Bounds for the maximal and Riesz potential operators with variable fractionality

Abstract

We prove $L^{p(\cdot)}$-to-$L^{q(\cdot)}$ bounds for variable versions of the fractional maximal $M^{α(\cdot)}$ and Riesz potential $I^{α(\cdot)}$ operators. The changing fractionality in these operators is given by averaging the function $α(\cdot)$ over balls. The bounds for $M^{α(\cdot)}$ are in terms of a three-exponent Muckenhoupt condition relating $p(\cdot),q(\cdot),$ and $α(\cdot)$, while the bounds for $I^{α(\cdot)}$ are in terms of the boundedness of $M^{α(\cdot)}$ and a packing condition on $α(\cdot).$ These bounds hold under Hardy--Littlewood maximal function boundedness and Muckenhoupt conditions on the individual exponents $p(\cdot),q(\cdot),α(\cdot).$ The proofs are based on an adaptation of sparse domination to variable fractionality and an embedding into variable sequential spaces.