  In this paper, we study the optimal time decay rates for the higher order
spatial derivatives of solutions to a diffusive Oldroyd-B model. As pointed out
in the Section 1.2 of Huang-Wang-Wen-Zi (J. Differential Equations 306:
456-491, 2022), how to estiblish the optimal decay estimate for the
highest-order spatial derivatives of the solution to this model is still an
open problem. Motivated by Wang-Wen (Sci. China Math. 65: 1199-1228, 2022), we
give a positive answer to this problem via some delicate analyses on the low
and high frequency parts of the solution.
