Isabelle (proof assistant)
The Isabelle automated theorem prover is a higher-order logic (HOL) theorem prover, written in Standard ML and Scala. As an LCF-style theorem prover, it is based on a small logical core (kernel) to increase the trustworthiness of proofs without requiring yet supporting explicit proof objects. Isabelle is available inside a flexible system framework allowing for logically safe extensions, which comprise both theories as well as implementations for code-generation, documentation, and specific support for a variety of formal methods. It can be seen as an IDE for formal methods. In recent years, a substantial number  of theories and system extensions have been collected in the Isabelle "Archive of Formal Proofs" (Isabelle AFP) Isabelle was named by Lawrence Paulson after Gérard Huet's daughter. The Isabelle theorem prover is free software, released under the revised BSD license. Features. Isabelle is generic: it provides a meta-logic (a weak type theory), which is used to encode object logics like first-order logic (FOL), higher-order logic (HOL) or Zermelo–Fraenkel set theory (ZFC). The most widely used object logic is Isabelle/HOL, although significant set theory developments were completed in Isabelle/ZF. Isabelle's main proof method  is a higher-order version of resolution, based on higher-order unification. Though interactive, Isabelle features efficient automatic reasoning tools, such as a term rewriting engine and a tableaux prover, various decision procedures, and, through the Sledgehammer proof-automation interface, external satisfiability modulo theories (SMT) solvers (including CVC4) and resolution-based automated theorem provers (ATPs), including E and SPASS (the Metis proof method reconstructs resolution proofs generated by these ATPs). It also features two model finders (counterexample generators): Nitpick and Nunchaku. Isabelle features locales which are modules that structure large proofs. A locale fixes types, constants, and assumptions within a  specified scope so that they do not have to be repeated for every lemma. Isar ("intelligible semi-automated reasoning") is Isabelle's formal proof language. It is inspired by the Mizar system. Example proof. Isabelle allows proofs to be written in two different styles, the procedural and the declarative. Procedural proofs specify a series of tactics (theorem proving functions/procedures) to apply; while reflecting the procedure that a human mathematician might apply to proving a result, they are typically hard to read as they do not describe the outcome of these steps. Declarative proofs (supported by Isabelle's proof language , Isar), on the other hand, specify the actual mathematical operations to be performed, and are therefore more easily read and checked by humans. The procedural style has been deprecated in recent versions of Isabelle. For example, a declarative proof by contradiction in Isar that the square root of two is not rational can be written as follows. Applications. Isabelle has been used to aid formal methods for the specification, development and verification of software and hardware systems. Isabelle has been used to formalize numerous theorems from mathematics and computer science, like Gödel's completeness theorem, Gödel's theorem about the  consistency of the axiom of choice, the prime number theorem, correctness of security protocols, and properties of programming language semantics. Many of the formal proofs are, as mentioned, maintained in the Archive of Formal Proofs, which contains (as of 2019) at least 500 articles with over 2 million lines of proof in total. Larry Paulson keeps a list of research projects that use Isabelle. Alternatives. Several languages and systems provide similar functionality: