Questions about Gödel's incompleteness theorems and related topics.

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5answers
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Understanding ω-consistent and ω-incomplete theory

A theory $K$ is said to be $ω$-consistent if, for every formula $B(x)$ of $K$, if $﹁ B(n)$ is a theorem in $K$ for every natural number $n$, then it is not the case that $(∃x)B(x)$ is a theorem in ...
4
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3answers
209 views

Concrete example for diagonal lemma

Diagonal lemma says that in a theory with enough assumption for any formula $A(x)$ there exist a sentence $B$ such that $B$ $\iff$ $A(\#(B))$ is a theorem in that theory, in which $\#(B)$ represents ...
8
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1answer
165 views

How did Kurt Gödel's Incompleteness Theorem affect the mathematical world?

Hi I am looking not to understand the Incompleteness Theorem, but to find out more about how and what this has effected the mathematics world. I am in high school, in Honors Algebra II, and I am ...
4
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6answers
776 views

prove that it's not provable

Is it possible to prove that you can't prove some theorem? For example: Prove that you can't prove the Riemann hypothesis. I have a feeling it's not possible but I want to know for sure. Thanks in ...
2
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2answers
141 views

clarify the term “arithmetics” when talking about Gödel's incompleteness theorems

I am not quite sure what really is meant when talking about "arithmetics" in context of Gödel's incompleteness theorems. How I so far understand it: Gödel proved that every sufficiently powerful ...
4
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1answer
264 views

Under the impact of Gödel's incompleteness theorems, are the conclusions of statistics reliable?

After I read the Gödel's incompleteness theorems, I am confused about the following: Could you tell me if the conclusions of mathematical statistics are reliable? Gödel's incompleteness theorems ...
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2answers
80 views

How is Goedel's 1st incompleteness theorem related to the Axioms of a theory

i am thinking of various connections and formulations of Goedel's 1st incompleteness theorem. Apart from connections to Turing's Halting Problem and Algorithmic Complexity Theory, i am looking for ...
3
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2answers
145 views

Gödel's incompleteness theorems: where to learn? Is there a straightforward relation between the two?

What would be a good textbook or paper to learn the proofs of the two Gödel's incompleteness theorems from? I would prefer it to be as close to the original proofs as possible. I have not tried to ...
5
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2answers
262 views

Why doesn't “$V$ is a model of $ZF$” imply consistency of ZF?

One can prove that $V$, Von Neumann's universe, satisfies all of $ZF$ axioms, so it's a model of $ZF$. But I can't see why this doesn't imply the consistency of that theorem. I'm aware that ...
20
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2answers
1k views

What is the prerequisite knowledge for learning Godel's incompleteness theorem

I am very interested in learning the incompleteness theorem and its proof. But first I must know what things I need to learn first. My current knowledge consists of basic high school education and the ...
1
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1answer
179 views

Is Douglas Hofstadter's version of Godel's proof utter nonsense?

Is Douglas Hofstadter's version of Godel's proof, which he offers in his book Godel, Escher, Bach, utter nonsense? Hofstadter goes to great length to disguise the fact that there are two distinct ...
2
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0answers
51 views

Maths, esp. Godel, and poetry

At the risk of being an interloper: I'm a poet with a bit of mathematical training. Right now I've got a grant from the Arts Council of Northern Ireland to write a collection (loosely) based on the ...
3
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2answers
103 views

If $T$ proves any incorrect $\forall$-rudimentary sentence, then $T$ is inconsistent

A theory $T$ in the language of arithmetic is called $\omega$-inconsistent if for some formula $F(x)$, $\exists x F(x)$ is a theorem of $T$, but so is $\neg F(n)$ for each natural number $n$. ...
1
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1answer
103 views

Must every decidable theory be axiomatizable?

Note: By "theory" I mean a set of sentences, not assumed to be closed under logical consequence (otherwise the question would be trivial). Comments/ideas: There's a well-known result that every ...
5
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2answers
154 views

Modern book on Gödel's incompleteness theorems in all technical details

Is there a modern book on Gödel's incompleteness theorems that goes into each and every technical aspect of the proof of them (a classical one, if such exists)? I'm not interested in popular ...
0
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1answer
86 views

Peano's theory of arithmetic and Gödel's 1st Incompleteness Theorem

Let $\mathcal{N}$ be Peano's 1st order theory of arithmetic and $\mathscr{A}$ it's standard model (which we assume exists). Infer from Gödel's 1st Incompleteness Theorem that there exists a closed ...
2
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1answer
69 views

Does second-order arithmetic (Z2) prove soundness and uniform reflection for first-order arithmetic (PA)?

(1) Does full second-order arithmetic (Z2) prove soundness and uniform reflection schemas for first-order arithmetic (PA)? That is, do we have for all formulas $\phi$: $$ \underset \phi \forall \; ...
2
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1answer
84 views

Con(PA) implies consistency of $\mathsf{PA}$ + ¬Con($\mathsf{PA}$)

The Wikipedia article for $\omega$-consistency says "Now, assuming PA is really consistent, it follows that $\mathsf{PA}$ + ¬Con($\mathsf{PA}$) is also consistent, for if it were not, then PA would ...
5
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2answers
206 views

Interaction of completeness and second incompleteness theorems

So I was reading the Wikipedia article on Godel's completeness theorem, the section on its relation to completeness. It says that completeness gives the existence of a model of arithmetic $\mathcal M ...
3
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1answer
60 views

The effects of requiring a recursive vs. a recursively enumberable axiomatization in the incompleteness theorem

I believe that the (paraphrased) original statement of Gödels first incompleteness theorem (including Rosser's trick) is If T is a sufficiently strong recursive axiomatization of the natural ...
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1answer
45 views

Logic soundness and completeness

I have to do a number of similar type questions, but I am having trouble grasping the general concepts around soundness and completeness. I have read up on the general definitions and think I ...
2
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2answers
124 views

What percentage of formulas is unprovable in a given axiomatic system?

I am trying to use language I am not familiar with, so bear with me. If I make no sense, I try to be clearer. Assume we are given a formal language. Assume $S$ is the set of every well-formed formula ...
0
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1answer
38 views

Kolmogorov (Kolmogoroff-) Complexity, Contradiction with Invariance Theorem.

Fix some programming languages $S$ which is rich enough such that one can write interpreters for $S$ in $S$. Define $$ K(w) := \mbox{length of a shortest program producing $w$}. $$ Now fix some ...
3
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1answer
136 views

What other unprovable theorems are there? [duplicate]

Gödel's incompleteness theorem presents us with the possibility of having theorems that are neither provable nor disprovable in a given axiomatic set. Already we have the continuum hypothesis which ...
6
votes
2answers
447 views

Gödel's (in)completeness theorems and the axiomatization of Euclidean geometry

In David Hilbert's 1899 Grundlagen der Geometrie, Hilbert gives a rigorous axiomatization of Euclidean geometry. As I understand it, some of Hilbert's axioms must be expressed in second order logic ...
9
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1answer
188 views

What was the planned topic of Gödel's second paper on incompleteness?

Gödel's incompleteness theorems first appeared together in a paper titled (translated to English) "On formally undecidable propositions of Principia Mathematica and related systems I," with the Roman ...
4
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3answers
627 views

Classifying Types of Paradoxes: Liar's Paradox, Et Alia

The well-known Liar's Paradox "This statement is false" leads to a recursive contradiction: If the statement is interpreted to be true then it is actually false, and if it is interpreted to be false ...
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2answers
142 views

Question about computability of true/provable formulas

I would like to clarify some things related to the computability of the sets of all theorems and true formulas for the formal arithmetic. Consider the theory $T$ of formal arithmetic (the theory of ...
6
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3answers
2k views

are there non-standard models of arithmetic in second order arithmetic?

non-standard models of arithmetic in second order arithmetic? Background: According to Godel's theorem, if we have, in a given consistent system S, a non-provable wff. A, then we can extend the ...
2
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2answers
165 views

What does the completeness of a system mean for the provability of certain statments?

Through the 16th to 19th centuries mathematicians tried to prove the Euclids parallel postulate from Euclid's other four postulates. In the beginning of the 19th century the mathematics community ...
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2answers
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Examples of statements which are true but not provable

Speaking informally and working, for example, in Peano Arithmetic (PA), we know that the essence of the Gödel's first incompleteness theorem is that there are true statements (in our model PA), which ...
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1answer
107 views

How can a statement be known to be undecidable in ZFC without ZFC being inconsistent?

I'm attempting to understand the answer to the question, Is there a statement whose undecidability is undecidable (as in independent, not a decision problem)? The answer appears to be "Yes". However, ...
8
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0answers
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Elementary references on Robinson Arithmetic, Prim. Recursive fns etc.

I'm in the middle of revising my freely available and much-downloaded introductory notes Gödel Without (Too Many) Tears. (They are a sort of cut down version of part of my Gödel book, and I'm ...
2
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1answer
168 views

Any “natural” examples of true statements in number theory not provable in 2nd order systems?

I know that there are a few theorems in number theory that are somehow known to be true, but have been shown not to be provable in first-order Peano arithmetic (PA). Have any so-called "natural" ...
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2answers
76 views

Is this possibility ruled out by Godel's Incompleteness Theorem?

From Wikipedia: "The first incompleteness theorem states that no consistent system of axioms whose theorems can be listed by an "effective procedure" (e.g., a computer program, but it could be any ...
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1answer
140 views

Questions on a sentence in ZFC asserting that ZFC has a model

Question 1. Let $\operatorname{con}(\mathsf{ZFC})$ be a sentence in $\mathsf{ZFC}$ asserting that $\mathsf{ZFC}$ has a model. Let $S$ be the theory $\mathsf{ZFC}+\operatorname{con}(\mathsf{ZFC})$. ...
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1answer
87 views

Expressibility; Incompleteness of Peano Arithmetic

I'm working through Peter Smith's book, 'An Introduction to Godel's Theorems'. One small issue I've encountered is how the notion of expressibility is used to prove the incompleteness of Peano ...
8
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1answer
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Gödelian incompleteness; Smullyan's Puzzle

I am currently doing exercises on the Gödelian theorems; and we are confronted with the introductory puzzle of R. Smullyan's book, which is as follows: Suppose we have a machine which prints strings ...
8
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3answers
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Why Euclidean geometry cannot be proved incomplete by Gödel's incompleteness theorems?

Wikipedia mentioned the limitation of Gödel's theorems. According to it, Euclidean geometry doesn't satisfy the hypotheses of Gödel's incompleteness theorems, that is, it cannot define natural ...
5
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1answer
181 views

Explain/illustrate Goedel's theorems and possible implications to non-mathematicians

I am asked to give a talk about (a) mathematical practice, (b) axiomatization, (c) Gödel's theorems and (d) possible antimechanist arguments based on the incompleteness theorems (as mentioned in P ...
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6answers
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Does Gödel's Incompleteness Theorem really say anything about the limitations of theoretical physics?

Stephen Hawking believes that Gödel's Incompleteness Theorem makes the search for a 'Theory of Everything' impossible. He reasons that because there exist mathematical results that cannot be proven, ...
6
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2answers
152 views

Formalizing proof of Godel sentence's non-provability?

In the question What does it mean for something to be true but not provable in peano arithmetic? Henning Makholm states, "...he [Godel] gave a (formalizable, with a few additional technical ...
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1answer
44 views

Can we determine which statements are incomplete due to Godel?

Due to Godel's incompleteness theorems we know that there are true statements in a system that cannot be proven with that system. My questions are 1) can we tell which statements in a system are the ...
56
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13answers
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5
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1answer
114 views

Impossibility of theories proving consistency of each other?

By Godel's second incompleteness theorem, a consistent theory (to which the theorem applies) cannot prove itself consistent. I learned that it's also impossible to have a pair of consistent theories ...
5
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3answers
663 views

P vs NP and Gödel

I apologize for the, perhaps, silly question. My impression, as a layman, is that Gödel Incompleteness Theorem should rule out the possibility that P=NP. Is that true or there are deeper technical ...
5
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4answers
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Is every theorem of PA true in the standard model of number theory $N$?

My understanding is that every theorem $\phi$ of $PA$ is true in $N$ because $N$ is a model for $PA$, $N\models PA$. By completeness of first order logic, "$PA\vdash\phi$" implies that "if $N\models ...
2
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2answers
84 views

Why does undecidability of arithmetic not follow from that of first-order logic?

As far as I understand, first-order arithmetic incorporates first-order logic. It is a fact that a first-order logic with at least two binary predicates is undecidable. Doesn't this imply immediately ...
9
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1answer
285 views

Freeman Dyson's example of an unprovable truth

Freeman Dyson has claimed that $$\nexists m,n \operatorname{Reversed}(2^n) = m ^ 5 $$ (where $\operatorname{Reversed}(l)$ just is the reverse of the digits of $l$ in base 10), is probably an ...
0
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1answer
43 views

Are there complete formal systems?

I am relatively new to logic, so forgive my ignorance. My question is: Are there any formal systems that are deemed complete? If so, how did we prove they were complete and which are they?