 513–520 (2007)
5. Amir, D.: Characterizations of inner product spaces. In: Operator Theory: Advances and Applications, vol. 20. Birkhäuser Verlag, Basel (1986)