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My mis-statement in class about the standard resolution refutation proof error
- To: Rao Kambhampati <email@example.com>
- Subject: My mis-statement in class about the standard resolution refutation proof error
- From: Subbarao Kambhampati <firstname.lastname@example.org>
- Date: Tue, 10 Apr 2012 21:34:28 -0700
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- Sender: email@example.com
In class, I said that a standard error that I had pointed out to you during resolution refutation class was still made by 3-4 people in the class.
Actually I mis-spoke.
The error that students did in the exam was to say that
~(A V B) resolves with A VB to give empty clause.
This is semantically correct (~(A V B) is indeed inconsistent with A V B and allows us to derive false) but syntactically wrong in that ~(A V B) is not in the clausal form.
If you put it in clausal form you will get two clauses, ~A and ~B which can then be resolved with AV B in sequence to get empty clause.
The error I had pointed out in the class earlier was a more egregious one in that it is both syntactically and semantically incorrect.
This one involves saying ~A V ~B resolves with A V B giving empty clause.
Here ~A V ~B is not actually inconsistent with A V B (The world A=True, B =False, is a model of both formulas, for example).