Modular inequalities for the Calderón operator

Abstract

If $P,Q:[0,\infty)\to$ are increasing functions and $T$ is the Calderón operator defined on positive or decreasing functions, then optimal modular inequalities $\int P(Tf)\leq C\int Q(f)$ are proved. If $P=Q$, the condition on $P$ is both necessary and sufficient for the modular inequality. In addition, we establish general interpolation theorems for modular spaces.