How can we derive cross covariance $R_\mathrm{xy}(t_1,t_2)=R_\mathrm{yx}^*(t_2,t_1)$?

Question

In random process, cross covariance is nonnegative definite like $$R_\mathrm{xy}(t_1,t_2)=\mathbf{E}(\mathrm{X}(t_1)\mathrm{Y}^*(t_2))=R_\mathrm{yx}^*(t_2,t_1)$$ I'm wondering how it can be derived. Thank you.

Answer 1

$$\mathrm{X}(t_1)\mathrm{Y}^*(t_2)=\left(\mathrm{Y}(t_2)\mathrm{X}^*(t_1)\right)^*$$