Sparse Endpoint Estimates for Bochner-Riesz Multipliers on the Plane

For $0< \lambda < \frac{1}2$, let $B_{\lambda }$ be the Bochner-Riesz multiplier of index $\lambda$ on the plane. Associated to this multiplier is the critical index $1 < p_\lambda = \frac{4} {3+2 \lambda } < \frac{4}3$. We prove a sparse bound for $B_{\lambda }$ with indices $(p_\lambda , q)$, where $p_\lambda ' < q < 4$. This is a further quantification of the endpoint weak $L^{p_\lambda}$ boundedness of $B_{\lambda }$, due to Seeger. Indeed, the sparse bound immediately implies new endpoint weighted weak type estimates for weights in $A_1 \cap RH_{\rho }$, where $\rho > \frac4 {4 - 3 p_{\lambda }}$.