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Quantifier elimination
Quantifier elimination
Quantifier elimination
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Quantifier elimination is a concept of simplification used in mathematical logic, model theory, and theoretical computer science. Informally, a quantified statement " such that ..." can be viewed as a question "When is there an such that ...?", and the statement without quantifiers can be viewed as the answer to that question.[1]

One way of classifying formulas is by the amount of quantification. Formulas with less depth of quantifier alternation are thought of as being simpler, with the quantifier-free formulas as the simplest. A theory has quantifier elimination if for every formula , there exists another formula without quantifiers that is equivalent to it (modulo this theory).

Examples

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An example from mathematics says that a single-variable quadratic polynomial has a real root if and only if its discriminant is non-negative:[1]

Here the sentence on the left-hand side involves a quantifier , whereas the equivalent sentence on the right does not.

Examples of theories that have been shown decidable using quantifier elimination are Presburger arithmetic,[2][3][4][5][6] algebraically closed fields, real closed fields,[7][8] atomless Boolean algebras, term algebras, dense linear orders,[7] abelian groups,[9] Rado graphs, as well as many of their combinations such as Boolean algebra with Presburger arithmetic, and term algebras with queues.

Quantifier elimination for the theory of the real numbers as an ordered additive group is Fourier–Motzkin elimination; for the theory of the field of real numbers it is the Tarski–Seidenberg theorem.[7]

Quantifier elimination can also be used to show that "combining" decidable theories leads to new decidable theories (see Feferman–Vaught theorem).

Algorithms and decidability

[edit]

If a theory has quantifier elimination, then a specific question can be addressed: Is there a method of determining for each ? If there is such a method we call it a quantifier elimination algorithm. If there is such an algorithm, then decidability for the theory reduces to deciding the truth of the quantifier-free sentences. Quantifier-free sentences have no variables, so their validity in a given theory can often be computed, which enables the use of quantifier elimination algorithms to decide validity of sentences.

[edit]

Various model-theoretic ideas are related to quantifier elimination, and there are various equivalent conditions.

Every first-order theory with quantifier elimination is model complete. Conversely, a model-complete theory, whose theory of universal consequences has the amalgamation property, has quantifier elimination.[10]

The models of the theory of the universal consequences of a theory are precisely the substructures of the models of .[10] The theory of linear orders does not have quantifier elimination. However the theory of its universal consequences has the amalgamation property.

Basic ideas

[edit]

To show constructively that a theory has quantifier elimination, it suffices to show that we can eliminate an existential quantifier applied to a conjunction of literals, that is, show that each formula of the form:

where each is a literal, is equivalent to a quantifier-free formula. Indeed, suppose we know how to eliminate quantifiers from conjunctions of literals, then if is a quantifier-free formula, we can write it in disjunctive normal form

and use the fact that

is equivalent to

Finally, to eliminate a universal quantifier

where is quantifier-free, we transform into disjunctive normal form, and use the fact that is equivalent to

Relationship with decidability

[edit]

In early model theory, quantifier elimination was used to demonstrate that various theories possess properties like decidability and completeness. A common technique was to show first that a theory admits elimination of quantifiers and thereafter prove decidability or completeness by considering only the quantifier-free formulas. This technique can be used to show that Presburger arithmetic is decidable.

Theories could be decidable yet not admit quantifier elimination. Strictly speaking, the theory of the additive natural numbers did not admit quantifier elimination, but it was an expansion of the additive natural numbers that was shown to be decidable. Whenever a theory is decidable, and the language of its valid formulas is countable, it is possible to extend the theory with countably many relations to have quantifier elimination (for example, one can introduce, for each formula of the theory, a relation symbol that relates the free variables of the formula).[citation needed]

Example: Nullstellensatz for algebraically closed fields and for differentially closed fields.[clarification needed]

See also

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Notes

[edit]

References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Quantifier elimination

View on Grokipedia
from Grokipedia
Quantifier elimination is a technique in and that enables the transformation of any formula, potentially involving quantifiers such as universal (∀) and existential (∃), into an equivalent quantifier-free formula within specific theories, typically those admitting quantifier elimination. This property simplifies logical expressions by reducing them to combinations of atomic predicates—such as equations and inequalities—connected by operators like conjunction (∧), disjunction (∨), and negation (¬). Theories such as and real closed fields admit quantifier elimination. The concept was first introduced by Mojżesz Presburger in 1929 for and later generalized by in the mid-20th century, who demonstrated in 1948 that the theory of real closed fields—encompassing the real numbers with addition, multiplication, order, and constants 0 and 1—admits quantifier elimination, implying the decidability of its theory. Tarski's work extended to algebraically closed fields, showing that any elementary predicate in these structures is logically equivalent to a quantifier-free one, a result formalized through algebraic methods and model-theoretic principles. This breakthrough, later refined in Tarski's 1951 publication, provided an algorithmic procedure for elimination, though with high computational complexity, often doubly exponential in the number of variables. Quantifier elimination has profound implications across mathematics and , facilitating the solution of problems in , , and program verification by converting complex quantified statements into manageable quantifier-free forms. For instance, it underpins the Tarski-Seidenberg theorem, which guarantees the existence of such equivalents over the reals and supports applications like cylindrical algebraic decomposition (CAD) for semi-algebraic set analysis. In , it aids in deciding for logical formulas over ordered fields and has been applied to problems, such as static output feedback stabilization. Despite its theoretical power, practical implementations like QEPCAD software highlight ongoing challenges in efficiency for high-dimensional cases.

Definition and Fundamentals

Definition

In first-order logic, the syntax includes atomic formulas, which are expressions of the form P(t1,…,tn)P(t_1, \dots, t_n) where PP is a predicate symbol and tit_i are terms, or equality statements t1=t2t_1 = t_2. Quantifiers consist of the existential quantifier ∃\exists and the universal quantifier ∀\forall, which bind variables within their scope to express existence or universality. Quantifier-free formulas are then formed by combining atomic formulas using boolean connectives such as conjunction ∧\wedge, disjunction ∨\vee, ¬\neg, and implication →\to. A theory TT in a LL has quantifier elimination if, for every ϕ(x1,…,xn,y1,…,ym)\phi(x_1, \dots, x_n, y_1, \dots, y_m) of LL, there exists a quantifier-free ψ(y1,…,ym)\psi(y_1, \dots, y_m) such that, in every model of TT, ϕ\phi and ψ\psi define the same relations on the yy-parameters. Equivalently, T⊢∀x1…xn ϕ(x1,…,xn,y1,…,ym)↔ψ(y1,…,ym)T \vdash \forall x_1 \dots x_n \, \phi(x_1, \dots, x_n, y_1, \dots, y_m) \leftrightarrow \psi(y_1, \dots, y_m). This property facilitates the transformation of any quantified formula into an equivalent quantifier-free formula relative to TT. Both existential and universal quantifiers are handled through this equivalence: an existential quantifier ∃x ϕ(x,y)\exists x \, \phi(x, y) can be reduced to a universal one via the logical duality ∃x ϕ(x,y)↔¬∀x ¬ϕ(x,y)\exists x \, \phi(x, y) \leftrightarrow \neg \forall x \, \neg \phi(x, y), allowing iterative elimination until no quantifiers remain. Quantifier elimination ensures that the models of TT are preserved, as every first-order definable relation in those models coincides with a quantifier-free definable one, thereby reducing the expressive power of the theory to its quantifier-free fragment without loss of semantic content. This reduction simplifies analysis and proof within the theory. Quantifier elimination is closely related to model completeness, a property that every formula is equivalent to an existential one over the theory, and implies model completeness.

Basic Principles

Quantifier elimination in first-order theories often begins with syntactic manipulations to handle quantifiers systematically. For existential quantifiers, formulas are typically transformed into and then into (DNF), allowing the existential quantifier to distribute over disjunctions. This reduces the problem to eliminating the quantifier from a conjunction of literals, where the resulting quantifier-free formula is the disjunction of the eliminated forms from each conjunct. Universal quantifiers are addressed through logical duality, leveraging the equivalence ∀x ϕ(x)≡¬∃x ¬ϕ(x)\forall x \, \phi(x) \equiv \neg \exists x \, \neg \phi(x). This transforms into an existential one, enabling reduction to the existential case after applying twice, thereby preserving the focus on existential elimination procedures. On the model-theoretic side, quantifier elimination is closely tied to the properties of model completeness and the amalgamation property. A TT is model-complete if every is equivalent modulo TT to an existential , meaning that for any models M⊆N\mathcal{M} \subseteq \mathcal{N} of TT, every with parameters from M\mathcal{M} that holds in N\mathcal{N} already holds in M\mathcal{M}. The amalgamation property ensures that for any two models sharing a common submodel, there exists a third model into which both embed over the submodel. Specifically, a TT admits quantifier elimination if and only if it is model-complete and its universal fragment T∀T_\forall (the set of consequences of TT) has the amalgamation property. In cases where a lacks quantifier elimination in its original , it may still admit it after expanding the language with definable relations or predicates, such as order relations, which capture additional structure necessary for the equivalence to quantifier-free formulas. This expansion preserves the 's models while enabling the required elimination.

Theories Exhibiting Quantifier Elimination

Presburger Arithmetic

Presburger arithmetic is the first-order theory of the natural numbers with the successor function, addition, and order relation, but without multiplication. Although often studied over the integers (ℤ) with addition and order, the theory over natural numbers (ℕ) with successor, addition, and order is equivalent. The language consists of symbols for variables, logical connectives, quantifiers, equality, the constant 0, the successor function S, addition +, and the order relation <, allowing the expression of linear arithmetic statements over the non-negative integers. This theory captures properties of integer addition but avoids the undecidability introduced by multiplication, as seen in full Peano arithmetic. Quantifier elimination in Presburger arithmetic proceeds by transforming prenex formulas with existential quantifiers into equivalent quantifier-free formulas, leveraging the structure of linear Diophantine equations and inequalities. Specifically, for a formula of the form ∃x φ(x, \bar{y}), where φ involves linear combinations like sums of coefficients times variables plus constants, the elimination reduces to conditions on the coefficients and free variables \bar{y} using greatest common divisors (gcd) and congruence relations. The process often expands the language temporarily with divisibility predicates (e.g., k \mid t for integer k > 0 and term t), which can be eliminated afterward, yielding a purely quantifier-free result in the original language. For a more general case, the formula ∃x_1 \dots ∃x_k \left( \sum_{i=1}^k a_i x_i = b \right), with b a free variable or constant, eliminates to the condition that the gcd of the coefficients {a_1, \dots, a_k} divides b, as the image of the linear map is the ideal generated by this gcd in the integers. In 1929, Mojżesz Presburger established the decidability of this theory through a quantifier elimination procedure, demonstrating that every formula is equivalent to a quantifier-free one and providing an effective bound on the size of the eliminated formula, though modern analyses show the complexity can reach triple-exponential time. This result highlighted Presburger arithmetic as a decidable fragment of arithmetic, foundational for automated theorem proving and verification in linear integer constraints.

Real Closed Fields

A real closed field is an that is elementarily equivalent to the field of real numbers, characterized by the properties that every positive element has a and every univariate of odd degree has at least one root in the field. This definition ensures a unique ordering on the field, where the positive elements are precisely the nonzero squares, and it captures the properties essential for algebraic and analytic reasoning over the reals. The real numbers themselves form the prototypical example of a real closed field, and any can be extended to a real closed field, known as its real closure. The theory of real closed fields admits quantifier elimination, as established by the Tarski-Seidenberg theorem, a fundamental concept in real algebraic geometry. This theorem states that every first-order in the language of ordered fields is equivalent to a quantifier-free over any real closed field. Geometrically, it asserts that a set defined by polynomial equations and inequalities in an (n+1)-dimensional space can be projected onto an n-dimensional space, and the resulting set will still be definable by polynomial identities and inequalities; this projection property directly implies quantifier elimination over the reals, effectively providing an algorithm to determine if a given set of polynomial inequalities has a real solution. first proved this result in the 1930s, with a complete exposition appearing in his 1951 , where he demonstrated that quantifiers can be systematically eliminated using the field's algebraic properties. Abraham Seidenberg provided an alternative, more algebraic proof in the 1950s, avoiding some of Tarski's model-theoretic machinery and emphasizing elimination via resultants and discriminants. This theorem implies model completeness for the theory, allowing complex existential statements to be reduced to combinations of inequalities and equalities. Quantifier elimination in real closed fields is particularly powerful for describing semi-algebraic sets, which are finite unions of sets defined by polynomial inequalities and equalities; the elimination process yields a quantifier-free description of their projections via sign conditions on a finite set of polynomials. For instance, consider the existential formula ∃x , p(x) = 0, where p(x) = ax^2 + bx + c is a quadratic polynomial with a ≠ 0; this is equivalent to the quantifier-free condition that the discriminant b^2 - 4ac ≥ 0. More generally, for an existential formula ∃x \bigwedge_{i=1}^n p_i(x) > 0 \wedge \bigwedge_{j=1}^m q_j(x) = 0 involving polynomials p_i and q_j, quantifier elimination produces an equivalent quantifier-free formula consisting of sign conditions on resultants of the q_j and discriminants derived from the system. These reductions enable precise geometric and analytic characterizations without bound variables, foundational for real algebraic geometry.

Algorithms and Methods

Fourier-Motzkin Elimination

Fourier-Motzkin elimination is a classical for performing quantifier elimination in the theory of linear real arithmetic, specifically for eliminating existential quantifiers from prenex formulas consisting of conjunctions of linear inequalities over the real numbers. The method works by successively eliminating one variable at a time, projecting the defined by the inequalities onto the remaining variables, resulting in an equivalent quantifier-free formula that is equisatisfiable with the original. This projection preserves the semantics of by ensuring that the eliminated can take a real value satisfying the constraints if and only if the projected inequalities hold. The algorithm was first described by in as part of his work on solving systems of linear inequalities, and it was independently rediscovered and refined by Theodore Motzkin in his 1936 Ph.D. thesis. To eliminate an existentially quantified variable xx from a of the form ∃x⋀k=1n(akx+bk⋅y+ck≥0)\exists x \bigwedge_{k=1}^n (a_k x + \mathbf{b}_k \cdot \mathbf{y} + c_k \geq 0), where y\mathbf{y} represents the remaining free variables, bk\mathbf{b}_k are vectors, and ak,ck∈Ra_k, c_k \in \mathbb{R}, the inequalities are partitioned into three sets based on the aka_k of xx:
  • PP: inequalities with ak>0a_k > 0 (providing lower bounds on xx),
  • NN: inequalities with ak<0a_k < 0 (providing upper bounds on xx),
  • ZZ: inequalities with ak=0a_k = 0 (independent of xx).
The set ZZ is retained unchanged in the output. If P=∅P = \emptyset or N=∅N = \emptyset, the projection includes only ZZ along with any implied contradictions (e.g., a strict inequality like 0>00 > 0). Otherwise, for every pair of inequalities p∈Pp \in P and n∈Nn \in N, a new inequality is generated by combining them to cancel the coefficient of xx: specifically, multiply pp by −an-a_n (positive) and nn by apa_p (positive), then add to obtain (−an)(bp⋅y+cp)+ap(bn⋅y+cn)≥0(-a_n)(\mathbf{b}_p \cdot \mathbf{y} + c_p) + a_p (\mathbf{b}_n \cdot \mathbf{y} + c_n) \geq 0. This yields up to ∣P∣⋅∣N∣|P| \cdot |N| new inequalities, which, together with ZZ, form the projected system. The process is repeated for each quantified variable until the formula is quantifier-free. A key challenge in the procedure is redundancy, as the number of generated inequalities can grow quadratically per elimination step (at most (n/2)2(n/2)^2 for nn input inequalities), leading to a doubly exponential worst-case complexity in the number of variables—specifically, O((n/2)2d)O((n/2)^{2^d}) where dd is the number of variables to eliminate and nn the initial number of inequalities—making it impractical for high dimensions without optimizations like redundancy removal. Despite this, the algorithm is complete for deciding satisfiability in linear real arithmetic and has been implemented in various computer algebra systems, such as Redlog, where it serves as a foundational method for handling linear quantifier elimination tasks. For illustration, consider the ∃x1(x1−x2≤0∧x1−x3≤0∧−x1+x2+2x3≤0∧−x3≤−1)\exists x_1 \left( x_1 - x_2 \leq 0 \land x_1 - x_3 \leq 0 \land -x_1 + x_2 + 2x_3 \leq 0 \land -x_3 \leq -1 \right), rewritten in standard form as inequalities ≥0\geq 0. Eliminating x1x_1 (with positive coefficients in the first two, negative in the third) pairs the inequalities to produce new constraints 2x3≤02x_3 \leq 0 and x2+x3≤0x_2 + x_3 \leq 0, along with the independent −x3+1≥0-x_3 + 1 \geq 0. Further eliminating x3x_3 from these yields a contradiction (e.g., 1≤01 \leq 0), showing the original is unsatisfiable.

Virtual Substitution

Virtual substitution is a method for quantifier elimination in the theory of real closed fields, providing an effective procedure for the Tarski-Seidenberg theorem by parameterizing solutions through the substitution of virtual test terms such as and critical points of polynomials, thereby reducing quantified formulas to a finite disjunction of quantifier-free cases. This approach excels in handling nonlinear polynomials by leveraging algebraic techniques to isolate relevant solution sets without enumerating all possible combinations. The procedure for eliminating an existential quantifier ∃x φ(x, y), where φ is a quantifier-free involving polynomials in x and parameters y over the reals, involves constructing an elimination set E consisting of pairs (γ, t), where γ is a guard condition (e.g., conditions on coefficients or discriminants) and t is a virtual test term (e.g., rational functions or expressed algebraically). The quantified formula is then equivalent to the disjunction over E of γ ∧ φ(x ← t, y). For a equation p(x, y) = 0, the method computes resultants or discriminants with respect to x to identify , isolating them via algebraic means such as , and substitutes bounds or critical points to cover intervals where the polynomial's or zero set behaves consistently. In the nonlinear case, this extends to higher degrees by recursively applying substitution tables that handle up to quadratic terms per elimination step, with guards ensuring coverage of all real solutions. Virtual substitution was developed by Volker Weispfenning in 1988, initially for the linear case, and later extended to nonlinear polynomials as part of broader quantifier elimination frameworks, achieving single exponential time complexity for linear formulas and double exponential for general cases with bounded degree. Implementations like those in the Redlog system demonstrate its practicality, often outperforming general methods for problems with low quantifier alternations. Consider the formula ∃x (x² + y < 0). To eliminate the quantifier, virtual substitution identifies the critical point of the polynomial x² + y at x = 0, where the minimum value is y. The guard condition is true (no special conditions needed for the vertex), and substitution of the test term x = 0 yields y < 0. For y ≥ 0, the minimum value x² + y ≥ y ≥ 0, so no real x satisfies the strict inequality, confirming no solution. Root isolation for the related equation x² + y = 0 (i.e., x² = -y) uses Sturm sequences or discriminant analysis on the univariate polynomial in x to determine that real roots exist only if y < 0, with sign changes indicating the intervals where the inequality holds, leading to the quantifier-free equivalent y < 0. Compared to Fourier-Motzkin elimination, virtual substitution handles nonlinearity more efficiently through targeted algebraic and computations, avoiding the full projection of intermediate varieties that leads to in the number of inequalities. This makes it particularly suitable for parametric problems where y represents free variables, reducing the output size significantly for low-degree instances.

Implications for Decidability

Proving Decidability

Quantifier elimination provides a direct method for establishing the decidability of theories by reducing arbitrary formulas to equivalent quantifier-free ones, whose can then be checked using decidable procedures for combinations of atomic formulas, such as propositional logic solvers or direct evaluation in the theory's . Specifically, if a theory admits quantifier elimination and its quantifier-free fragment is decidable, then the entire is decidable, as any sentence can be transformed into a quantifier-free equivalent whose determines the original sentence's validity. While the converse does not always hold—decidability does not imply quantifier elimination—quantifier elimination simplifies proofs of decidability by providing an explicit algorithmic reduction. A seminal historical application of this principle is Presburger's 1929 proof of decidability for the theory of natural numbers with addition (), where he developed a quantifier elimination procedure that reduces formulas to a finite set of cases analyzable via congruence relations, thereby establishing an algorithm to decide truth. In a similar vein, Alfred Tarski's 1948 work on the theory of real closed fields demonstrated decidability through quantifier elimination, showing that every formula is equivalent to a quantifier-free one involving only inequalities, with the procedure later optimized for computational efficiency while preserving the core logical implication. For instance, the of dense linear orders without endpoints admits quantifier elimination, reducing formulas to quantifier-free combinations of betweenness relations (e.g., [a<b](/page/ListofFrenchcomposers)[a < b](/page/List_of_French_composers)), which are decidable via simple evaluations, thereby yielding overall decidability for the .

Complexity Considerations

Quantifier elimination procedures, while theoretically complete for certain theories, face significant computational challenges due to their inherent . The Fourier-Motzkin elimination , used for eliminating existential quantifiers in systems of linear inequalities over the reals, has a double exponential in the number of variables, arising from the in the number of inequalities at each elimination step, which can reach up to 2O(n2)2^{O(n^2)} in the worst case. Cylindrical algebraic decomposition (CAD), a cornerstone method for quantifier elimination in the theory of real closed fields, originally exhibits triply exponential complexity in the number of quantifier alternations in . Improvements by McCallum in the , particularly through refined projection operators, reduced this to doubly exponential complexity for well-oriented input formulas, enabling more practical applications in higher dimensions. QE-based decision procedures for the theory of real closed fields are when the dimension (number of variables) is fixed, reflecting the tractability of low-dimensional instances via optimized geometric methods. In general, however, these procedures require due to the need to manage exponentially growing intermediate representations across arbitrary dimensions. Recent advancements up to 2025 in SMT solvers such as Z3 incorporate lazy quantifier elimination techniques, including model-based projection and on-demand instantiation for nonlinear real arithmetic, which mitigate practical complexity by avoiding full upfront elimination and focusing on relevant instances during search. A specific complexity bound for QE in real closed fields, as established by Davenport and collaborators, shows that for a prenex with kk quantifiers, where ss bounds the coefficients' size and dd the polynomials' degrees, the runtime is O((sd)2k)O((sd)^{2^k}), highlighting the sensitivity to the quantifier prefix length.

Historical Development and Applications

Historical Milestones

The origins of quantifier elimination trace back to early methods for solving systems of inequalities, with introducing an elimination technique for linear inequalities in 1827 as part of his work on and . This approach, now known as Fourier-Motzkin elimination, was independently rediscovered and refined by Theodore Motzkin in 1936 to handle systems more efficiently. A significant milestone came in 1929 when Mojżesz Presburger established the decidability of the first-order theory of natural numbers under addition (Presburger arithmetic) by demonstrating a quantifier elimination procedure, proving the theory complete and consistent. In the 1930s, Alfred Tarski initiated foundational work on the theory of real closed fields, discovering that it admits quantifier elimination, though the full proof appeared in his 1948 publication providing a decision method for elementary algebra and geometry. Tarski's result implied the decidability of the theory but was complex; Abraham Seidenberg offered a simplified proof in 1954 using algebraic geometry techniques. During the 1950s, developed the notion of model completeness and showed that theories admitting quantifier elimination are model complete. In 1975, George E. Collins developed cylindrical algebraic decomposition as an effective algorithm for quantifier elimination over real closed fields, dramatically improving computational feasibility compared to Tarski's method. By the , quantifier elimination had become a central tool in , enabling key results such as the Ax-Kochen-Ershov theorems of 1965, which established quantifier elimination for the theory of p-adic fields after expanding the language with predicates for Henselian valuations. The 1980s and 1990s saw extensions to o-minimal structures, where Anand Pillay and collaborators proved quantifier elimination in expanded languages, revealing the tame geometry of definable sets in ordered structures like the reals with restricted analytic functions.

Modern Applications

In , quantifier elimination (QE) plays a crucial role in (SMT) solvers such as Z3 and CVC5, particularly for handling nonlinear real arithmetic (NRA) in tasks. Z3 supports QE tactics for linear real arithmetic and extends to approximate methods for NRA through techniques like virtual substitution and numerical optimization, enabling the resolution of quantified constraints in verification problems. Similarly, CVC5 incorporates a dedicated QE solver that tracks quantifier instantiations during SMT solving, supporting NRA via integration with linearization and polynomial approximations. Recent benchmarks up to 2025 demonstrate that while direct QE in Z3 and CVC5 solves around 145 and 103 instances respectively on termination verification benchmarks involving 335 C programs, specialized extensions like PolyQEnt outperform them by solving up to 154 instances with reduced average runtimes (e.g., 1.6 seconds versus 19.4 seconds for Z3). In synthesis tasks from 32 PolySynth benchmarks, CVC5 solves 0 instances directly, while Z3 handles 24, highlighting the challenges and ongoing improvements in QE for high-dimensional NRA. In , QE over real closed fields (RCF) is employed in the analysis of hybrid systems, where it facilitates computations by projecting existential quantifiers from continuous dynamics constraints. Tools like SpaceEx perform scalable analysis for hybrid systems with piecewise-affine dynamics using polyhedral approximations, and QE is integrated in complementary frameworks such as KeYmaera X to verify safety properties by eliminating variables in differential dynamic logic formulas over RCF. For instance, SpaceEx enables analysis of hybrid systems with over 100 variables using scalable polyhedral approximations for projections, while KeYmaera X employs exact QE over RCF for precise verification of safety properties in systems with fewer variables. In and , QE supports parameter learning in probabilistic models by solving problems over real-valued distributions, as seen in 2020s advancements in verification. For instance, QE techniques are applied to encode and verify robustness properties of quantized neural networks as SMT problems over RCF, eliminating quantifiers from input perturbation constraints to ensure adversarial safety. A key development is the integration of for heuristic QE, such as using and to optimize variable ordering in cylindrical algebraic (CAD), which reduces in high-dimensional problems. In a debiased of 41,370 problems, models achieve up to 60.43% accuracy in selecting optimal orderings, outperforming traditional heuristics and enabling QE on larger systems. A specific application arises in path planning, where QE eliminates variables from collision avoidance constraints defined over the reals, generating quantifier-free safety proofs for trajectories. By deriving explicit formulas from implicit quantified conditions (e.g., ensuring no overlap between a vehicle's path and obstacles), this method automates verification for planar maneuvers in systems like UAVs and collision avoidance, with runtimes under 1 second compared to hours of manual effort or gigabytes of memory for full CAD-based QE.

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