Norms of Minimal Projections

Abstract

It is proved that the projection constants of two- and three-dimensional spaces are bounded by $4/3$ and $(1+\sqrt{5})/2$, respectively. These bounds are attained precisely by the spaces whose unit balls are the regular hexagon and dodecahedron. In fact, a general inequality for the projection constant of a real or complex $n$-dimensional space is obtained and the question of equality therein is discussed.