Herbrand-Confluence

Herbrand-Confluence

We consider cut-elimination in the sequent calculus for classical first-order logic. It is well known that this system, in its most general form, is neither confluent nor strongly normalizing. In this work we take a coarser (and mathematically more realistic) look at cut-free proofs. We analyze which witnesses they choose for which quantifiers, or in other words: we only consider the Herbrand-d...

Herbrand-Confluence for Cut Elimination in Classical First Order Logic

We consider cut-elimination in the sequent calculus for classical first-order logic. It is well known that this system, in its most general form, is neither confluent nor strongly normalizing. In this work we take a coarser (and mathematically more realistic) look at cut-free proofs. We analyze which witnesses they choose for which quantifiers, or in other words: we only consider the Herbrand-d...

Herbrand Logic

The Herbrand Topos

We define a new topos, the Herbrand topos, inspired by the modified realizability topos and our earlier work on Herbrand realizability. We also introduce the category of Herbrand assemblies and characterise these as the ¬¬-separated objects in the Herbrand topos. In addition, we show that the category of sets is included as the category of ¬¬-sheaves and prove that the inclusion functor preserv...

Remarks on Herbrand Analyses

We show that for theories T + with function parameters, in general (1) T + ` A , A =⇒ / T + ` A (even if A does not contain function parameters and T + is an open theory), where A is the Herbrand normal form of A and A is a Herbrand realization of A . A similar result holds for first order theories T if the index functions (used in the definition of A from A) are allowed to occur in instances o...