1. The Pythagoras number of some affine algebras and local algebras;Choi;J. Reine Angew. Math.,1982
2. Pythagoras numbers of real algebroid curves and Gram matrices;Quarez;J. Algebra,2001
3. J.M. Ruiz, Aspectos aritméticos y geométricos del problema decimoséptimo de Hilbert para gérmenes analı́ticos, Tesis Doctoral, Univ. Complutense de Madrid, 1983