CO(1–0) vs. [CI](1–0) luminosity (top) and CO(2–1) vs. [CI](1–0) luminosity (bottom) for the ULIRGs in our sample. The best-fit relations are shown as dashed orange lines. The best-fit parameters are reported at the bottom-right corner of the plots. We also display the Pearson correlation coefficients (ρ) and their associated p-values. The solid black lines in both panels represent the corresponding relations reported by Jiao et al. (2017) for a sample of 71 (U)LIRGs, and the dotted lines represent the relations of Jiao et al. (2019) for a sample of 15 nearby spiral galaxies, between and .