This paper presents the first study of the complexity of the optimization problem for integer linear-exponential programs which extend classical integer linear programs with the exponential function \(x \mapsto 2^x\) and the remainder function \((x,y) \mapsto (x \bmod 2^y)\). The problem of deciding if such a program has a solution was recently shown to be NP-complete in [Chistikov et al., ICALP’24]. The optimization problem instead asks for a solution that maximizes (or minimizes) a linear-exponential objective function, subject to the constraints of an integer linear-exponential program.

We investigate a fragment of Linear Temporal Logic (LTL) comprising the tomorrow (X) and eventually (F) modalities, and present a singly exponential time algorithm for the pastification problem within this fragment. The pastification problem consists of constructing, for a given LTL formula, an equivalent formula that exclusively employs past temporal operators. While the best known algorithms for this task in full LTL–and in the fragment under consideration–exhibit triply exponential time complexity, our approach achieves optimal complexity for this fragment. The proposed algorithm proceeds in two main stages: (i) the input formula is first translated into a tailored normal form, and then (ii) a pure past formula is synthesized from a tree-like structure derived from the normalized formula. With minor adaptations, the algorithm extends to handle the fragment of LTL featuring the tomorrow and globally modalities. We provide an implementation of the algorithm in the temporal reasoning tool BLACK, and report on an experimental evaluation of its performance.1

This paper introduces a generic framework that provides sufficient conditions for guaranteeing polynomial-time decidability of fixed-negation fragments of first-order theories that adhere to certain fixed-parameter tractability requirements. It enables deciding sentences of such theories with arbitrary existential quantification, conjunction and a fixed number of negation symbols in polynomial time. It was recently shown by Nguyen and Pak [SIAM J. Comput. 51(2): 1--31 (2022)] that an even more restricted such fragment of Presburger arithmetic (the first-order theory of the integers with addition and order) is NP-hard. In contrast, by application of our framework, we show that the fixed negation fragment of weak Presburger arithmetic, which drops the order relation from Presburger arithmetic in favour of equality, is decidable in polynomial time. We give two further examples of instantiations of our framework, showing polynomial-time decidability of the fixed negation fragments of weak linear real arithmetic and of the restriction of Presburger arithmetic in which each inequality contains at most one variable.

Lower bounds provide essential insights into the minimal computational resources required for algorithm execution. This paper focuses on logical theories, a domain where estimating resources is particularly difficult, and provides a novel, fully-automated method for computing lower bounds on memory usage, serving as a proxy for the computational resources required to perform logical reasoning. Specifically, the paper focuses on computing lower bounds on the size of the minimal deterministic finite automaton that encodes the solution set of a given Presburger arithmetic (also known as linear integer arithmetic) formula. The lower bounds are accompanied by independently verifiable certificates which also support a union-like operation that can be used to increase the computed bounds.We conducted an extensive empirical evaluation of our method using over 5 000 formulae from the quantifier-free fragment of Presburger arithmetic, sourced from the SMT-LIB repository. The results show that our method often produces lower bounds that are close to the actual size of the minimal deterministic finite automaton. Moreover, it succeeds in computing non-trivial bounds even for instances that are out of reach (by several orders of magnitude) for the existing state-of-the-art automata-based tools for solving Presburger arithmetic.

We give a quantifier elimination procedure for one-parametric Presburger arithmetic, the extension of Presburger arithmetic with the function x ↦ t ⋅ x, where t is a fixed free variable ranging over the integers. This resolves an open problem proposed in [Bogart et al., Discrete Analysis, 2017]. As conjectured in [Goodrick, Arch. Math. Logic, 2018], quantifier elimination is obtained for the extended structure featuring all integer division functions x ↦ ⌊x/(f(t))⌋, one for each integer polynomial f. Our algorithm works by iteratively eliminating blocks of existential quantifiers. The elimination of a block builds on two sub-procedures, both running in non-deterministic polynomial time. The first one is an adaptation of a recently developed and efficient quantifier elimination procedure for Presburger arithmetic, modified to handle formulae with coefficients over the ring ℤ[t] of univariate polynomials. The second is reminiscent of the so-called "base t division method" used by Bogart et al. As a result, we deduce that the satisfiability problem for the existential fragment of one-parametric Presburger arithmetic (which encompasses a broad class of non-linear integer programs) is in NP, and that the smallest solution to a satisfiable formula in this fragment is of polynomial bit size.

This paper investigates ∃ℝ(ξ^ℤ), that is the extension of the existential theory of the reals by an additional unary predicate ξ^ℤ for the integer powers of a fixed computable real number ξ > 0. If all we have access to is a Turing machine computing ξ, it is not possible to decide whether an input formula from this theory is satisfiable. However, we show an algorithm to decide this problem when - ξ is known to be transcendental, or - ξ is a root of some given integer polynomial (that is, ξ is algebraic). In other words, knowing the algebraicity of ξ suffices to circumvent undecidability. Furthermore, we establish complexity results under the proviso that ξ enjoys what we call a polynomial root barrier. Using this notion, we show that the satisfiability problem of ∃ℝ(ξ^ℤ) is - in ExpSpace if ξ is an algebraic number, and - in 3Exp if ξ is a logarithm of an algebraic number, Euler’s e, or the number π, among others. To establish our results, we first observe that the satisfiability problem of ∃ℝ(ξ^ℤ) reduces in exponential time to the problem of solving quantifier-free instances of the theory of the reals where variables range over ξ^ℤ. We then prove that these instances have a small witness property: only finitely many integer powers of ξ must be considered to find whether a formula is satisfiable. Our complexity results are shown by relying on well-established machinery from Diophantine approximation and transcendental number theory, such as bounds for the transcendence measure of numbers. As a by-product of our results, we are able to remove the appeal to Schanuel’s conjecture from the proof of decidability of the entropic risk threshold problem for stochastic games with rational probabilities, rewards and threshold [Baier et al., MFCS, 2023]: when the base of the entropic risk is e and the aversion factor is a fixed algebraic number, the problem is (unconditionally) in Exp.

Abstract This paper focuses on succinctness results for fragments of Linear Temporal Logic with Past ( $$\textsf{LTL}$$ LTL ) devoid of binary temporal operators like until, and provides methods to establish them. We prove that there is a family of cosafety languages $$(\mathcal {L}_n)_{n \ge 1}$$ ( L n ) n ≥ 1 such that $$\mathcal {L}_n$$ L n can be expressed with a pure future formula of size $$\mathcal {O}(n)$$ O ( n ) , but it requires formulae of size $$2^{\varOmega (n)}$$ 2 Ω ( n ) to be captured with past formulae. As a by-product, such a succinctness result shows the optimality of the pastification algorithm proposed in [Artale et al., KR, 2023]. We show that, in the considered case, succinctness cannot be proven by relying on the classical automata-based method introduced in [Markey, Bull. EATCS, 2003]. In place of this method, we devise and apply a combinatorial proof system whose deduction trees represent $$\textsf{LTL}$$ LTL formulae. The system can be seen as a proof-centric (one-player) view on the games used by Adler and Immerman to study the succinctness of $$\textsf{CTL}$$ CTL .

This paper provides an NP procedure that decides whether a linear-exponential system of constraints has an integer solution. Linear-exponential systems extend standard integer linear programs with exponential terms $2^x$ and remainder terms ${(x \bmod 2^y)}$. Our result implies that the existential theory of the structure $(\mathbb{N},0,1,+,2^{(\cdot)},V_2(\cdot,\cdot),\leq)$ has an NP-complete satisfiability problem, thus improving upon a recent EXPSPACE upper bound. This theory extends the existential fragment of Presburger arithmetic with the exponentiation function $x \mapsto 2^x$ and the binary predicate $V_2(x,y)$ that is true whenever $y \geq 1$ is the largest power of $2$ dividing $x$. Our procedure for solving linear-exponential systems uses the method of quantifier elimination. As a by-product, we modify the classical Gaussian variable elimination into a non-deterministic polynomial-time procedure for integer linear programming (or: existential Presburger arithmetic).

We study the non-linear extension of integer programming with greatest common divisor constraints of the form gcd(f, g) ~ d, where f and g are linear polynomials, d is a positive integer, and ~ is a relation among ≤, = ≠, = and ≥. We show that the feasibility problem for these systems is in NP, and that an optimal solution minimizing a linear objective function, if it exists, has polynomial bit length. To show these results, we identify an expressive fragment of the existential theory of the integers with addition and divisibility that admits solutions of polynomial bit length. It was shown by Lipshitz [Trans. Am. Math. Soc., 235, pp. 271-283, 1978] that this theory adheres to a local-to-global principle in the following sense: a formula Φ is equi-satisfiable with a formula Ψ in this theory such that Ψ has a solution if and only if Ψ has a solution modulo every prime p. We show that in our fragment, only a polynomial number of primes of polynomial bit length need to be considered, and that the solutions modulo prime numbers can be combined to yield a solution to Φ of polynomial bit length. As a technical by-product, we establish a Chinese-remainder-type theorem for systems of congruences and non-congruences showing that solution sizes do not depend on the magnitude of the moduli of non-congruences.

We investigate expansions of Presburger arithmetic, i.e., the theory of the integers with addition and order, with additional structure related to exponentiation: either a function that takes a number to the power of $2$, or a predicate for the powers of $2$. The latter theory, denoted $\mathrm{PresPower}$, was introduced by Büchi as a first attempt at characterizing the sets of tuples of numbers that can be expressed using finite automata; Büchi's method does not give an elementary upper bound, and the complexity of this theory has been open. The former theory, denoted as $\mathrm{PresExp}$, was shown decidable by Semenov; while the decision procedure for this theory differs radically from the automata-based method proposed by Büchi, Semenov's method is also non-elementary. And in fact, the theory with the power function has a non-elementary lower bound. In this paper, we show that while Semenov's and Büchi's approaches yield non-elementary blow-ups for $\mathrm{PresPower}$, the theory is in fact decidable in triply exponential time, similarly to the best known quantifier-elimination algorithm for Presburger arithmetic. We also provide a $\mathrm{NExpTime}$ upper bound for the existential fragment of $\mathrm{PresExp}$, a step towards a finer-grained analysis of its complexity. Both these results are established by analyzing a single parameterized satisfiability algorithm for $\mathrm{PresExp}$, which can be specialized to either the setting of $\mathrm{PresPower}$ or the existential theory of $\mathrm{PresExp}$. Besides the new upper bounds for the existential theory of $\mathrm{PresExp}$ and $\mathrm{PresPower}$, we believe our algorithm provides new intuition for the decidability of these theories, and for the features that lead to non-elementary blow-ups.

We study the expressivity and complexity of two modal logics interpreted on finite forests and equipped with standard modalities to reason on submodels. The logic \(\mathsf {ML} ({\color{black}{{\vert\!\!\vert\!\vert}}})\) extends the modal logic K with the composition operator \({\color{black}{{\vert\!\!\vert\!\vert}}}\) from ambient logic whereas \(\mathsf {ML} (\mathbin {\ast })\) features the separating conjunction \(\mathbin {\ast }\) from separation logic. Both operators are second-order in nature. We show that \(\mathsf {ML} ({\color{black}{{\vert\!\!\vert\!\vert}}})\) is as expressive as the graded modal logic \(\mathsf {GML}\) (on trees) whereas \(\mathsf {ML} (\mathbin {\ast })\) is strictly less expressive than \(\mathsf {GML}\) . Moreover, we establish that the satisfiability problem is Tower -complete for \(\mathsf {ML} (\mathbin {\ast })\) , whereas it is (only) AExp Pol -complete for \(\mathsf {ML} ({\color{black}{{\vert\!\!\vert\!\vert}}})\) , a result that is surprising given their relative expressivity. As by-products, we solve open problems related to sister logics such as static ambient logic and modal separation logic.

Abstract We give a new quantifier elimination procedure for Presburger arithmetic extended with a unary counting quantifier $$\exists ^{= x} y\, \mathrm {\Phi }$$ ∃ = x y Φ that binds to the variable $$x$$ x the number of different $$y$$ y satisfying $$\mathrm {\Phi }$$ Φ . While our procedure runs in non-elementary time in general, we show that it yields nearly optimal elementary complexity results for expressive counting extensions of Presburger arithmetic, such as the threshold counting quantifier $$\exists ^{\ge c} y\, \mathrm {\Phi }$$ ∃ ≥ c y Φ that requires that the number of different y satisfying $$\mathrm {\Phi }$$ Φ be at least $$c\in \mathbb {N}$$ c ∈ N , where c can succinctly be defined by a Presburger formula. Our results are cast in terms of what we call the monadically-guarded fragment of Presburger arithmetic with unary counting quantifiers, for which we develop a 2ExpSpace decision procedure.

Abstract We study a family of modal logics interpreted on tree-like structures, and featuring local quantifiers $$\exists ^{k}p$$ ∃ k p that bind the proposition p to worlds that are accessible from the current one in at most k steps. We consider a first-order and a second-order semantics for the quantifiers, which enables us to relate several well-known formalisms, such as hybrid logics, $$\textsf {S5Q}$$ S 5 Q and graded modal logic. To better stress these connections, we explore fragments of our logics, called herein round-bounded fragments. Depending on whether first or second-order semantics is considered, these fragments populate the hierarchy $${2\textsc {NExp} \subset 3\textsc {NExp} \subset \cdots }$$ 2 NE X P ⊂ 3 NE X P ⊂ ⋯ or the hierarchy $${2\textsc {AExp}_{pol} \subset 3\textsc {AExp}_{pol} \subset \cdots }$$ 2 AE X P pol ⊂ 3 AE X P pol ⊂ ⋯ , respectively. For formulae up-to modal depth k, the complexity improves by one exponential.

This paper resolves two open problems on linear integer arithmetic (LIA), also known as Presburger arithmetic. First, we give a triply exponential geometric decision procedure for LIA, i.e., a procedure based on manipulating semilinear sets. This matches the running time of the best quantifier elimination and automata-based procedures. Second, building upon our first result, we give a doubly exponential upper bound on the Vapnik–Chervonenkis (VC) dimension of sets definable in LIA, proving a conjecture of D. Nguyen and I. Pak [Combinatorica 39, pp. 923–932, 2019].

The Boolean satisfiability problem plays a central role in computational complexity and is often used as a starting point for showing NP lower bounds. Generalisations such as Succinct SAT, where a Boolean formula is succinctly represented as a Boolean circuit, have been studied in the literature in order to lift the Boolean satisfiability problem to higher complexity classes such as NEXP. While, in theory, iterating this approach yields complete problems for k-NEXP for all k > 0, using such iterations of Succinct SAT is at best tedious when it comes to proving lower bounds. The main contribution of this paper is to show that the Boolean satisfiability problem has another canonical generalisation in terms of higher-order Boolean functions that is arguably more suitable for showing lower bounds beyond NP. We introduce a family of problems HOSAT(k,d), k ≥ 0, d ≥ 1, in which variables are interpreted as Boolean functions of order at most k and there are d quantifier alternations between functions of order exactly k. We show that the unbounded HOSAT problem is TOWER-complete, and that HOSAT(k,d) is complete for the weak k-EXP hierarchy with d alternations for fixed k,d ≥ 1 and d odd. We illustrate the usefulness of HOSAT by characterising the complexity of weak Presburger arithmetic, the first-order theory of the integers with addition and equality but without order. It has been a long-standing open problem whether weak Presburger arithmetic has the same complexity as standard Presburger arithmetic. We answer this question affirmatively, even for the negation-free fragment and the Horn fragment of weak Presburger arithmetic.

We describe a set of simple features that are sufficient in order to make the satisfiability problem of logics interpreted on trees Tower -hard. We exhibit these features through an Auxiliary Logic on Trees (ALT ), a modal logic that essentially deals with reachability of a fixed node inside a forest and features modalities from sabotage modal logic to reason on submodels. After showing that ALT admits a Tower -complete satisfiability problem, we prove that this logic is captured by four other logics that were independently found to be Tower -complete: two-variables separation logic, quantified computation tree logic, modal logic of heaps and modal separation logic. As a by-product of establishing these connections, we discover strict fragments of these logics that are still non-elementary.

Given an existential formula Φ of linear arithmetic over p-adic integers together with valuation constraints, we study the p-universality problem which consists of deciding whether Φ is satisfiable for all primes p, and the analogous problem for the closely related existential theory of Büchi arithmetic. Our main result is a coNEXP upper bound for both problems, together with a matching lower bound for existential Büchi arithmetic. On a technical level, our results are obtained from analysing properties of a certain class of p-automata, finite-state automata whose languages encode sets of tuples of natural numbers.

We present the first complete axiomatisation for quantifier-free separation logic. The logic is equipped with the standard concrete heaplet semantics and the proof system has no external feature such as nominals/labels. It is not possible to rely completely on proof systems for Boolean BI as the concrete semantics needs to be taken into account. Therefore, we present the first internal Hilbert-style axiomatisation for quantifier-free separation logic. The calculus is divided in three parts: the axiomatisation of core formulae where Boolean combinations of core formulae capture the expressivity of the whole logic, axioms and inference rules to simulate a bottom-up elimination of separating connectives, and finally structural axioms and inference rules from propositional calculus and Boolean BI with the magic wand.

Abstract Modal separation logics are formalisms that combine modal operators to reason locally, with separating connectives that allow to perform global updates on the models. In this work, we design Hilbert-style proof systems for the modal separation logics $\text {MSL}(\ast ,\langle \neq \rangle )$ and $\text {MSL}(\ast ,\Diamond )$, where $\ast $ is the separating conjunction, $\Diamond $ is the standard modal operator and $\langle \neq \rangle $ is the difference modality. The calculi only use the logical languages at hand (no external features such as labels) and can be divided in two main parts. First, normal forms for formulae are designed and the calculi allow to transform every formula into a formula in normal form. Second, another part of the calculi is dedicated to the axiomatization for formulae in normal form, which may still require non-trivial developments but is more manageable.

The list segment predicate ls used in separation logic for verifying programs with pointers is well suited to express properties on singly-linked lists. We study the effects of adding ls to the full quantifier-free separation logic with the separating conjunction and implication, which is motivated by the recent design of new fragments in which all these ingredients are used indifferently and verification tools start to handle the magic wand connective. This is a very natural extension that has not been studied so far. We show that the restriction without the separating implication can be solved in polynomial space by using an appropriate abstraction for memory states, whereas the full extension is shown undecidable by reduction from first-order separation logic. Many variants of the logic and fragments are also investigated from the computational point of view when ls is added, providing numerous results about adding reachability predicates to quantifier-free separation logic.

We present a general approach to axiomatise separation logics with heaplet semantics with no external features such as nominals/labels. To start with, we design the first (internal) Hilbert-style axiomatisation for the quantifier-free separation logic. We instantiate the method by introducing a new separation logic with essential features: it is equipped with the separating conjunction, the predicate ls, and a natural guarded form of first-order quantification. We apply our approach for its axiomatisation. As a by-product of our method, we also establish the exact expressive power of this new logic and we show PSpace-completeness of its satisfiability problem.

Abstract We describe a set of simple features that are sufficient in order to make the satisfiability problem of logics interpreted on trees Tower-hard. We exhibit these features through an Auxiliary Logic on Trees (), a modal logic that essentially deals with reachability of a fixed node inside a forest and features modalities from sabotage modal logic to reason on submodels. After showing that admits a Tower-complete satisfiability problem, we prove that this logic is captured by four other logics that were independently found to be Tower-complete: two-variables separation logic, quantified computation tree logic, modal logic of heaps and modal separation logic. As a by-product of establishing these connections, we discover strict fragments of these logics that are still non-elementary.

We study the expressivity and complexity of two modal logics interpreted on finite forests and equipped with standard modalities to reason on submodels. The logic ML(|) extends the modal logic K with the composition operator | from ambient logic, whereas ML(*) features the separating conjunction * from separation logic. Both operators are second-order in nature. We show that ML(|) is as expressive as the graded modal logic GML (on trees) whereas ML(*) is strictly less expressive than GML. Moreover, we establish that the satisfiability problem is Tower-complete for ML(*), whereas it is (only) AExpPol-complete for ML(|), a result which is surprising given their relative expressivity. As by-products, we solve open problems related to sister logics such as static ambient logic and modal separation logic.

The list segment predicate $$\mathtt {ls}$$ used in separation logic for verifying programs with pointers is well-suited to express properties on singly-linked lists. We study the effects of adding $$\mathtt {ls}$$ to the full propositional separation logic with the separating conjunction and implication, which is motivated by the recent design of new fragments in which all these ingredients are used indifferently and verification tools start to handle the magic wand connective. This is a very natural extension that has not been studied so far. We show that the restriction without the separating implication can be solved in polynomial space by using an appropriate abstraction for memory states whereas the full extension is shown undecidable by reduction from first-order separation logic. Many variants of the logic and fragments are also investigated from the computational point of view when $$\mathtt {ls}$$ is added, providing numerous results about adding reachability predicates to propositional separation logic.

We study an extension of propositional separation logic that can specify robustness properties, such as acyclicity and garbage freedom, for automatic verification of stateful programs with singly-linked lists. We show that its satisfiability problem is PSpace-complete, whereas modest extensions of the logic are shown to be Tower-hard. As separating implication, reachability predicates (under some syntactical restrictions) and a unique quantified variable are allowed, this logic subsumes several PSpace-complete separation logics considered in previous works.