Liu, Y;    Liu, XW;    Barlow, MJ;    Luo, SG;      (2004)    Chemical abundances of planetary nebulae from optical recombination lines – II. Abundances derived from collisionally excited lines and optical recombination lines.                   Monthly Notices of the Royal Astronomical Society , 353  (4)   pp. 1251-1285.    10.1111/j.1365-2966.2004.08156.x <https://doi.org/10.1111/j.1365-2966.2004.08156.x>.       Green open access