A Magari algebra is a pair where is a Boolean algebra and is an operator satisfying:
and .
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Remark
Suppose that is a ‘Gödelian’ theory containing enough arithmetics in order to admit a formula internalizing consistency. induces an operator on the Lindenbaum-Tarski algebra of provable equivalence classes of formulas of by .
Then the condition M2 for can be viewed as expressing the non-provability of the consistency of the theory since says that is consistent whereas says that is consistent i.e. not (For more details see Beklemishev-Gabelaia 2012).
Properties
From M1 it follows that is monotone: implies .
Moreover, satisfies the following transitivity property:
To see this, consider where the last equation follows from the commutativity of together with M1.
Set whence sofar we have got : . Now we want to show that :
From M2 plus the definition of : . But since the expanded left side is a containing a factor . Whence but since and is monotone. Whence all in all we get and we are done.