A Variational Framework for the Algorithmic Complexity of PDE Solutions

TL;DR

This paper introduces a variational framework that uses gradient flows to analyze the computability and complexity of PDE solutions, connecting structural PDE properties to their algorithmic solvability and resource requirements.

Contribution

It presents a novel optimization-based approach to study PDE computability and complexity, linking PDE structural features to solution difficulty and computational resource scaling.

Findings

Identifies conditions under which PDE solutions are polynomial-time computable.

Characterizes regimes with super-polynomial complexity blowup.

Links PDE structural properties to computational complexity.

Abstract

Partial Differential Equations (PDEs) are fundamental tools for modeling physical phenomena, yet most PDEs of practical interest cannot be solved analytically and require numerical approximations. The feasibility of such numerical methods, however, is ultimately constrained by the limitations of existing computation models. Since digital computers constitute the primary physical realizations of numerical computation, and Turing machines define their theoretical limits, the question of Turing computability of PDE solutions arises as a problem of fundamental theoretical significance. The Turing computability of PDE solutions provides a rigorous framework to distinguish equations that are, in principle, algorithmically solvable from those that inherently encode undecidable or non-computable behavior. Once computability is established, complexity theory extends the analysis by quantifying the computational resources required to approximate the corresponding PDE solutions. In this work, we present a novel framework based on least-squares variational formulations and their associated gradient flows to study the computability and complexity of PDE solutions from an optimization perspective. Our approach enables the approximation of PDE solution operators via discrete gradient flows, linking structural properties of the PDE, such as coercivity, ellipticity, and convexity, to the inherent complexity of their solutions. This framework characterizes both regimes where PDEs admit effective numerical solvers in polynomial-time and those exhibiting complexity blowup, where the input data possess polynomial-time complexity, yet the solution itself scales super-polynomially.