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Elimination of unknowns for systems of algebraic differential-difference equations. (English) Zbl 1458.12003

Designing methods for solving equations is probably the original problem area of algebra. When we solve systems of linear equations, we triangulate the system, i.e., we generate an equivalent (same solutions) system in which some variables are eliminated from some of the equations. Elimination of variables is also the goal we try to obtain when we apply methods of resultants or Gröbner bases to algebraic equations. Can we pursue such a goal also for algebraic differential-difference equations? The authors of this paper show that we can.
A central fact was proven by Gauss in the Fundamental Theorem of Algebra: every non-constant univariate polynomial with complex coefficients has at least one complex root. Hilbert has extended this to the Nullstellensatz: every ideal \(I\) in an \(n\)-variate polynomial ring over an algebraically closed field \(K\) which does not contain a non-zero constant has a root in \(K^n\). In other words, \(I\) has a common root if and only if \(1 \not\in I\). The Nullstellensatz can even be made “effective” by giving a bound \(B\) such that \(1\) is not in \(I\) if and only if 1 cannot be generated as a sum of multiples from a finite basis of \(I\) by employing multiplies of degree bounded by \(B\).
In this paper the authors show that such effective bounds for deciding the solvability of systems can also be derived for equations in differential-difference rings.

MSC:

12H05 Differential algebra
12H10 Difference algebra
03C10 Quantifier elimination, model completeness, and related topics
14Q20 Effectivity, complexity and computational aspects of algebraic geometry

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