Section: Research Program
Proof nets, atomic flows, and combinatorial proofs
Proof nets graphlike presentations of sequent calculus proofs such that all "trivial rule permutations" are quotiented away. Ideally the notion of proof net should be independent from any syntactic formalism, but most notions of proof nets proposed in the past were formulated in terms of their relation to the sequent calculus. Consequently we could observe features like “boxes” and explicit “contraction links”. The latter appeared not only in Girard's proof nets [45] for linear logic but also in Robinson's proof nets [68] for classical logic. In this kind of proof nets every link in the net corresponds to a rule application in the sequent calculus.
Only recently, due to the rise of deep inference, new kinds of proof nets have been introduced that take the formula trees of the conclusions and add additional “flowgraph” information (see e.g., [54][2] leading to the notion of atomic flow and [49]. On one side, this gives new insights in the essence of proofs and their normalization. But on the other side, all the known correctness criteria are no longer available.
Combinatorial proofs [52] are another form syntaxindependent proof presentation which separates the multiplicative from the additive behaviour of classical connectives.
The following research questions investigated by members of the Parsifal team:

Finding (for classical and intuitionistic logic) a notion of canonical proof presentation that is deductive, i.e., can effectively be used for doing proof search.

Studying the normalization of proofs using atomic flows and combinatorial proofs, as they simplify the normalization procedure for proofs in deep inference, and additionally allow to get new insights in the complexity of the normalization.

Studying the size of proofs in the combinatorial proof formalism.