[1] F.Deutsch, “Best Approximation in Inner Product Spaces,”Springer, Berlin, 2001.
[2] D.Fang,X.Luo and Chong Li, “Nonlinear Simultaneous Approximation in Complete Lattice Banach Spaces,”Taiwanese Journal of Mathematics, 2008.
[3] W. C. Charles, “Linear Algebra,” 1968, p. 62.
[4] V. Prasolov and V. M. Tikhomirov, “Geometry,” American Mathematical Society, 2001, p. 22.