Section: Research Program
Proof nets and atomic flows
Proof nets and atomic flows are abstract (graphlike) presentations of 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 [54] for linear logic but also in Robinson's proof nets [74] 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., [5] , [4] and [58] . 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.
This directly leads to the following research questions investigated by members of the Parsifal team:

Finding (for classical logic) a notion of proof nets that is deductive, i.e., can effectively be used for doing proof search. An important property of deductive proof nets must be that the correctness can be checked in linear time. For the classical logic proof nets by Lamarche and Straßburger [5] this takes exponential time (in the size of the net).

Studying the normalization of proofs in classical logic using atomic flows. Although there is no correctness criterion they allow to simplify the normalization procedure for proofs in deep inference, and additionally allow to get new insights in the complexity of the normalization.