Making proofs without Modus Ponens: An introduction to the combinatorics and complexity of cut elimination

Modus Ponens says that if you know $A$ and you know that $A$ implies $B$, then you know $B$. This is a basic rule that we take for granted and use repeatedly, but there is a gem of a theorem in logic by Gentzen to the effect that it is not needed in some logical systems. It is fun to say, ``You can make proofs without lemmas'' to mathematicians and watch how they react, but our true intention here is to let go of logic as a reflection of reasoning and move towards combinatorial aspects. Proofs contain basic problems of algorithmic complexity within their framework, and there is strong geometric and dynamical flavor inside them.