Monte Carlo convergence rates for $k$th moments in Banach spaces

We formulate standard and multilevel Monte Carlo methods for the $k$th moment $\mathbb{M}^k_\varepsilon[ξ]$ of a Banach space valued random variable $ξ\colonΩ\to E$, interpreted as an element of the $k$-fold injective tensor product space $\otimes^k_\varepsilon E$. For the standard Monte Carlo estimator of $\mathbb{M}^k_\varepsilon[ξ]$, we prove the $k$-independent convergence rate $1-\frac{1}{p}$ in the $L_q(Ω;\otimes^k_\varepsilon E)$-norm, provided that (i) $ξ\in L_{kq}(Ω;E)$ and (ii) $q\in[p,\infty)$, where $p\in[1,2]$ is the Rademacher type of $E$. By using the fact that Rademacher averages are dominated by Gaussian sums combined with a version of Slepian's inequality for Gaussian processes due to Fernique, we moreover derive corresponding results for multilevel Monte Carlo methods, including a rigorous error estimate in the $L_q(Ω;\otimes^k_\varepsilon E)$-norm and the optimization of the computational cost for a given accuracy. Whenever the type of the Banach space $E$ is $p=2$, our findings coincide with known results for Hilbert space valued random variables. We illustrate the abstract results by three model problems: second-order elliptic PDEs with random forcing or random coefficient, and stochastic evolution equations. In these cases, the solution processes naturally take values in non-Hilbertian Banach spaces. Further applications, where physical modeling constraints impose a setting in Banach spaces of type $p<2$, are indicated.

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