# Estimation of Smooth Functionals of Location Parameter in Gaussian and Poincaré Random Shift Models

Estimation of Smooth functionals

## Abstract

Let E be a separable Banach space and let $$f:E\mapsto {\mathbb {R}}$$ be a smooth functional. We discuss a problem of estimation of f(𝜃) based on an observation X = 𝜃 + ξ, where 𝜃E is an unknown parameter and ξ is a mean zero random noise, or based on n i.i.d. observations from the same random shift model. We develop estimators of f(𝜃) with sharp mean squared error rates depending on the degree of smoothness of f for random shift models with distribution of the noise ξ satisfying Poincaré type inequalities (in particular, for some log-concave distributions). We show that for sufficiently smooth functionals f these estimators are asymptotically normal with a parametric convergence rate. This is done both in the case of known distribution of the noise and in the case when the distribution of the noise is Gaussian with covariance being an unknown nuisance parameter.

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## Notes

1. 1.

The definition and relevant properties of j-th order Fréchet derivatives could be found, e.g., in Cartan (1971), Sections 2.1, 5.1, 5.3.

2. 2.

In fact, a more general result was proved in Koltchinskii and Zhilova (2020) for spaces Cs,γ(E) of functionals of smoothness s whose derivatives are allowed to grow as ∥𝜃γ. For simplicity, we consider here only the case of γ = 0 (both f and its derivatives are uniformly bounded).

3. 3.

Note that these asymptotic relationships hold in the case when E = En depends on n, for instance, when $$E={\mathbb {R}}^{d}$$ with d = dn.

4. 4.

A standard way to write Poincaré inequality for r.v. in $${\mathbb {R}}^{d}$$ is:

$$\begin{array}{@{}rcl@{}} \text{Var}(g(\xi)) \leq C {\mathbb{E}} \|\nabla g(\xi)\|^{2}, \end{array}$$

where ∇g is the gradient of function g that, by Rademacher’s theorem, exists a.s. for all locally Lipschitz functions and ∥⋅∥ is the standard Euclidean norm.

5. 5.

Here $$\|\cdot \|_{\psi _{1}}$$ is the Orlicz ψ-norm corresponding to the sub-exponential tails; see Vershynin (2018), Chapter 2 for the definitions.

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## Acknowledgements

V. Koltchinskii was supported in part by NSF Grant DMS-1810958. M. Zhilova was supported in part by NSF Grant DMS-1712990.

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Koltchinskii, V., Zhilova, M. Estimation of Smooth Functionals of Location Parameter in Gaussian and Poincaré Random Shift Models. Sankhya A (2021). https://doi.org/10.1007/s13171-020-00232-1

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### Keywords

• Smooth functionals
• Efficiency
• Random shift model
• Poincaré inequality
• Normal approximation.

### AMS (2000) subject classification.

• Primary 62H12; Secondary 62G20
• 62H25
• 60B20