# On singular Frobenius for second order linear partial differential equations

## TL;DR

This paper extends Frobenius methods to second order linear PDEs, introducing geometric and algebraic tools to solve classical equations and model ODEs as PDEs with explicit solutions.

## Contribution

It develops a new Frobenius-inspired framework for second order PDEs, including regular singularity, indicial conic, and convergence theorems, broadening solution classes.

## Key findings

- Solved classical PDEs like heat, wave, and Laplace equations using the new method.
- Established geometric interpretation of Frobenius conditions via indicial conic.
- Constructed PDE models for classical ODEs with explicit polynomial solutions.

## Abstract

The main subject of this paper is the study of analytic second order linear partial differential equations. We aim to solve the classical equations and some more, in the real or complex analytical case. This is done by introducing methods inspired by the method of Frobenius method for second order linear ordinary differential equations. We introduce a notion of Euler type partial differential equation. To such a PDE we associate an indicial conic, which is an affine plane curve of degree two. Then comes the concept of regular singularity and finally convergence theorems, which must necessarily take into account the type of PDE (parabolic, elliptical or hyperbolic) and a nonresonance condition. This condition gives a new geometric interpretation of the original condition between the roots of the original Frobenius theorem for second order ODEs. The interpretation is something like, a certain reticulate has or not vertices on the indexical conic. Finally, we retrieve the solution of all the classical PDEs by this method (heat diffusion, wave propagation and Laplace equation), and also increase the class of those that have explicit algorithmic solution to far beyond those admitting separable variables. The last part of the paper is dedicated to the construction of PDE models for the classical ODEs like Airy, Legendre, Laguerre, Hermite and Chebyshev by two different means. One model is based on the requirement that the restriction of the PDE to lines through the origin must be the classical ODE model. The second is based on the idea of having symmetries on the PDE model and imitating the ODE model. We study these PDEs and obtain their solutions, obtaining for the framework of PDEs some of the classical results, like existence of polynomial solutions (Laguerre, Hermite and Chebyshev polynomials).