Isabelle (proof assistant)

Original author(s) Lawrence Paulson
Initial release 1986[1]
Stable release
Isabelle2016 (February 2016)
Written in Standard ML and Scala
Operating system Linux, Windows, Mac OS X
Type Mathematics
License BSD license (core system)

The Isabelle theorem prover is an interactive theorem prover, a Higher Order Logic (HOL) theorem prover. It is an LCF-style theorem prover (written in Standard ML), so it is based on a small logical core to ease logical correctness. 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). Isabelle's main proof method is a higher-order version of resolution, based on higher-order unification. Though interactive, Isabelle also features efficient automatic reasoning tools, such as a term rewriting engine and a tableaux prover, as well as various decision procedures. 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. The Isabelle theorem prover is free software, released under the revised BSD license.

Isabelle was named by Lawrence Paulson after Gérard Huet's daughter.[2]

Example proof

Isabelle allows proofs to be written in two different styles, the procedural and the declarative. Procedural proofs specify a series of tactics or 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. The Archive of Formal Proofs also recommends the declarative style.

For example, a declarative proof by contradiction in Isar that the square root of two is not rational can be written as follows.

theorem sqrt2_not_rational:
  "sqrt (real 2) ∉ ℚ"
  let ?x = "sqrt (real 2)"
  assume "?x ∈ ℚ"
  then obtain m n :: nat where
    sqrt_rat: "¦?x¦ = real m / real n" and lowest_terms: "coprime m n"
    by (rule Rats_abs_nat_div_natE)
  hence "real (m^2) = ?x^2 * real (n^2)" by (auto simp add: power2_eq_square)
  hence eq: "m^2 = 2 * n^2" using of_nat_eq_iff power2_eq_square by fastforce
  hence "2 dvd m^2" by simp
  hence "2 dvd m" by simp
  have "2 dvd n" proof-
    from ‹2 dvd m› obtain k where "m = 2 * k" ..
    with eq have "2 * n^2 = 2^2 * k^2" by simp
    hence "2 dvd n^2" by simp
    thus "2 dvd n" by simp
  with ‹2 dvd m› have "2 dvd gcd m n" by (rule gcd_greatest)
  with lowest_terms have "2 dvd 1" by simp
  thus False using odd_one by blast


Isabelle has been used to aid formal methods for the specification, development and verification of software and hardware systems.

Larry Paulson keeps a list of research projects that use Isabelle.


  1. Paulson, L. C. (1986). "Natural deduction as higher-order resolution". The Journal of Logic Programming. 3 (3): 237. doi:10.1016/0743-1066(86)90015-4.
  2. Gordon, Mike (1994-11-16). "1.2 History". Isabelle and HOL. Cambridge AR Research (The Automated Reasoning Group). Retrieved 2016-04-28.
  3. Philip Wadler's "An Angry Half-Dozen" (1998) attributes this result to: Albert J. Camilleri. "A hybrid approach to verifying liveness in a symmetric multiprocessor". 10th International Conference on Theorem Proving in Higher-Order Logics, Elsa Gunter and Amy Felty, editors, Murray Hill, New Jersey, August 1997. Lecture Notes in Computer Science (LNCS) Vol. 1275, Springer Verlag, 1997
  4. Klein, Gerwin; Elphinstone, Kevin; Heiser, Gernot; Andronick, June; Cock, David; Derrin, Philip; Elkaduwe, Dhammika; Engelhardt, Kai; Kolanski, Rafal; Norrish, Michael; Sewell, Thomas; Tuch, Harvey; Winwood, Simon (October 2009). "seL4: Formal verification of an OS kernel" (PDF). 22nd ACM Symposium on Operating System Principles. Big Sky, Montana, US. pp. 207–200.


External links

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