Endpoint estimates for higher order Marcinkiewicz multipliers

We consider Marcinkiewicz multipliers of any lacunary order defined by means of uniformly bounded variation on each lacunary Littlewood--Paley interval of some fixed order $\tau\geq 1$. We prove the optimal endpoint bounds for such multipliers as a corollary of a more general endpoint estimate for a class of multipliers introduced by Coifman, Rubio de Francia and Semmes and further studied by Tao and Wright. Our methods also yield the best possible endpoint mapping property for higher order H\"ormander-Mihlin multipliers, namely multipliers which are singular on every point of a lacunary set of order $\tau$. These results can be considered as endpoint versions of corresponding results of Sj\"ogren and Sj\"olin. Finally our methods generalize a weak square function characterization of the space $L\log^{1/2}L$ in terms of a square function introduced by Tao and Wright: we realize such a weak characterization as the dual of the Chang--Wilson--Wolff inequality, thus giving corresponding weak square function characterizations for the spaces $L\log^{\tau/2}L$ for general integer orders $\tau\geq 1$.