Proof Pearl: A Probabilistic Proof for the Girth-Chromatic Number Theorem

Proper conflict-free list-coloring, odd minors, subdivisions, and layered treewidth

Formal verification involves the use of logical and computational methods to establish claims that are expressed in precise mathematical terms. These can include ordinary mathematical theorems, as well as claims that pieces of hardware or software, network protocols, and mechanical and hybrid systems meet their specifications. In practice, there is not a sharp distinction between verifying a piece of mathematics and verifying the correctness of a system: formal verification requires describing hardware and software systems in mathematical terms, at which point establishing claims as to their correctness becomes a form of theorem proving. Conversely, the proof of a mathematical theorem may require a lengthy computation, in which case verifying the truth of the theorem requires verifying that the computation does what it is supposed to do. The gold standard for supporting a mathematical claim is to provide a proof, and twentieth-century developments in logic show most if not all conventional proof methods can be reduced to a small set of axioms and rules in any of a number of foundational systems. With this reduction, there are two ways that a computer can help establish a claim: it can help find a proof in the first place, and it can help verify that a purported proof is correct. Automated theorem proving focuses on the “finding” aspect. Resolution theorem provers, tableau theorem provers, fast satisfiability solvers, and so on provide means of establishing the validity of formulas in propositional and first-order logic. Other systems provide search procedures and decision procedures for specific languages and domains, such as linear or nonlinear expressions over the integers or the real numbers. Architectures like SMT (“satisfiability modulo theories”) combine domain-general search methods with domain-specific procedures. Computer algebra systems and specialized mathematical software packages provide means of carrying out mathematical computations, establishing mathematical bounds, or finding mathematical objects. A calculation can be viewed as a proof as well, and these systems, too, help establish mathematical claims. Automated reasoning systems strive for power and efficiency, often at the expense of guaranteed soundness. Such systems can have bugs, and it can be difficult to ensure that the results they deliver are correct. In contrast, interactive theorem proving focuses on the “verification” aspect of theorem proving, requiring that every claim is supported by a proof in a suitable axiomatic foundation. This sets a very high standard: every rule of inference and every step of a calculation has to be justified by appealing to prior definitions and theorems, all the way down to basic axioms and rules. In fact, most such systems provide fully elaborated “proof objects” that can be communicated to other systems and checked independently. Constructing such proofs typically requires much more input and interaction from users, but it allows us to obtain deeper and more complex proofs. The Lean Theorem Prover aims to bridge the gap between interactive and automated theorem proving, by situating automated tools and methods in a framework that supports user interaction and the construction of fully specified axiomatic proofs. The goal is to support both mathematical reasoning and reasoning about complex systems, and to verify claims in both domains.

Um resultado de Erdos demonstra a existência de grafos com número cromático e cintura arbitrariamente grandes. Temos então que um clique suficientemente grande contém um grafo com número cromático e cintura grandes como subgrafo, porém muitos grafos de interesse não necessariamente contém cliques grandes, então é interessante encontrar outra condição que garanta a existência de subgrafos com número cromático e cintura grandes. Uma conjectura de Erdos e Hajnal diz que todo grafo com número cromático suficientemente grande contém um subgrafo com número cromático e cintura grandes. O objetivo deste trabalho é estudar tal conjectura. O texto começa com uma breve apresentação de construções livres de triângulos. Em particular, é demonstrada uma construção de Codenotti, Pudlák e Resta, por meio de planos projetivos. O tópico principal do texto começa com uma demonstração de Rödl de que todo grafo com número cromático suficientemente grande contém um subgrafo livre de triângulos e com número cromático grande. Em sequência, apresentaremos uma demonstração de que grafos com número cromático suficientemente grande contém algum circuito ímpar grande. Apresentaremos também um resultado de Mohar e Wu, que demonstra que a família dos grafos de Kneser respeita a conjectura de Erdos e Hajnal. Outro resultado apresentado é de Gábor Tardos, demonstrando que a família dos shift graphs respeita a conjectura de Erdos e Hajnal. E por fim apresentaremos alguns breves resultados sobre os type graphs, mostrando casos que respeitam a conjectura de Erdos e Hajnal.

Automated theorem provers for first-order logic usually operate on sets of first-order clauses. It is well-known that the translation of a formula in full first-order logic to a clausal normal form (CNF) can crucially affect performance of a theorem prover. In our recent work we introduced a modification of first-order logic extended by the first class boolean sort and syntactical constructs that mirror features of programming languages. We called this logic FOOL. Formulas in FOOL can be translated to ordinary first-order formulas and checked by first-order theorem provers. While this translation is straightforward, it does not result in a CNF that can be efficiently handled by state-of-the-art theorem provers which use superposition calculus. In this paper we present a new CNF translation algorithm for FOOL that is friendly and efficient for superposition-based first-order provers. We implemented the algorithm in the Vampire theorem prover and evaluated it on a large number of problems coming from formalisation of mathematics and program analysis. Our experimental results show an increase of performance of the prover with our CNF translation compared to the naive translation.

We study the computational efficiency of approaches, based on Hilbert's Nullstellensatz, which use systems of linear equations for detecting non-colorability of graphs having large girth and chromatic number. We show that for every non-k-colorable graph with n vertices and girth g > 4k, the algorithm is required to solve systems of size at least n Ω(g) in order to detect its non-k-colorability.

In proof checkers and theorem provers (e.g. Coq [4] and ProPre [13]) recursive definitions of functions are shown to terminate automatically. In standard non-formalised termination proofs of recursive functions, a decreasing measure is sometimes used. Such a decreasing measure is usually difficult to find.By observing the proof trees of the proofs of termination of recursive functions in ProPre (the system used in Coq’s proofs of termination), [14] finds a decreasing measure which could be used to show termination in the standard non-formalised way. This is important because it establishes a method to find decreasing measures that help in showing termination. As the ProPre system made heavy use of structural rather than inductive rules, an extended more powerful version has been built with new proof trees based on new rules.In this article, we show that the ordinal measures found in [14] lose the decreasing property in the extended ProPre system and then, set out to show that the extended ProPre system will still be suitable for finding measures required by other systems (e.g. NQTHM). We do this by showing that exist other measures that can be associated to the proof trees developed in the extended ProPre system that respect the decreasing property. We also show that the new parameterised measure functions preserve the decreasing property up to a simple condition.

: This paper describes a compositional proof strategy for verifying properties of requirements specifications. The proof strategy, which may be applied using either a model checker or a theorem prover, uses known state invariants to prove state and transition invariants. Two proof rules are presented: a standard incremental proof rule analogous to Manna and Pnueli's incremental proof rule and a compositional proof rule. The advantage of applying the compositional rule is that it decomposes a large verification problem into smaller problems which often can be solved more efficiently than the larger problem. The steps needed to implement the compositional rule are described, and the results of applying the proof strategy to two examples, a simple cruise control system and a realworld Navy system, are presented. In the Navy example, compositional verification using either theorem proving or model checking was three times faster than verification based on the standard incremental (noncompositional) rule. In addition to the two above rules for proving invariants, a new compositional proof rule is presented for circular assume-guarantee proofs of invariants. While in principle the strategy and rules described for proving invariants may be applied to any state-based specification with parallel composition of components, the specifications in the paper are expressed in the SCR (Software Cost Reduction) tabular notation, the auxiliary invariants used in the proofs are automatically generated invariants, and the verification is supported by the SCR tools.

The present paper is devoted to the study of the properties of distance graphs in Euclidean space. We prove, in particular, the existence of distance graphs with chromatic number of exponentially large dimension and without cliques of dimension 6. In addition, under a given constraint on the cardinality of the maximal clique, we search for distance graphs with extremal large values of the chromatic number. The resulting estimates are best possible within the framework of the proposed method, which combines probabilistic techniques with the linear-algebraic approach.

Interactive theorem provers (ITPs) exploit the collaboration between humans and computers, enabling proof of complex theorems. Further, ITPs allow extraction of provably correct implementations from proofs. However, often, the extracted code interface with external libraries containing real-life complexities—proprietary library calls, remote/cloud APIs, complex models like ML models, inline assembly, highly non-linear arithmetic, vector instructions etc. We refer to such functions/operations as closed-box components. For such components, the user has to provide appropriate assumed lemmas to model the behavior of these functions. However, we found instances where these assumed lemmas are inconsistent with the actual semantics of these closed-box components. Hence, even correct-by-construction code extracted from an ITP may still behave incorrectly when interfaced with such closed-box components.To this end, we propose StarFuzz, that allows the $$\text {F}^\star $$ F ⋆ interactive theorem prover to provide better end-to-end assurance on the application— even when interfaced with the closed-box components. Under the hood, StarFuzz rides on Sādhak, an SMT solver that combines fuzz testing to allow satisfiability checking over closed-box components. On the $$\text {F}^\star $$ F ⋆ library that includes external implementations in OCaml, StarFuzz discovered four bugs—one bug that revealed an error on the assumed lemmas for a closed-box function, and three bugs in the external implementations of these components.

The theorem of Sylow is proved in Isabelle HOL. We follow the proof by Wielandt that is more general than the original and uses a nontrivial combinatorial identity. The mathematical proof is explained in some detail, leading on to the mechanization of group theory and the necessary combinatorics in Isabelle. We present the mechanization of the proof in detail, giving reference to theorems contained in an appendix. Some weak points of the experiment with respect to a natural treatment of abstract algebraic reasoning give rise to a discussion of the use of module systems to represent abstract algebra in theorem provers. Drawing from that, we present tentative ideas for further research into a section concept for Isabelle.

Triangle-free graphs with large chromatic numbers

Let Q (n, χ) denote the minimum clique size an n-vertex graph can have if its chromatic number is χ. Using Ramsey graphs we give an exact, albeit implicit, formula for the case χ ≥ (n + 3)/2.

The proofs of the Church-Rosser theorems for β, η and β ∪ η reduction in untyped λ-calculus are formalized in Isabelle/HOL, an implementation of Higher Order Logic in the generic theorem prover Isabelle. For β-reduction, both the standard proof and the variation by Takahashi are given and compared. All proofs are based on a general theory of commutating relations which supports an almost geometric style of confluence proofs.

An experiment of the cover set induction method inRRL is presented with a mechanical proof of Ramsey's theorem in graph theory. The proof is similar to the proof obtained by Kaufmann using the Boyer-Moore theorem prover. We show that this similarity is not unusual, because there is a close relationship between the Boyer-Moore logic and the algebraic specification of abstract data types on which the cover set induction method is based. (This implies that many proofs done by the Boyer-Moore theorem prover can be reproduced byRRL.) Our experiment shows thatRRL can automatically prove all the lemmas in Ramsey's theorem, while the Boyer-Moore theorem prover needs several user's hints and takes much longer (CPU time) to finish.

Developments in graph colouring theory were motivated by the four-colour problem and Heawood's theorem. Both of these were originally formulated as map-colouring problems that can be expressed as colouring graphs embedded on surfaces. This chapter gives an overview of the abundance of results concerning the chromatic number of graphs that are embedded on surfaces . Introduction In 1852 Francis Guthrie asked whether the regions of every planar map can be coloured with four colours in such a way that no two regions of the map with common boundary receive the same colour. In effect, by duality Guthrie was asking whether every planar graph is 4-colourable. This easily stated problem became known as the famous four-colour problem. Attempts to solve it led to many important results in graph theory, but the problem itself remained unsolved for more than a century. It was finally answered in the positive by Appel and Haken [9], [10], [12]. More about its proof comes later in this chapter. For more than a century, the four-colour problem was one of the driving forces that led to developments in graph theory, and specifically graph colouring theory. Its generalization – the Heawood problem – was the main motivation for developments in topological graph theory (see [51] or [49]). Both of these were originally formulated as map-colouring problems, asking about colouring the faces of an embedded graph so that adjacent faces receive different colours. Every map-colouring problem can be expressed as a graph-colouring problem by considering the dual graph of the map. Its vertices correspond to the faces of the map, and two vertices are adjacent if the corresponding faces are adjacent on the surface. In this chapter we discuss the colouring of graphs embedded on surfaces. We start by describing some of the ideas in the proof of the four-colour theorem and give a proof of the list-colouring version of the five-colour theorem that is due to Thomassen.