arXiv subjects

Markus Tempelmayr

Publications and source records attributed to Markus Tempelmayr.

8 recordsLinked to original sources

arXiv2026

Weak universality for stochastic reaction-diffusion models with long-range correlated noise

We study the large-scale behaviour of a family of stochastic reaction-diffusion equations driven by long-range correlated noise in a weakly nonlinear regime. Depending on the decay of correlations of the noise and the strength of the nonlinearity, the universal behaviour is governed by a version of the dynamical $\Phi^p$ model with long-range correlated noise, and with a coupling constant determined by the reaction term of the microscopic model. Our main result establishes the stochastic estimates and convergence of models required in the theory of regularity structures. We adapt the multiindex-based approach to regularity structures using a suitable expansion of the reaction term tailored to the law of the noise. This yields a systematic weak universality result, allowing for a particularly simple identification of the macroscopic limit throughout the full subcritical regime and beyond Gaussian noise. The method appears robust and applicable to a broader class of singular stochastic PDEs.

math.PR↗

arXiv2025

A priori bounds for stochastic porous media equations via regularity structures

We prove a priori bounds for solutions of singular stochastic porous media equations with multiplicative noise in their natural $L^1$-based regularity class. We consider the first singular regime, i.e.~noise of space-time regularity $\alpha-2$ for $\alpha\in(2/3,1)$, and prove modelledness of the solution in the sense of regularity structures with respect to the solution of the corresponding linear stochastic heat equation. The proof relies on the kinetic formulation of the equation and a novel renormalized energy inequality. A careful analysis allows to balance the degeneracy of the diffusion coefficient against sufficiently strong damping of the multiplicative noise for small values of the solution.

math.AP↗

arXiv2024

Lecture notes on Malliavin calculus in regularity structures

Malliavin calculus provides a characterization of the centered model in regularity structures that is stable under removing the small-scale cut-off. In conjunction with a spectral gap inequality, it yields the stochastic estimates of the model. This becomes transparent on the level of a notion of model that parameterizes the solution manifold, and thus is indexed by multi-indices rather than trees, and which allows for a more geometric than combinatorial perspective. In these lecture notes, this is carried out for a PDE with heat operator, a cubic nonlinearity, and driven by additive noise, reminiscent of the stochastic quantization of the Euclidean $\phi^4$ model. More precisely, we informally motivate our notion of the model $(\Pi,\Gamma)$ as charts and transition maps, respectively, of the nonlinear solution manifold. These geometric objects are algebrized in terms of formal power series, and their algebra automorphisms. We will assimilate the directional Malliavin derivative to a tangent vector of the solution manifold. This means that it can be treated as a modelled distribution, thereby connecting stochastic model estimates to pathwise solution theory, with its analytic tools of reconstruction and integration. We unroll an inductive calculus that in an automated way applies to the full subcritical regime.

math.PR↗

arXiv2023

Stochastic estimates for the thin-film equation with thermal noise

We construct and derive uniform stochastic estimates on the renormalised model for a class of fourth-order conservative quasilinear singular SPDEs in arbitrary dimension $d\geq 1$ and in the full subcritical regime of noise regularity. The prototype of the class of equations we study is the so-called thin-film equation with thermal noise, also commonly referred to in the literature as the stochastic thin-film equation. We derive an explicit expression for the form of the counterterm as a function of the film mobility which is in surprising agreement with the form conjectured in Remark 9.1 of Math. Comp. 92 (2023), 1931-976.

math.AP↗

arXiv2023

Characterizing models in regularity structures: a quasilinear case

We give a novel characterization of the centered model in regularity structures which persists for rough drivers even as a mollification fades away. We present our result for a class of quasilinear equations driven by noise, however we believe that the method is robust and applies to a much broader class of subcritical equations. Furthermore, we prove that a convergent sequence of noise ensembles, satisfying uniformly a spectral gap assumption, implies the corresponding convergence of the associated models. Combined with the characterization, this establishes a universality-type result.

math.PR↗

arXiv2023

The structure group for quasi-linear equations via universal enveloping algebras

We consider the approach of replacing trees by multi-indices as an index set of the abstract model space $\mathsf{T}$ introduced by Otto, Sauer, Smith and Weber to tackle quasi-linear singular SPDEs. We show that this approach is consistent with the postulates of regularity structures when it comes to the structure group $\mathsf{G}$. In particular, $\mathsf{G}\subset{\rm Aut}(\mathsf{T})$ arises from a Hopf algebra $\mathsf{T}^+$ and a comodule $Δ\colon\mathsf{T}\rightarrow \mathsf{T}^+\otimes\mathsf{T}$. In fact, this approach, where the dual $\mathsf{T}^*$ of the abstract model space $\mathsf{T}$ naturally embeds into a formal power series algebra, allows to interpret $\mathsf{G}^*\subset{\rm Aut}(\mathsf{T}^*)$ as a Lie group arising from a Lie algebra $\mathsf{L} \subset{\rm End}(\mathsf{T}^*)$ consisting of derivations on this power series algebra. These derivations in turn are the infinitesimal generators of two actions on the space of pairs (nonlinearities, functions of space-time mod constants). These actions are shift of space-time and tilt by space-time polynomials. The Hopf algebra $\mathsf{T}^+$ arises from a coordinate representation of the universal enveloping algebra ${\rm U}(\mathsf{L})$ of the Lie algebra $\mathsf{L}$. The coordinates are determined by an underlying pre-Lie algebra structure of the derived algebra of $\mathsf{L}$. Strong finiteness properties, which are enforced by gradedness and the restrictive definition of $\mathsf{T}$, allow for this purely algebraic construction of $\mathsf{G}$. We also argue that there exist pre-Lie algebra and Hopf algebra morphisms between our structure and the tree-based one in the cases of branched rough paths (Grossman-Larson, Connes-Kreimer) and of the generalized parabolic Anderson model.

math-ph↗

arXiv2023

Lecture notes on tree-free regularity structures

These lecture notes are intended as reader's digest of recent work on a diagram-free approach to the renormalized centered model in Hairer's regularity structures. More precisely, it is about the stochastic estimates of the centered model, based on Malliavin calculus and a spectral gap assumption. We focus on a specific parabolic partial differential equation in quasi-linear form driven by (white) noise. We follow a natural renormalization strategy based on preserving symmetries, and carefully introduce Hairer's notion of a centered model, which provides the coefficients in a formal series expansion of a general solution. We explain how the Malliavin derivative in conjunction with Hairer's re-expansion map allows to reformulate this definition in a way that is stable under removing the small-scale regularization. A few exemplary proofs are provided, both of analytic and of algebraic character. The working horse of the analytic arguments is an ``annealed'' Schauder estimate and related Liouville principle, which is provided. The algebra of formal power series, in variables that play the role of coordinates of the solution manifold, and its algebra morphisms are the key algebraic objects.

math.PR↗

arXiv2021

A diagram-free approach to the stochastic estimates in regularity structures

In this paper, we explore the version of Hairer's regularity structures based on a greedier index set than trees, as introduced by Otto, Sauer, Smith and Weber. More precisely, we construct and stochastically estimate the renormalized model avoiding the use of Feynman diagrams but still in a fully automated, i. e. inductive way. This is carried out for a class of quasi-linear parabolic PDEs driven by noise in the full singular but renormalizable range. We assume a spectral gap inequality on the (not necessarily Gaussian) noise ensemble. The resulting control on the variance of the model naturally complements its vanishing expectation arising from the BPHZ-choice of renormalization. We capture the gain in regularity on the level of the Malliavin derivative of the model by describing it as a modelled distribution. Symmetry is an important guiding principle and built-in on the level of the renormalization Ansatz. Our approach is analytic and top-down rather than combinatorial and bottom-up.

math.PR↗