Abstract Ces\`aro spaces. II. Optimal range

Ces\`aro spaces are investigated from the optimal domain and optimal range point of view. There is a big difference between the cases on $[0, \infty)$ and on $[0, 1]$, as we can see in Theorem 1. Moreover, we present an improvement of Hardy inequality on $[0, 1]$ which plays an important role in these considerations.