Abstract Cesàro spaces. Duality

We study abstract Cesàro spaces CX  , which may be regarded as generalizations of Cesàro sequence spaces cespcesp and Cesàro function spaces Cesp(I)Cesp(I) on I=[0,1]I=[0,1] or I=[0,∞)I=[0,∞), and also as the description of optimal domain from which Cesàro operator acts to X  . We find the dual of such spaces in a very general situation. What is however even more important, we do it in the simplest possible way. Our proofs are more elementary than the known ones for cespcesp and Cesp(I)Cesp(I). This is the point how our paper should be seen, i.e. not as a generalization of known results, but rather like grasping and exhibiting the general nature of the problem, which is not so easily visible in previous publications. Our results show also an interesting phenomenon that there is a big difference between duality in the cases of finite and infinite intervals.