Über die Existenz gemeinsamer Elemente bester Approximation bezüglich zweier Normen. N. Hadjisavvas; B. Nassopoulos — 1987 Manuscripta mathematica
On the Renorming of a Vector Space Endowed with Two Norms and the Simultaneous Best Approximation Problem. N. Hadjisavvas; Nassopoulos B. — 1988 Mathematische Annalen