1. J.Alonso, Ortogonalidad en espacios normados. Thesis, Univ. of Extremadura 1984.
2. J. Alonso andC. Ben�tez, Orthogonality in normed linear spaces: a survey. Part I: Main properties. Extracta Math.3, 1?15 (1988).
3. J. Alonso andC. Ben�tez, Orthogonality in normed linear spaces: a survey. Part II: Relations between main orthogonalities. Extracta Math.4, 121?131 (1989).
4. G. Birkhoff, Orthogonality in linear metric spaces. Duke Math. J.1, 169?172 (1935).
5. M.Del Rio, Ortogonalidad en espacios normados y caracterizaci�n de espacios prehilbertianos. Thesis, Univ. de Santiago de Compostela 1975.