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README.md

README.md

Reading Sequent calculus in wikipedia

I read this article for making the validator of proofs in LK/LJ. So I would occasionally skip the part which is irrelavant to LK or LJ.

  • Hilbert style proof calculus
    • Each line is an unconditional tautology = theorem.
  • Gentzen style proof calculus
    • Each line is a conditional tautology. Conditions are on the left.
    • Natural deduction
      • every line has exactly one assertion on the right.
    • Sequent calculus
      • every line has zero or more assertions on the right.

Hilbert style has only a few rules, but needs many axioms. Gentzen style have more rules than Hilbert style, but needs fewer axioms.

  • LK was introduced in 1934.
  • LJ was introduced in 1935.

The cut elimination theorem on LK and LJ is called Gentzen's Hauptsatz. It is later used to prove the consistency of Peano arithmetics.

  • Sequent calculi (ie LK and LJ) are also called Gentzen's systems.

(Hilbert's system seems a little different from sequent calculi)

  • Natural deduction
    • Judgement is a form, A0, ..., An |- B
  • Sequent Calculus
    • Sequent is a form, A0, ..., An |- B0, ..., Bn
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