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We consider the problem of estimating expectations with respect to a target distribution with an unknown normalising constant, and where even the un-normalised target needs to be approximated at finite resolution. This setting is ubiquitous across science and engineering applications, for example in the context of Bayesian inference where a physics-based model governed by an intractable partial differential equation (PDE) appears in the likelihood. A multi-index sequential Monte Carlo (MISMC) method is used to construct ratio estimators which provably enjoy the complexity improvements of multi-index Monte Carlo (MIMC) as well as the efficiency of sequential Monte Carlo (SMC) for inference. In particular, the proposed method provably achieves the canonical complexity of $$\hbox {MSE}^{-1}$$ MSE - 1 , while single-level methods require $$\hbox {MSE}^{-\xi }$$ MSE - ξ for $$\xi >1$$ ξ > 1 . This is illustrated on examples of Bayesian inverse problems with an elliptic PDE forward model in 1 and 2 spatial dimensions, where $$\xi =5/4$$ ξ = 5 / 4 and $$\xi =3/2$$ ξ = 3 / 2 , respectively. It is also illustrated on more challenging log-Gaussian process models, where single-level complexity is approximately $$\xi =9/4$$ ξ = 9 / 4 and multilevel Monte Carlo (or MIMC with an inappropriate index set) gives $$\xi = 5/4 + \omega $$ ξ = 5 / 4 + ω , for any $$\omega > 0$$ ω > 0 , whereas our method is again canonical. We also provide novel theoretical verification of the product-form convergence results which MIMC requires for Gaussian processes built in spaces of mixed regularity defined in the spectral domain, which facilitates acceleration with fast Fourier transform methods via a cumulant embedding strategy, and may be of independent interest in the context of spatial statistics and machine learning.
We consider the problem of estimating expectations with respect to a target distribution with an unknown normalising constant, and where even the un-normalised target needs to be approximated at finite resolution. This setting is ubiquitous across science and engineering applications, for example in the context of Bayesian inference where a physics-based model governed by an intractable partial differential equation (PDE) appears in the likelihood. A multi-index sequential Monte Carlo (MISMC) method is used to construct ratio estimators which provably enjoy the complexity improvements of multi-index Monte Carlo (MIMC) as well as the efficiency of sequential Monte Carlo (SMC) for inference. In particular, the proposed method provably achieves the canonical complexity of $$\hbox {MSE}^{-1}$$ MSE - 1 , while single-level methods require $$\hbox {MSE}^{-\xi }$$ MSE - ξ for $$\xi >1$$ ξ > 1 . This is illustrated on examples of Bayesian inverse problems with an elliptic PDE forward model in 1 and 2 spatial dimensions, where $$\xi =5/4$$ ξ = 5 / 4 and $$\xi =3/2$$ ξ = 3 / 2 , respectively. It is also illustrated on more challenging log-Gaussian process models, where single-level complexity is approximately $$\xi =9/4$$ ξ = 9 / 4 and multilevel Monte Carlo (or MIMC with an inappropriate index set) gives $$\xi = 5/4 + \omega $$ ξ = 5 / 4 + ω , for any $$\omega > 0$$ ω > 0 , whereas our method is again canonical. We also provide novel theoretical verification of the product-form convergence results which MIMC requires for Gaussian processes built in spaces of mixed regularity defined in the spectral domain, which facilitates acceleration with fast Fourier transform methods via a cumulant embedding strategy, and may be of independent interest in the context of spatial statistics and machine learning.
Cyber-physical system security presents unique challenges to conventional measurement science and technology. Anomaly detection in software-assisted physical systems, such as those employed in additive manufacturing or in DNA synthesis, is often hampered by the limited available parameter space of the underlying mechanism that is transducing the anomaly. As a result, the formulation of anomaly detection for such systems often leads to inverse or ill-posed problems, requiring statistical treatments. Here, we present Bayesian inference of unknown parameters associated with a generic actuator considered as a representative vital element of a cyber-physical system. Via a series of experimental input-output measurements, a transfer function for the actuator is obtained numerically, which serves as our model for the proposed method. Linear, nonlinear, and delayed dynamics may be assumed for the actuator response. By devising a code-based malicious signal, we study the efficacy of Bayesian inference for its potential to produce a detection, including uncertainty quantification, with a remarkably small number of input data points. Our approach should be adaptable to a variety of real-time cyber-physical anomaly detection scenarios.
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