# A generalization of Picard-Lindelof theorem/ the method of characteristics to systems of PDE

## TL;DR

This paper extends the Picard-Lindelof theorem and the method of characteristics to a broader class of first-order PDE systems, providing a framework for existence and uniqueness of solutions under Lipschitz or $C^r$ conditions.

## Contribution

It generalizes classical methods to nonlinear systems and hyperbolic quasilinear PDEs, introducing a discretization approach for constructing unique solutions.

## Key findings

- Established local existence and uniqueness for generalized PDE systems.
- Developed a discretization method to approximate solutions.
- Discussed extensions to nonlinear and parameter-dependent systems.

## Abstract

We generalize Picard-Lindelof theorem/ the method of characteristics to the following system of PDE: $C_{il}(x,y) {\partial y_i / \partial x_l} + {\partial y_i / \partial x_m} = D_i(x,y)$. With a Lipschitz or $C^r$ $C_{il},D_i: [-a, a]^{m} \times [-b, b]^{n} \rightarrow \mathbb{R}$ and initial condition $I_i: [-\bar{a}, \bar{a}]^{m-1} \rightarrow (-b,b)$, $\bar{a} \leq a$, we obtain a local unique Lipschitz or $C^r$ solution $f$, respectively that satisfies the initial condition, $f_i (v, 0 ) = I_i(v)$, $v \in [-\bar{a}, \bar{a}]^{m-1}$. To construct the solution we set bounds on the value of the solution by discretizing the domain of the solution along the direction perpendicular to the initial condition hyperplane. As the number of discretization hyperplanes is taken to infinity the upper and lower bounds of the solution approach each other, hence this gives a unique function for the solution ($Ufs$). A locality condition is derived based on the constants of the problem. The dependence of $C_{il}$, $D_i$ and $I_i$ on parameters, the generalization to nonlinear systems of PDE and the application to hyperbolic quasilinear systems of first order PDE in two independent variables is discussed.