Listen in while I have an actual insight on air!  After a week of head scratching, I figure out -- while chattering! -- what I hope is both a correct and intuitive example of multiplicative disjunction.

I explain the basic idea of multiplicative versus additive proof rules, and consider multiplicative conjunction (tensor), addition conjunction (&, "with"), additive disjunction -- but leaving multiplicative disjunction (par) for next time!

We discuss briefly the central ideas of linear logic, where by default assumptions must be used exactly once. 

In this episode I discuss the basic structural rules of weakening, contraction, and exchange, and speculate on their dark origin.

Cut elimination for sequent calculus is more involved that normalization of detours for natural deduction.  There are more cases of cuts that must be transformed than correspond to detours (introductions followed by eliminations).  In this episode, I explain why that is.

We saw in the last few episodes that proofs in natural deduction can be simplified by removing detours, which occur when an introduction inference is immediately followed by an elimination inference on the introduced formula.  What corresponds to this for sequent calculus proofs?  The answer is cut elimination.  This episode describes the cut rule and what is meant by a cut-elimination procedure.  We will talk more about such a procedure in the next episode.

We talk about normalizing detours -- which are when an introduction inference is immediately followed by an elimination inference -- for the implication rules.  Under Curry-Howard, this actually corresponds to beta-reduction, and could make the proof bigger (though less complex in a certain sense).

This episode explains the idea of normalization of proofs in natural deduction.  We want to eliminate so-called detours in proofs, which occur when an introduction is immediately followed by an elimination.

Sequent calculus is a different style in which proof systems can be formulated, where for each connective, we have a left rule for introducing it (in the conclusion of the rule) in the left part of a sequent G => D (i.e., in G), and similarly a right rule for introducing it in the right part (D).  The beauty of sequent calculus is disjunction is handled without any departure from the general form for sequent calculus rules (unlike disjunction in natural deduction, as we discussed last time).  

We discuss the problem of the or-elimination rule in natural deduction, which does not have the correct form for natural deduction inferences.  It is a research problem to fix this!

Don't forget to get in touch with me if you want to do the mini-course over Zoom next month, on normalization.