| Summary: | Linear rules have played an increasing role in structural proof theory in
recent years. It has been observed that the set of all sound linear inference
rules in Boolean logic is already coNP-complete, i.e. that every Boolean
tautology can be written as a (left- and right-)linear rewrite rule. In this
paper we study properties of systems consisting only of linear inferences. Our
main result is that the length of any 'nontrivial' derivation in such a system
is bound by a polynomial. As a consequence there is no polynomial-time
decidable sound and complete system of linear inferences, unless coNP=NP. We
draw tools and concepts from term rewriting, Boolean function theory and graph
theory in order to access some required intermediate results. At the same time
we make several connections between these areas that, to our knowledge, have
not yet been presented and constitute a rich theoretical framework for
reasoning about linear TRSs for Boolean logic.
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