%0 Journal Article %T Polynomial Approximation and ¦Ø_¦Õ^r(f,t) Twenty Years Later %A Z. Ditzian %J Surveys in Approximation Theory %D 2007 %I %X About twenty years ago the measure of smoothness ¦Ø_¦Õ^r(f,t) was introduced and related to the rate of polynomial approximation. In this article we survey developments about this and related concepts since that time. %U http://www.math.technion.ac.il/sat/papers/9/