Corrected constraints on big bang nucleosynthesis in a modified gravity model of $f(R)\propto R^n$

Big bang nucleosynthesis in a modified gravity model of $f(R)\propto R^n$ is investigated. The only free parameter of the model is a power-law index $n$. We find cosmological solutions in a parameter region of $1< n \leq (4+\sqrt{6})/5$\mathrm{.}Wecalculateabundancesof$^4$He,D,$^3$He,$^7$Li,and$^6$Li during big bang nucleosynthesis. We compare the results with the latest observational data. It is then found that the power-law index is constrained to be $(n-1)=(-0.86\pm 1.19)\times {10}^{-4}$ (95 % C.L.) mainly from observations of deuterium abundance as well as ${}^4$He abundance.