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rigorous and formal proof



This below is the answer of Claus Zinn for my question concerning
difference between "rigorous" and "formal" proof.

-- Roman Matuszewski
   (romat@mizar.org)
==================================================================

The difference between 'rigorous and 'formal' proofs is not that easy,
I guess. It may even be they that the english usage for these terms refer
to the same. One attempt would be:
        - rigorous and formal proofs both are written in a formal
          language
        - the rigorous proof is written in a language above inference
          level, independant of some calculus.
          E.g., a MIZAR proof is a rigourous proof.
        - the formal proof is written in a language at inference level
          and the language depends only on the calculi used.
          E.g. the resolution graph returned by OTTER is a proof in a
          formal language.

Note that, as far as I understand MIZAR, the rigorous proof written
in the MIZAR language is never translated into a formal proof; because
MIZAR verifies the proof step by step; and for each step a formal
proof is found.

As a matter of fact, neither formal nor even rigorous proofs, as
defined above, can be find in mathematical textbooks.  Mathematicians
have a different understanding of what a rigorous proof is: they say,
for example, that Landau gives a rigorous treatment in his book
'grundlagen der analysis' on analysis.

All the best,

Claus

--
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