We formalize the Berlekamp-Zassenhaus algorithm for factoring square-free integer polynomials in Isabelle/HOL. We further adapt an existing formalization of Yun's square-free factorization algorithm to integer polynomials, and thus provide an efficient and certified factorization algorithm for arbitrary univariate polynomials.The algorithm first performs a factorization in the prime field GF(p) and then performs computations in the ring of integers modulo p k , where both p and k are determined at runtime. Since a natural modeling of these structures via dependent types is not possible in Isabelle/HOL, we formalize the whole algorithm using Isabelle's recent addition of local type definitions.Through experiments we verify that our algorithm factors polynomials of degree 100 within seconds.

Abstract. The LLL basis reduction algorithm was the first polynomialtime algorithm to compute a reduced basis of a given lattice, and hence also a short vector in the lattice. It thereby approximates an NP-hard problem where the approximation quality solely depends on the dimension of the lattice, but not the lattice itself. The algorithm has several applications in number theory, computer algebra and cryptography.In this paper, we develop the first mechanized soundness proof of the LLL algorithm using Isabelle/HOL. We additionally integrate one application of LLL, namely a verified factorization algorithm for univariate integer polynomials which runs in polynomial time.

We formally verify the Berlekamp-Zassenhaus algorithm for factoring square-free integer polynomials in Isabelle/HOL. We further adapt an existing formalization of Yun's squarefree factorization algorithm to integer polynomials, and thus provide an efficient and certified factorization algorithm for arbitrary univariate polynomials. The algorithm first performs factorization in the prime field GF(p) and then performs computations in the ring of integers modulo p k , where both p and k are determined at runtime. Since a natural modeling of these structures via dependent types is not possible in Isabelle/HOL, we formalize the whole algorithm using locales and local type definitions. Through experiments we verify that our algorithm factors polynomials of degree up to 500 within seconds.

Model checking algorithms are typically complex graph algorithms, whose correctness is crucial for the usability of a model checker. However, establishing the correctness of such algorithms can be challenging and is often done manually. Mechanising the verification process is crucially important, because model checking algorithms are often parallelised for efficiency reasons, which makes them even more error-prone. This paper shows how the VerCors concurrency verifier is used to mechanically verify the parallel nested depth-first search (NDFS) graph algorithm of Laarman et al. [25]. We also demonstrate how having a mechanised proof supports the easy verification of various optimisations of parallel NDFS. As far as we are aware, this is the first automated deductive verification of a multi-core model checking algorithm. This research has been performed while working at the University of Twente.

Modern program analyzers translate imperative programs to an intermediate formal language like integer transition systems (ITSs), and then analyze properties of ITSs. Because of the high complexity of the task, a number of incorrect proofs are revealed annually in the Software Verification Competitions. In this paper, we establish the trustworthiness of termination and safety proofs for ITSs. To this end we extend our Isabelle/HOL formalization IsaFoR by formalizing several verification techniques for ITSs, such as invariant checking, ranking functions, etc. Consequently the extracted certifier CeTA can now (in)validate safety and termination proofs for ITSs. We also adapted the program analyzers T2 and AProVE to produce machine-readable proof certificates, and as a result, most termination proofs generated by these tools on a standard benchmark set are now certified.