Thomas Sturm

Real Quantifier Elimination In Practice

We consider polynomials and rational functions which are invariant under the action of a finite linear group. The aim is to give a survey over the knowledge on some structural properties of such rings and fields of invariants. Particular emphasis lies on the modular case, where the characteristic of the ground field divides the group order.

A Case Study on the Parametric Occurrence of Multiple Steady States

We consider the problem of determining multiple steady states for positive real values in models of biological networks. Investigating the potential for these in models of the mitogen-activated protein kinases (MAPK) network has consumed considerable effort using special insights into the structure of corresponding models. Here we apply combinations of symbolic computation methods for mixed equality/inequality systems, specifically virtual substitution, lazy real triangularization and cylindrical algebraic decomposition. We determine multistationarity of an 11-dimensional MAPK network when numeric values are known for all but potentially one parameter. More precisely, our considered model has 11 equations in 11 variables and 19 parameters, 3 of which are of interest for symbolic treatment, and furthermore positivity conditions on all variables and parameters.

Parametric Systems of Linear Congruences

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.

Parametric quantified SAT solving

We generalize successful algorithmic ideas for quantified satisfiability solving to the parametric case where there are parameters in the input problem. The output is then not necessarily a truth value but more generally a propositional formula in the parameters of the input. Since one can naturally embed propositional logic into first-order logic over Boolean algebras, our work amounts from a model-theoretic point of view to a quantifier elimination procedure for initial Boolean algebras. Our work is completely and efficiently implemented in the logic package Redlog contained in the open source computer algebra system Reduce. We describe this implementation and discuss computation examples pointing at possible applications of our work to configuration problems in the automotive industry.

Symbolic Versus Numerical Computation and Visualization of Parameter Regions for Multistationarity of Biological Networks

We investigate models of the mitogenactivated protein kinases (MAPK) network, with the aim of determining where in parameter space there exist multiple positive steady states. We build on recent progress which combines various symbolic computation methods for mixed systems of equalities and inequalities. We demonstrate that those techniques benefit tremendously from a newly implemented graph theoretical symbolic preprocessing method. We compare computation times and quality of results of numerical continuation methods with our symbolic approach before and after the application of our preprocessing.

Effective Quantifier Elimination for Presburger Arithmetic with Infinity

We consider Presburger arithmetic extended by infinity. For this we give an effective quantifier elimination and decision procedure which implies also the completeness of our extension. The asymptotic worst-case complexity of our procedure is bounded by a function that is triply exponential in the input word length, which is known to be a tight bound for regular Presburger arithmetic. Possible application areas include quantifier elimination and decision procedures for Boolean algebras with cardinality constraints, which have recently moved into the focus of computer science research for software verification, and deductive database queries.

Parametric Qualitative Analysis of Ordinary Differential Equations: Computer Algebra Methods for Excluding Oscillations (Extended Abstract) (Invited Talk)

Investigating oscillations for parametric ordinary differential equations (ODEs) has many applications in science and engineering but is a very hard problem. Already for two dimensional polynomial systems this question is related to Hilbert’s 16th problem, which is still unsolved [1].

Supporting global numerical optimization of rational functions by generic symbolic convexity tests

Convexity is an important property in nonlinear optimization since it allows to apply efficient local methods for finding global solutions. We propose to apply symbolic methods to prove or disprove convexity of rational functions over a polyhedral domain. Our algorithms reduce convexity questions to real quantifier elimination problems. Our methods are implemented and publicly available in the open source computer algebra system Reduce. Our long term goal is to integrate Reduce as a workhorse for symbolic computations into a numerical solver.

Automatic verification of the adequacy of models for families of geometric objects

We consider parametric families of semi-algebraic geometric objects, each implicitly defined by a first-order formula. Given an unambiguous description of such an object family and an intended alternative description we automatically construct a first-order formula which is true if and only if our alternative description uniquely describes geometric objects of the reference description. We can decide this formula by applying real quantifier elimination. In the positive case we furthermore derive the defining first-order formulas corresponding to our new description. In the negative case we can produce sample points establishing a counterexample for the uniqueness. We demonstrate our method by automatically proving uniqueness theorems for characterizations of several geometric primitives and simple complex objects. Finally, we focus on tori, characterizations of which can be applied in spline approximation theory with toric segments. Although we cannot yet practically solve the fundamental open questions in this area within reasonable time and space, we demonstrate that they can be formulated in our framework. In addition this points at an interesting and practically relevant challenge problem for automated deduction in geometry in general.