# Elimination of unknowns for systems of algebraic differential-difference equations

## TL;DR

This paper develops effective elimination theorems for algebraic differential-difference equations, providing a computable bound for deriving consequences involving only certain variables, and relates algebraic consistency to the existence of solutions in rings of meromorphic functions.

## Contribution

It introduces a computable function for eliminating variables in differential-difference systems and connects algebraic consistency with solutions in rings of meromorphic functions.

## Key findings

- Established a bound B(r,s) for variable elimination.
- Proved the equivalence of algebraic consistency and solutions in meromorphic function rings.
- Provided a method to determine when a differential-difference system has solutions.

## Abstract

We establish effective elimination theorems for differential-difference equations. Specifically, we find a computable function $B(r,s)$ of the natural number parameters $r$ and $s$ so that for any system of algebraic differential-difference equations in the variables $\mathbf{x} = x_1, \ldots, x_q$ and $\mathbf{y} = y_1, \ldots, y_r$ each of which has order and degree in $\mathbf{y}$ bounded by $s$ over a differential-difference field, there is a non-trivial consequence of this system involving just the $\mathbf{x}$ variables if and only if such a consequence may be constructed algebraically by applying no more than $B(r,s)$ iterations of the basic difference and derivation operators to the equations in the system. We relate this finiteness theorem to the problem of finding solutions to such systems of differential-difference equations in rings of functions showing that a system of differential-difference equations over $\mathbb{C}$ is algebraically consistent if and only if it has solutions in a certain ring of germs of meromorphic functions.

---
Source: https://tomesphere.com/paper/1812.11390