@article{Li-Gre-2007,
    author={Jing-Rebecca Li and Leslie Greengard },
    title={On the numerical solution of the heat equation I: fast solvers 
           in free space },
    doi={10.1016/j.jcp.2007.06.021 },
    journal={Journal of Computational Physics },
    year={2007 },
    month={10},
    volume={226 (2) },
    pages={1891--1901},
}