Tagged Questions

Questions about mathematical logic, including model theory, proof theory, computability theory (a.k.a. recursion theory), and non-standard logics. Questions which merely seek to apply logical or formal reasoning to other areas of mathematics should not use this tag. Consider using one of the ...

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Basic: Sequent definition, and-introduction, and iff

I am reading through "Mathematical Logic by Ian Chiswell & Wilfred Hodges"(amazon, and publisher) So far have it has covered $\land$-Introduction and $\land$-Elimination Sadly this text only has ...
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A Puzzle on Infinity: How to guess the color of hats? [duplicate]

Infinitely many (i.e. $\omega$ - many) people each have either a white hat or black hat on their heads. Each person can see everyone's hats except their own. Each person simultaneously announces a ...
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Is this non-constant function periodic for every definable number?

Given the set $\mathbb{D}$ which contains all definable real numbers. The definition must not be infinite long. E.g. it contains $12$, $-3$, $\frac{1}{12}$, $\sqrt{2}$, $\pi^2$, $i+e$, Chaitin's ...
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Software for solving first-order logic

Is there any class of software that can help me with the following problem in first order logic: given $\phi$ a formula with a "hole" in it (a subformula which is undetermined) and a particular set of ...
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What does “consistency” mean if formal systems are inherently meaningless?

In the book Gödel's Proof by Ernest Nagel and James R. Newman, the authors insist that formal systems are to be considered as meaningless mechanical systems, which yield theorems by merely applying ...
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Assume that Γ is a a Henkin theory. For any two constants c,d, either $\Gamma \vdash c=d$ or $\Gamma \vdash c \neq d$. There are two constants a,b such that $\Gamma \vdash a\neq b$.Show that Γ is a ...
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vacuous truth -> empty set is both included and not included in every set?

I understand the concept of vacuous truth and its use in showing that the empty set is a subset of every set. Based on my understanding of vacuous truth (for example https://en.wikipedia.org/wiki/...
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$1+1=2$…but Why? [duplicate]

A study that was carried on recently showed that even babies at the age of few months know that $1+1=2$. My question is : is this a fact that can be proved, or is it a just a postulate as those in ...
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How to simplify using algebra laws

Simplify the following by using algebra laws. (i) X’.Y’ + X.Y.Z. + X’.Y + X.Y My attempt: X’.Y’ + Y(X.Y.Z + X'Y + X.Y) X’.Y’ + (X.Z + X' + X) X’(X’.Y’ + X') + X.Z + X Y’ + X' + X.Z + X Y’ + X' +...
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can you help me to transform ∀x FO logical formula to it equivalent ¬∃ formula?

i have this formula ∀x ∀y (A(x,y) V A(y,x) → B(x,c1) ∧ B(y,c2) ∧ c1≠c2) to the equivalent formula that start by ¬∃x ¬∃ y ? you will find the question here ...
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An infinite set of axioms in ZF? What does that mean?

Before write this question, I lookeded around enough in this forum for a possible answer and although there are many similar questions, I couldn't find one answer which understand or satisfies me. I ...
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Answer key to Peter Smith, “An Introduction to Formal Logic”, exercise 13.C.11

If A, B are tautologically inconsistent, then so are $\neg A$ and $\neg B$ This statement is from question C11 at http://www.logicmatters.net/resources/pdfs/answers/Exercises13.pdf, which the answer ...
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The positive introspection axiom

I am studying modal logic with the textbook 'Reasoning about Knowledge' Fagin et al. 1995 The positive introspection axiom is taken as something that can be proved with the possible worlds model of ...
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Is my proof to show that $\mathcal{P}(A) \subseteq\mathcal{P}(B) \implies A \subseteq B$ correct? $\mathcal{P}$ refers to the power set.
Suppose $A$ and $B$ are sets, and that $x$ is an arbitrary element of $A$. The definition of the given $\mathcal{P}(A) \subseteq \mathcal{P}(B)$ means \forall y[(y \in \mathcal{P}(A) \rightarrow y \...