arXiv subjects

Alois Pichler

Publications and source records attributed to Alois Pichler.

At least 19 recordsLinked to original sources

arXiv2025

Quantization Of Probability Measures In Maximum~Mean~Discrepancy Distance

Accurate approximation of probability measures is essential in numerical applications. This paper explores the quantization of probability measures using the maximum mean discrepancy (MMD) distance as a guiding metric. We first investigate optimal approximations by determining the best weights, followed by addressing the problem of optimal facility locations. To facilitate efficient computation, we reformulate the nonlinear objective as expectations over a product space, enabling the use of stochastic approximation methods. For the Gaussian kernel, we derive closed-form expressions to develop a deterministic optimization approach. By integrating stochastic approximation with deterministic techniques, our framework achieves precise and efficient quantization of continuous distributions, with significant implications for machine learning and signal processing applications.

math.OC↗

arXiv2025

Higher-Order Stochastic Dominance Constraints in Optimization

This contribution examines optimization problems that involve stochastic dominance constraints. These problems have uncountably many constraints. We develop methods to solve the optimization problem by reducing the constraints to a finite set of test points needed to verify stochastic dominance. This improves both theoretical understanding and computational efficiency. Our approach introduces two formulations of stochastic dominance$\unicode{x2013}$one employs expectation operators and another based on risk measures$\unicode{x2013}$allowing for efficient verification processes. Additionally, we develop an optimization framework incorporating these stochastic dominance constraints. Numerical results validate the robustness of our method, showcasing its effectiveness for solving higher-order stochastic dominance problems, with applications to fields such as portfolio optimization.

math.OC↗

arXiv2025

StochasticDominance.jl: A Julia Package for Higher Order Stochastic Dominance

Stochastic dominance is a fundamental concept in decision-making under uncertainty and quantitative finance, yet its practical application is hindered by computational intractability due to infinitely many constraints. We introduce the Julia package StochasticDominance, an open-source tool that efficiently verifies and optimizes under higher-order stochastic dominance constraints. Our approach builds on recent theoretical advancements that reduce infinite constraints to a finite number, making higher-order stochastic dominance more accessible. This package provides a user-friendly, black-box solution, enabling researchers and practitioners to incorporate stochastic dominance constraints seamlessly into their optimization frameworks.

math.OC↗

arXiv2025

Models Parametric Analysis via Adaptive Kernel Learning

Any applied mathematical model contains parameters. The paper proposes to use kernel learning for the parametric analysis of the model. The approach consists in setting a distribution on the parameter space, obtaining a finite training sample from this distribution, solving the problem for each parameter value from this sample, and constructing a kernel approximation of the parametric dependence on the entire set of parameter values. The kernel approximation is obtained by minimizing the approximation error on the training sample and adjusting kernel parameters (width) on the same or another independent sample of parameters. This approach to learning complex dependencies is called kernel learning (or kernel SVM). Traditionally, kernel learning is considered in the so-called Reproducing Kernel Hilbert Space (RKHS) with a fixed kernel. The novelty of our approach is that we consider the kernel learning in a broad subspace of square-integrable functions with a corresponding L_2-norm regularization. This subspace contains linear combinations of kernel functions with kernels of different shapes at different data points. The approach essentially uses a derived analytical representation of the L2-norm for kernel functions. Thus the approach substantially extends the flexibility of the traditional kernel SVM for account of the number of adjusted parameters and minimization of the training error not only over weights of kernels but also over their shapes. The important issue of selecting the optimal regularization parameter is resolved by minimizing the test error over this parameter. Numerical illustrations are provided.

math.OC↗

arXiv2024

Unbalanced Optimal Transport and Maximum Mean Discrepancies: Interconnections and Rapid Evaluation

This contribution presents substantial computational advancements to compare measures even with varying masses. Specifically, we utilize the nonequispaced fast Fourier transform to accelerate the radial kernel convolution in unbalanced optimal transport approximation, built upon the Sinkhorn algorithm. We also present accelerated schemes for maximum mean discrepancies involving kernels. Our approaches reduce the arithmetic operations needed to compute distances from $\mathcal O(n^2)$ to $\mathcal O(n\log n)$, opening the door to handle large and high-dimensional datasets efficiently. Furthermore, we establish robust connections between transportation problems, encompassing Wasserstein distance and unbalanced optimal transport, and maximum mean discrepancies. This empowers practitioners with compelling rationale to opt for adaptable distances.

math.OC↗

arXiv2024

On the Approximation of Kernel functions

Various methods in statistical learning build on kernels considered in reproducing kernel Hilbert spaces. In applications, the kernel is often selected based on characteristics of the problem and the data. This kernel is then employed to infer response variables at points, where no explanatory data were observed. The data considered here are located in compact sets in higher dimensions and the paper addresses approximations of the kernel itself. The new approach considers Taylor series approximations of radial kernel functions. For the Gauss kernel on the unit cube, the paper establishes an upper bound of the associated eigenfunctions, which grows only polynomially with respect to the index. The novel approach substantiates smaller regularization parameters than considered in the literature, overall leading to better approximations. This improvement confirms low rank approximation methods such as the Nyström method.

stat.ML↗

arXiv2024

Higher order measures of risk and stochastic dominance

Higher order risk measures are stochastic optimization problems by design, and for this reason they enjoy valuable properties in optimization under uncertainties. They nicely integrate with stochastic optimization problems, as has been observed by the intriguing concept of the risk quadrangles, for example. Stochastic dominance is a binary relation for random variables to compare random outcomes. It is demonstrated that the concepts of higher order risk measures and stochastic dominance are equivalent, they can be employed to characterize the other. The paper explores these relations and connects stochastic orders, higher order risk measures and the risk quadrangle. Expectiles are employed to exemplify the relations obtained.

q-fin.RM↗

arXiv2023

Soft Quantization using Entropic Regularization

The quantization problem aims to find the best possible approximation of probability measures on ${\mathbb{R}}^d$ using finite, discrete measures. The Wasserstein distance is a typical choice to measure the quality of the approximation. This contribution investigates the properties and robustness of the entropy-regularized quantization problem, which relaxes the standard quantization problem. The proposed approximation technique naturally adopts the softmin function, which is well known for its robustness in terms of theoretical and practicability standpoints. Moreover, we use the entropy-regularized Wasserstein distance to evaluate the quality of the soft quantization problem's approximation, and we implement a stochastic gradient approach to achieve the optimal solutions. The control parameter in our proposed method allows for the adjustment of the optimization problem's difficulty level, providing significant advantages when dealing with exceptionally challenging problems of interest. As well, this contribution empirically illustrates the performance of the method in various expositions.

math.PR↗

arXiv2023

Constrained Global Optimization by Smoothing

This paper proposes a novel technique called "successive stochastic smoothing" that optimizes nonsmooth and discontinuous functions while considering various constraints. Our methodology enables local and global optimization, making it a powerful tool for many applications. First, a constrained problem is reduced to an unconstrained one by the exact nonsmooth penalty function method, which does not assume the existence of the objective function outside the feasible area and does not require the selection of the penalty coefficient. This reduction is exact in the case of minimization of a lower semicontinuous function under convex constraints. Then the resulting objective function is sequentially smoothed by the kernel method starting from relatively strong smoothing and with a gradually vanishing degree of smoothing. The finite difference stochastic gradient descent with trajectory averaging minimizes each smoothed function locally. Finite differences over stochastic directions sampled from the kernel estimate the stochastic gradients of the smoothed functions. We investigate the convergence rate of such stochastic finite-difference method on convex optimization problems. The "successive smoothing" algorithm uses the results of previous optimization runs to select the starting point for optimizing a consecutive, less smoothed function. Smoothing provides the "successive smoothing" method with some global properties. We illustrate the performance of the "successive stochastic smoothing" method on test-constrained optimization problems from the literature.

math.OC↗

arXiv2023

Portfolio reshaping under 1st order stochastic dominance constraints by the exact penalty function methods

The paper addresses general constrained and non-linear optimization problems. For some of these notoriously hard problems, there exists a reformulation as an unconstrained, global optimization problem. We illustrate the transformation, and the performance of the reformulation for a non-linear problem in stochastic optimization. The problem is adapted from portfolio optimization with first order stochastic dominance constraints.

math.OC↗

arXiv2023

Expectiles In Risk Averse Stochastic Programming and Dynamic Optimization

This paper features expectiles in dynamic and stochastic optimization. Expectiles are a family of risk functionals characterized as minimizers of optimization problems. For this reason, they enjoy various unique stability properties, which can be exploited in risk averse management, in stochastic optimization and in optimal control. The paper provides tight relates of expectiles to other risk functionals and addresses their properties in regression. Further, we extend expectiles to a dynamic framework. As such, they allow incorporating a risk averse aspect in continuous-time dynamic optimization and a risk averse variant of the Hamilton-Jacobi-Bellman equations.

math.OC↗

arXiv2023

Nonequispaced Fast Fourier Transform Boost for the Sinkhorn Algorithm

This contribution features an accelerated computation of the Sinkhorn's algorithm, which approximates the Wasserstein transportation distance, by employing nonequispaced fast Fourier transforms (NFFT). The algorithm proposed allows approximations of the Wasserstein distance by involving not more than $\mathcal O(n\log n)$ operations for probability measures supported by~$n$ points. Furthermore, the proposed method avoids expensive allocations of the characterizing matrices. With this numerical acceleration, the transportation distance is accessible to probability measures out of reach so far. Numerical experiments using synthetic and real data affirm the computational advantage and superiority.

math.OC↗

arXiv2022

Mortality in Germany during the Covid-19 pandemic

The Covid-19 pandemic still causes severe impacts on society and the economy. This paper studies excess mortality during the pandemic years 2020 and 2021 in Germany empirically with a special focus on the life insurer's perspective. Our conclusions are based on official counts of German governmental offices on the living and deaths of the entire population. Conclusions, relevant for actuaries and specific insurance business lines, including portfolios of pension, life, and health insurance contracts, are provided.

q-fin.GN↗

arXiv2022

Wasserstein sensitivity of Risk and Uncertainty Propagation

When propagating uncertainty in the data of differential equations, the probability laws describing the uncertainty are typically themselves subject to uncertainty. We present a sensitivity analysis of uncertainty propagation for differential equations with random inputs to perturbations of the input measures. We focus on the elliptic diffusion equation with random coefficient and source term, for which the probability measure of the solution random field is shown to be Lipschitz-continuous in both total variation and Wasserstein distance. The result generalizes to the solution map of any differential equation with locally Hölder dependence on input parameters. In addition, these results extend to Lipschitz continuous quantities of interest of the solution as well as to coherent risk functionals of these applied to evaluate the impact of their uncertainty. Our analysis is based on the sensitivity of risk functionals and pushforward measures for locally Hölder mappings with respect to the Wasserstein distance of perturbed input distributions. The established results are applied, in particular, to the case of lognormal diffusion and the truncation of series representations of input random fields.

math.PR↗

arXiv2021

Uniform Function Estimators in Reproducing Kernel Hilbert Spaces

This paper addresses the problem of regression to reconstruct functions, which are observed with superimposed errors at random locations. We address the problem in reproducing kernel Hilbert spaces. It is demonstrated that the estimator, which is often derived by employing Gaussian random fields, converges in the mean norm of the reproducing kernel Hilbert space to the conditional expectation and this implies local and uniform convergence of this function estimator. By preselecting the kernel, the problem does not suffer from the curse of dimensionality. The paper analyzes the statistical properties of the estimator. We derive convergence properties and provide a conservative rate of convergence for increasing sample sizes.

math.ST↗

arXiv2021

Uncertainty Analysis for Drift-Diffusion Equations

We study evolution equations of drift-diffusion type when various parameters are random. Motivated by applications in pedestrian dynamics, we focus on the case when the total mass is, due to boundary or reaction terms, not conserved. After providing existence and stability for the deterministic problem, we consider uncertainty in the data. Instead of a sensitivity analysis we propose to measure functionals of the solution, so-called quantities of interest (QoI), by involving scalarizing statistics. For these summarizing statistics we provide probabilistic continuity results.

math.PR↗

arXiv2021

Mathematical Foundations of Distributionally Robust Multistage Optimization

Distributionally robust optimization involves various probability measures in its problem formulation. They can be bundled to constitute a risk functional. For this equivalence, risk functionals constitute a fundamental building block in distributionally robust stochastic programming. Multistage programming requires conditional versions of risk functionals to re-assess future risk after partial realizations and after preceding decisions. This paper discusses a construction of the conditional counterpart of a risk functional by passing its genuine characteristics to its conditional counterparts. The conditional risk functionals turn out to be different from the nested analogues of the original (law invariant) risk measure. It is demonstrated that the initial measure and its nested decomposition can be used in a distributionally robust multistage setting.

math.OC↗

arXiv2021

Foundations of Multistage Stochastic Programming

Multistage stochastic optimization problems are oftentimes formulated informally in a pathwise way. These are correct in a discrete setting and suitable when addressing computational challenges, for example. But the pathwise problem statement does not allow an analysis with mathematical rigor and is therefore not appropriate. This paper addresses the foundations. We provide a novel formulation of multistage stochastic optimization problems by involving adequate stochastic processes as control. The fundamental contribution is a proof that there exist measurable versions of intermediate value functions. Our proof builds on the Kolmogorov continuity theorem. A verification theorem is given in addition, and it is demonstrated that all traditional problem specifications can be stated in the novel setting with mathematical rigor. Further, we provide dynamic equations for the general problem, which is developed for various problem classes. The problem classes covered here include Markov decision processes, reinforcement learning and stochastic dual dynamic programming.

math.OC↗