This is a formula regarding getting expectation under the topic of Brownian Motion.
\begin{align} E[W(s)W(t)] &= E[W(s)(W(t) - W(s)) + W(s)^2] \\ &= E[W (s)]E[W (t) - W (s)] + E[W(s)^2] \\ &= 0+s\\ &=\min(s,t) \end{align}
How does $E[W (s)]E[W (t) - W (s)]$ turn into 0?
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