Stellar luminosity as a function of lithium abundance in stars for different models. Panel (a): for different diffusion coefficients of the flash-induced IGW mixing. Panel (b): for different diffusion parameters of the thermohaline mixing without IGWs. Panel (c): for different diffusion parameters of the thermohaline mixing with IGWs. The model with the flash-induced IGW mixing (solid lines, A² = 1) and without the flash-induced IGW mixing (dashed lines, A² = 0.0) are indicated in panel (b). yellow circles are RC stars and the green ones are red giant stars. The observational samples are from Kumar et al. (2020).