I am discovering that mathematicians cannot tell the difference between “proof by contradiction” and “proof of negation”. This is so for good reasons, but conflation of different kinds of proofs is bad mental hygiene which leads to bad teaching practice and confusion. For reference, here is a short explanation of the difference between proof of negation and proof by contradiction. Continue reading Proof of negation and proof by contradiction

# Category Archives: Logic

# Efficient computation with Dedekind reals

Two versions of this talk were given at Computability and complexity in analysis 2008 and at Mathematics, Algorithms and Proofs 2008.

Joint work with Paul Taylor.

**Abstract:** Cauchy’s construction of reals as sequences of rational approximations is the theoretical basis for a number of implementations of exact real numbers, while Dedekind’s construction of reals as cuts has inspired fewer useful computational ideas. Nevertheless, we can see the computational content of Dedekind reals by constructing them within Abstract Stone Duality (ASD), a computationally meaningful calculus for topology. This provides the theoretical background for a novel way of computing with real numbers in the style of logic programming. Real numbers are defined in terms of (lower and upper) Dedekind cuts, while programs are expressed as statements about real numbers in the language of ASD. By adapting Newton’s method to interval arithmetic we can make the computations as efficient as those based on Cauchy reals.

**Slides:** slides-map2008.pdf (obsolete version: slides-cca2008.pdf)

**Extended abstract:** abstract-cca2098.pdf

# The hydra game

Today I lectured about the Hydra game by Laurence Kirby and Jeff Paris (*Accessible Independence Results for Peano Arithmetic, Kirby and Paris, Bull. London Math. Soc. 1982; 14: 285-293*). For the occasion I implemented the game in Java. I am publishing the code for anyone who wants to play, or use it for teaching. Continue reading The hydra game

# Are small sentences of Peano arithmetic decidable?

Recently there has been a discussion (here, here, here, and here) on the Foundations of Mathematics mailing list about completeness of Peano arithmetic (PA) with respect to “small” sentences. Harvey Friedman made several conjectures of the following kind: “All true *small* sentences of PA are provable.” He proposed measures of smallness, such as counting the number of distinct variables or restricting the depth of terms. Here are some statistics concerning such statements.

Continue reading Are small sentences of Peano arithmetic decidable?

# Proof hacking

I recently lectured at an EST training workshop in Fischbachau, Germany. There were also a number of student talks, one of which was given by Luca Chiarabini from Munich. He talked about extraction of programs from proofs, using (a variant of) Curry-Howard isomorphism, also known as *propositions-as-types*. He had some very interesting ideas which were obviously related to old programming tricks, but he approached them from the logical point of view, rather than the programmer’s point of view. It got me thinking about how to write certain recursive programs as proofs. Since it is a nice application of program extraction, I want to share it with you here.