Variance of integrated Brownian motion

Let WtW_t be a standard Brownian motion. The variance Var⁡ ⁣(∫0tWs ds)\operatorname{Var}\!\left(\int_0^t W_s\,ds\right) has the form k t3k\,t^3. Find the constant kk.

Show hints (2)+
  1. The integral is Gaussian, mean 00: its variance is ∫0t∫0tCov⁡(Ws,Wu) ds du\int_0^t\int_0^t \operatorname{Cov}(W_s,W_u)\,ds\,du.
  2. Use Cov⁡(Ws,Wu)=min⁡(s,u)\operatorname{Cov}(W_s,W_u)=\min(s,u) and integrate.

Answer

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0.3333 (± 0.001)

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