On Approximation with Respect to Certain Quadratic Norms E. Akutowicz Indiana University Mathematics Journal 1962 text pdf 10.1512/iumj.1962.11.11010 10.1512/iumj.1962.11.11010 en Indiana Univ. Math. J. 11 (1962) 153 - 164 state-of-the-art mathematics http://iumj.org/access/