Better Answers to Real Questions

We consider existential problems over the reals. Extended quantifier elimination generalizes the concept of regular quantifier elimination by providing in addition answers, which are descriptions of possible assignments for the quantified variables. Implementations of extended quantifier elimination via virtual substitution have been successfully applied to various problems in science and engineering. So far, the answers produced by these implementations included infinitesimal and infinite numbers, which are hard to interpret in practice. We introduce here a post-processing procedure to convert, for fixed parameters, all answers into standard real numbers. The relevance of our procedure is demonstrated by application of our implementation to various examples from the literature, where it significantly improves the quality of the results

Algorithmic strategies for applicable real quantifier elimination

One of the most important algorithms for real quantifier elimination is the quantifier elimination by virtual substitution introduced by Weispfenning in 1988. In this thesis we present numerous algorithmic approaches for optimizing this quantifier elimination algorithm. Optimization goals are the actual running time of the implementation of the algorithm and the size of the output formula. Strategies for obtaining these goals include simplification of first-order formulas,reduction of the size of the computed elimination set, and condensing a new replacement for the virtual substitution. Local quantifier elimination computes formulas that are equivalent to the input formula only nearby a given point. We can make use of this restriction for further optimizing the quantifier elimination by virtual substitution. Finally we discuss how to solve a large class of scheduling problems by real quantifier elimination. To optimize our algorithm for solving scheduling problems we make use of the special form of the input formula and of additional information given by the description of the scheduling problemEines der bedeutendsten Verfahren zur reellen Quantorenelimination ist die Quantorenelimination mittels virtueller Substitution, die von Weispfenning 1988 eingeführt wurde. In der vorliegenden Arbeit werden zahlreiche algorithmische Strategien zur Optimierung dieses Verfahrens präsentiert. Optimierungsziele der Arbeit waren dabei die tatsächliche Laufzeit der Implementierung des Algorithmus sowie die Größe der Ausgabeformel. Zur Optimierung werden dabei die Simplifikation vonFormeln erster Stufe, die Reduktion der Größe der Eliminationsmenge sowie das Condensing, ein Ersatz für die virtuelle Substitution,untersucht. Lokale Quantorenelimination berechnet Formeln, die nur inder Nähe eines gegebenen Punktes äquivalent zur Eingabeformel ist. Diese Einschränkung erlaubt es, das Verfahren weiter zu verbessern.Als Anwendung des Eliminationsverfahren diskutieren wir abschließend, wie man eine große Klasse von Schedulingproblemen mittels reeller Quantorenelimination lösen kann. In diesem Fall benutzen wir die spezielle Struktur der Eingabeformel und zusätzliche Informationen über das Schedulingproblem, um die Quantorenelimination mittels virtueller Substitution problemspezifisch zu optimieren

Thirty Years of Virtual Substitution

International audienceIn 1988, Weispfenning published a seminal paper introducing a substitution technique for quantifier elimination in the linear theories of ordered and valued fields. The original focus was on complexity bounds including the important result that the decision problem for Tarski Algebra is bounded from below by a double exponential function. Soon after, Weispfenning's group began to implement substitution techniques in software in order to study their potential applicability to real world problems. Today virtual substitution has become an established computational tool, which greatly complements cylindrical algebraic decomposition. There are powerful implementations and applications with a current focus on satisfia-bility modulo theory solving and qualitative analysis of biological networks

Parametric systems of linear congruences

Abstract. Based on an extended quantifier elimination procedure for discretely valued fields, we devise algorithms for solving multivariate systems of linear congruences over the integers. This includes determining integer solutions for sets of moduli which are all power of a fixed prime, uniform p-adic integer solutions for parametric prime power moduli, lifting strategies for these uniform p-adic solutions for given primes, and simultaneous lifting strategies for finite sets of primes. The method is finally extended to arbitrary moduli.

Guarded Expressions in Practice

Computer algebra systems typically drop some degenerate cases when evaluating expressions, e.g., x=x becomes 1 dropping the case x = 0. We claim that it is feasible in practice to compute also the degenerate cases yielding guarded expressions. We work over real closed fields but our ideas about handling guarded expression can be easily transferred to other situations. Using formulas as guards provides a powerful tool for heuristically reducing the combinatorial explosion of cases: equivalent, redundant, tautological, and contradictive cases can be detected by simplification and quantifier elimination. Our approach allows to simplify the expressions on the basis of simplification knowledge on the logical side. The method described in this paper is implemented in the reduce package guardian, which is freely available on the www. 2 1. INTRODUCTION 1 Introduction It is meanwhile a well-known fact that evaluations obtained with the interactive use of computer algebra systems (cas) are n..

Local Quantifier Elimination

We introduce local quantifier elimination as a new variant of real quantifier elimination. Given a first-order formula and a real point we compute a quantifier-free formula which is not only for the given point equivalent to the input formula but also for all points in a semialgebraic set containing the specified point. The description of this semi-algebraic set is explicitly computed in the form of a conjunction of atomic formulas. Local quantifier elimination is in its application area, superior to both regular and generic quantifier elimination due to faster running times and shorter results

P-adic Constraint Solving

We automatically check for the feasibility of arbitrary boolean combinations of linear parametric p-adic constraints using a quantier elimination method. This can be done uniformly for all p. We focus on the necessary simplication methods. Our method is implemented within the computer algebra system reduce. We illustrate the applicability of this implementation to non-trivial problems including the solution of systems of linear congruences over the integers. 1 Introduction It is well-known that linear parametric constraint solving over the reals has numerous important applications in science and engineering. The same holds for corresponding integer and mixed real-integer problems. In this article we consider analogue problems over p-adic numbers instead of real numbers. This also has important though less obvious applications, mainly in class eld theory and Diophantine analysis [Dub92]. One can, for instance, weaken the problem of nding integer solutions to a Diophantine ..