Elimination of unknowns for systems of algebraic differential-difference equations

Abstract:

We establish effective elimination theorems for ordinary 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 ordinary differential-difference equations in the variables $\mathbfit {x} = x_1, \ldots , x_q$ and $\mathbfit {y} = y_1, \ldots , y_r$, each of which has order and degree in $\mathbfit {y}$ bounded by $s$ over a differential-difference field, there is a nontrivial consequence of this system involving just the $\mathbfit {x}$ 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.