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first-order and higher-order logic, model theory, set theory, proof theory, computability theory, formal languages, definability, interplay of syntax and semantics, constructive logic, intuitionism, philosophical logic, modal logic, completeness, Gödel incompleteness, decidability, undecidability, theories of truth, truth revision, consistency.

6 votes
5 answers
2k views

A book explaining power and limitations of Peano Axioms?

Are there books or survey articles explaining the subject to a non-expert? To clarify what I mean, here is a couple of issues that I would like to read about. (I am mainly interested in references but …
Sergei Ivanov's user avatar
58 votes
9 answers
8k views

How do they verify a verifier of formalized proofs?

In an unrelated thread Sam Nead intrigued me by mentioning a formalized proof of the Jordan curve theorem. I then found that there are at least two, made on two different systems. This is quite an ach …
Sergei Ivanov's user avatar
10 votes
Accepted

A decision problem concerning Diophantine inequalities

It is undecidable. If you could solve this, you could also solve Hilbert's 10th problem. Suppose we have an algorithm solving your problem for all $n$. Given a polynomial $p\in\mathbb[x_1,\dots,x_n]$, …
Sergei Ivanov's user avatar
6 votes

Is equivalence of functions built from nested exponentiations a decidable problem?

This is a justification of the algorithm suggested in Dan Turetsky's comment. Every expression in $E$ reduces to an expression in $E'$ which is the minumum language such that $x\in E'$ and $x^{(p_1*\ …
Sergei Ivanov's user avatar
20 votes
1 answer
907 views

A collection of intervals that can cover any measure zero set

This is a follow-up to this question (in fact, this is what originally motivated me to ask that one.) Let's say that a sequence $\{s_i\}$ of positive reals covers a set $X\subset\mathbb R$ if there i …
Sergei Ivanov's user avatar
27 votes
2 answers
2k views

A set that can be covered by arbitrarily small intervals

Let $X$ be a subset of the real line and $S=\{s_i\}$ an infinite sequence of positive numbers. Let me say that $X$ is $S$-small if there is a collection $\{I_i\}$ of intervals such that the length of …
Sergei Ivanov's user avatar