abstract
| - Herbrand structures have the advantage, computationally speaking, of being guided by the definability of all elements in them. A salient feature of the logics induced by them is that they internally
exhibit the induction scheme, thus providing a congenial, computationally-oriented framework for
formal inductive reasoning. Nonetheless, their enhanced expressivity renders any effective proof
system for them incomplete. Furthermore, the fact that they are not compact poses yet another prooftheoretic challenge. This paper offers several layers for coping with the inherent incompleteness and
non-compactness of these logics. First, two types of infinitary proof system are introduced—one
of infinite width and one of infinite height—which manipulate infinite sequents and are sound and
complete for the intended semantics. The restriction of these systems to finite sequents induces a
completeness result for finite entailments. Then, in search of effectiveness, two finite approximations
of these systems are presented and explored. Interestingly, the approximation of the infinite-width
system via an explicit induction scheme turns out to be weaker than the effective cyclic fragment of the
infinite-height system.
- Herbrand structures have the advantage, computationally speaking, of being guided by the definability of all elements in them. A salient feature of the logics induced by them is that they internally
exhibit the induction scheme, thus providing a congenial, computationally-oriented framework for
formal inductive reasoning. Nonetheless, their enhanced expressivity renders any effective proof
system for them incomplete. Furthermore, the fact that they are not compact poses yet another prooftheoretic challenge. This paper offers several layers for coping with the inherent incompleteness and
non-compactness of these logics. First, two types of infinitary proof system are introduced—one
of infinite width and one of infinite height—which manipulate infinite sequents and are sound and
complete for the intended semantics. The restriction of these systems to finite sequents induces a
completeness result for finite entailments. Then, in search of effectiveness, two finite approximations
of these systems are presented and explored. Interestingly, the approximation of the infinite-width
system via an explicit induction scheme turns out to be weaker than the effective cyclic fragment of the
infinite-height system.
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