These are some unpublished notes that I wrote, some of them might develop into papers later.

• "The Complexity of Satisfiability Problems with Two Occurrences", 4 pages, December 2003.

• "Exponential Incomparability of Tree-like and Ordered Resolution", 3 pages, December 2000, revised April 2001.

• "The Lattice of Propositional Proof Systems", 2 pages, February 1997, revised December 1998.

• "On Threshold Logic and Cutting Planes Proofs" (with Peter Clote), 8 pages, June 1997.

• "A Note on Eventually Complete Models of Type Theory and Quine's New Foundations" (with Ulrich Weigand), 6 pages, February 1997.

• "Elements Definable by Nonstandard $\Sigma_n$-Formulae in Models of Peano Arithmetic", 4 pages, August 1995.

• "The Axiom of Infinity in Zermelo Set Theory", 6 pages, January 1993.

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