A sum operator with applications to self-improving properties of Poincaré inequalities in metric spaces

We define a class of summation operators with applications to the self-improving nature of Poincaré-Sobolev estimates, in fairly general quasimetric spaces of homogeneous type. We show that these sum operators play the familiar role of integral operators of potential type (e.g., Riesz fractional integrals) in deriving Poincaré-Sobolev estimates in cases when representations of functions by such integral operators are not readily available. In particular, we derive norm estimates for sum operators and use these estimates to obtain improved Poincaré-Sobolev results.University of BolognaGruppo Nazionale per l'Analisi Matematica, la Probabilità e le loro Applicazioni (Istituto Nazionale di Alta Matematica