A Remark on Henkin Sentences and Their Contraries
John P. Burgess · Notre Dame Journal of Formal Logic · 2003
That the result of flipping quantifiers and negating what comes after, applied to branching-quantifier sentences, is not equivalent to the negation of the original has been known for as long as such sentences have been studied. It is here pointed out that this syntactic operation fails in the strongest possible sense to correspond to any operation on classes of models.