Extension without Cut

International audienceIn proof theory one distinguishes sequent proofs with cut and cut-free sequent proofs, while for proof complexity one distinguishes Frege-systems and extended Frege-systems. In this paper we show how deep inference can provide a uniform treatment for both classifications, such that we can define cut-free systems with extension, which is neither possible with Frege-systems, nor with the sequent calculus. We show that the propositional pigeon-hole principle admits polynomial-size proofs in a cut-free system with extension. We also define cut-free systems with substitution and show that the cut-free system with extension p-simulates the cut-free system with substitution