Qing Fang Tetsuro Yamamoto Superconvergence of finite difference approximations for convection-diffusion problems. 99-110 2001 8 Numer. Linear Algebra Appl. 2 https://doi.org/10.1002/1099-1506(200103)8:2<99::AID-NLA233>3.0.CO;2-8 db/journals/nla/nla8.html#FangY01