Huiyuan Li Jiachang Sun Yuan Xu Discrete Fourier Analysis, Cubature, and Interpolation on a Hexagon and a Triangle. 1653-1681 2008 46 SIAM J. Numer. Anal. 4 https://doi.org/10.1137/060671851 db/journals/siamnum/siamnum46.html#LiSX08