For questions on axioms, mathematical statements that are accepted as being true, usually without controversy.

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0answers
27 views

proving the axiom of addition and multiplication for real numbers $a$,$b$ and $c$

I have learnt about the axioms of multiplication and a few of addition of real numbers but I still have problems with proving the uniqueness of the equalities (i) $a+b+c=a+c+b=b+a+c=b+c+a=c+a+b=c+b+...
5
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4answers
8k views

What is the difference between an axiom and a postulate?

I hear about axioms in set theory and postulates in geometry, but they seem like the same thing. Do they mean the same thing but then are used in different instances or what? Is one word more ...
53
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7answers
3k views

Where are the axioms?

It is said that our current basis for mathematics are the ZFC-axioms. Question: Where are these axioms in our mathematics? When do we use them? I have now studied math for a year, and have yet to ...
28
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7answers
2k views

Is $\{0\}$ a field?

Consider the set $F$ consisting of the single element $I$. Define addition and multiplication such that $I+I=I$ and $I \times I=I$ . This ring satisfies the field axioms: Closure under addition. ...
3
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3answers
81 views

Infinite sums vs infinite unions

Why is it that For every set $S$, there exists a set $\bigcup S$. is something we take for granted (even though $S$ could be infinite), while For every sequence $a_1,a_2,\dots$ of numbers, ...
5
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5answers
153 views

an outline of “intuitive mathematics”?

This question is related to the third answer in this post. There seems to be a difference between the intuitive idea of a thing (such as a function) and "models" of that thing in mathematics (such ...
1
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2answers
1k views

Proof: Zero is less than one

How can one show/prove that $0<1$? $1$ is not actually defined to be greater than zero, but I think it can be proven. I already know that the real numbers are an ordered field and I am familiar ...
1
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0answers
89 views

Geometric basis for the real numbers

I am aware of the standard method of summoning the real numbers into existence -- by considering limits of convergent sequences of quotients. But I never actually think of real numbers in this way. I ...
2
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2answers
76 views

Why are the Separation axioms 'too weak to develop set theory with its usual operations and constructions'?

I was reading Jech's Set theory; there after introducing Russell's Paradox, he asserted: The safe way to eliminate paradoxes of this type is to abandon the Schema of Comprehension and keep its ...
14
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2answers
2k views

Axiom of Regularity

I am having difficulty understanding Axiom of Regularity : Every non-empty set $\rm A$ contains an element $\rm B$ which is disjoint from $\rm A.$ So from my understanding if I have a set like: ...
8
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2answers
933 views

What are the consequences if Axiom of Infinity is negated?

What mathematics can be built with standard ZFC with Axiom of Infinity replaced with its negation? Can the analysis be built? Is there special name for "ZFC without Infinity" set theory? I also ...
2
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1answer
49 views

Requirements for Diagonal Lemma

What are the axioms required for a formal system to be able to state the Diagonal Lemma or Fixed Point Theorem? If possible, could you please also relate with the following systems, if it applies to ...
0
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2answers
46 views

Prove in axiomatic system S

The question is to prove this in the axiomatic system S: $$(\forall x)(\forall y)((A_x \rightarrow R_{xy}) \rightarrow \neg A_y) \vdash (\forall x)(R_{xx} \rightarrow \neg A_x) $$ The problem is,...
2
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0answers
44 views

Induction Implies Well Ordering

Every proof that induction implies well ordering I have seen goes: assume $S\subset\mathbb{N}$ has no least element and let $T$ be its complement with respect to $\mathbb{N}$. Since $1$ is the ...
1
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1answer
40 views

Loops in Robinson arithmetic

In the Robinson arithmetic, I wonder how the recursive definition of addition and multiplication can be well-defined. Axiom 3 seems to prohibit other chains starting with an element which has no ...
0
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1answer
21 views

How to derive the five-segment axiom of Tarski's geometry from Hilbert's axioms?

We are trying to prove that in an arbitrary Hilbert plane (assuming Hilbert's axioms of Group I:Incidence, II:Order and III:Congruence https://en.wikipedia.org/wiki/Hilbert%27s_axioms), Tarski's ...
2
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1answer
519 views

How do you formally state the axiom of constructibility?

The Axiom of Constructibility states every set is constructible. My question is, how would one formally state this, since it seems to involve quite a bit of metamathematics? In particular, if one ...
2
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1answer
48 views

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 ...
1
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3answers
37 views

Why isn't there a need to specify a single $x$ in the axiom schema of replacement? [closed]

The axiom schema of replacement needs a function, defined by ${\forall}y({\exists}x:({\forall}z(P(y,z){\iff}(x=z))))$, where $f(y)=x$. My question is: why isn't ${\exists}$ replaced by ${\exists}!$? ...
0
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1answer
13 views

How can I show a set B with 8 elements and two operations (huntington axioms)

How can I show a set B with 8 elements and two operations, such that the axioms of huntington for boolean algebra holds? I found it with set of 2 elemtnts. but can't understand how to start with 8 ...
7
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1answer
78 views

Is there a category theory notion of the image of an axiom or predicate under a functor?

Let me first state that I am a category theory novice so your patience is appreciated. I might be making some very basic conceptional mistakes and I might just need a simpler language to do what I ...
2
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1answer
502 views

What is the rule for something divided by itself equaling 1?

Is there a name for the mathematical rule/axiom/property $x/x = 1?$ What are the conditions for it to apply? For instance, the rule does not apply where $x = 0$ or $x = \inf$. I saw one site that ...
1
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1answer
55 views

Axiom in Foundations, Extensionality

In my Foundations of Mathematics Textbook I encountered the following problem. The book states that for the domain of discourse $D = \{a,b,c\}$ and binary relation defined as $E = \{(a,b),\, (a,c)\}$ ...
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6answers
1k views

Meaning of the word “axiom”

One usually describes an axiom to be a proposition regarded as self-evidently true without proof. Thus, axioms are propositions we assume to be true and we use them in an axiomatic theory as premises ...
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2answers
72 views

Deducing $((\neg a \to \neg b) \to ((\neg a \to b) \to b)))$ from axioms

I have seen many questions here, using a different set of axioms than mine. Here is mine : $$1) (a \to (b \to a))$$ $$2) ((a \to (b \to c)) \to ((a \to b) \to (a \to c)))$$ $$3) ((\neg b \to \neg a) ...
3
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3answers
239 views

Why is this contradiction using axiom of constructibility incorrect?

Today I thought of this paradox and I'm trying to find the wrong assumption that causes it. Does anyone know what is wrong in the following argument: let $$A=\{x\in \mathbb{R}|\exists\phi:\forall a\...
2
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2answers
36 views

Propositional calculus axiom the other way around

I have the following axioms of propositional calculus (as well as modus ponens and the deduction theorem if needed): $$(a \to (b \to a)) \tag1$$ $$ (((a \to (b \to c)) \to ((a \to b) \to (a \to c))) \...
1
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0answers
53 views

Why is the area of a rectangle given as $l\times h$? [duplicate]

While this may seem like a simple question, I have found it very difficult to answer. My question is "Why is the area of a rectangle given as $l\times h$?" We define area as the "size of a two ...
1
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2answers
110 views

Proof that the minimal inductive set does not contain any more elements than $\{\emptyset, s(\emptyset), s(s(\emptyset)),s(s(s(\emptyset))),…\}$?

I am studying axiomatic (ZF) set theory and in particular the construction of the natural numbers from axiomatic set theory. Let me start by giving some introduction to my question. The axiom of ...
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1answer
29 views

Is the set of all limit ordinals a set? [closed]

It could be a limit ordinal too, then would be a class, but I'm not sure about is that an ordinal number
17
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3answers
1k views

Why does what I've written fail to define truth?

I stumbled across a set of axioms for first order logic a bit ago. Intrigued, I decided to try to write it all down and organise what I read. After I did that, it seemed to me as though one could ...
59
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9answers
6k views

Does mathematics require axioms?

I just read this whole article: http://web.maths.unsw.edu.au/~norman/papers/SetTheory.pdf which is also discussed over here: Infinite sets don't exist!? However, the paragraph which I found most ...
51
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10answers
13k views

Has there ever been an application of dividing by zero?

Regarding the expression $a/0$, according to Wikipedia: In ordinary arithmetic, the expression has no meaning, as there is no number which, multiplied by $0$, gives $a$ (assuming $a\not= 0$), and ...
2
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1answer
41 views

What does the word “Comprehension” mean in the Axiom of Comprehension?

I understand roughly what the Axiom of Comprehension means, that any predicate can be used to construct a set of the elements that satisfy the predicate. But in English terms, where does the word "...
16
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2answers
1k views

Proofs given in undergrad degree that need Continuum hypothesis?

Or alternative you need to assume CH is false. I know several proofs that use axiom of choice. Heine Borel theorem is the best example I can think off. Zorns lemma is heavily used in the non ...
1
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0answers
26 views

Applications of the Axiom of Regularity to non-set-theoretical Mathematics [duplicate]

In the beginning of my mathematics studies at university, we have learnt that nearly all of ordinary mathematics not dealing with proper classes can be formalized within ZFC, which is a famous ...
16
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11answers
3k views

What is an axiom in layman's terms? [closed]

I tried to explain to my kids what is an axiom in very nontechnical terminology (also known as layman's term) but I could not find anything that they could relate to easily. Do anyone have a good ...
35
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11answers
3k views

What is exactly the difference between a definition and an axiom?

I am wondering what the difference between a definition and an axiom. Isn't an axiom something what we define to be true? For example, one of the axioms of Peano Arithmetic states that $\forall n:0\...
3
votes
3answers
332 views

Defining the integers and rationals

What axiomatic system is most commonly used by modern mathematicians to describe the properties of the integers and the rationals? Properties like $a+0=a$, $a*1=a$, $a+b=b+a$, Also given these ...
0
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0answers
17 views

Scotts axiom, representation theorems for Qualitative -Numerical Probability function relations

Scotts theorem/axiom and other representation theorems give conditions under which a qualitative ordering (>= for at least as probable than) which satisfies certain constraints (total pre-order, ...
4
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1answer
73 views

Necessity of being rigorous

Disclamer I am no serious mathematician, just curious Context I recently discovered some set theories, ZFC and IZF in particular. It made me realize that I've studied math a whole year without ...
4
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2answers
676 views

Definition: Mathematical way to define the “left” and the “right”

How do we define the left and the right, (such as left/right handness) from a mathematical way of definition? How can we explain this definition to some inhabitant of a distant stellar system who has ...
2
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3answers
78 views

Axioms of Geometry?

I have taken the generic low level undergraduate classes, such as Calculus 1-3, Differential Equations, and Linear algebra. Since I never learned Geometry past a basic high school level, I thought it ...
2
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3answers
32 views

Inner Product proof, axiom 1

I hate to argue that my text book is wrong. That being said, I am going to try and do just that. The text book says that this IS a valid inner product, I disagree. The vectors u and v are defined ...
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3answers
1k views

Is the Bourbaki treatment of Set Theory outdated?

On page 5 of the following write up, the author asks why the Bourbaki did not notice that their system of Zermelo set theory with AC was inadequate for existing mathematics. Throughout the rest of the ...
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1answer
44 views

Prove axiom of infinity from ZF$-$Infinity +Zermelo's version of infinity

Assume ZF-infinity and assume that there is a set $X$ such that $\varnothing\in X$ and for all $t\in X$, $\{t\}\in X$. How to deduce axiom of infinity from these new axioms?
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2answers
59 views

Choice function without AC in special case [duplicate]

I read the Jech, Set theory, and saw following proposition. (☆) If S is a finite family of nonempty sets, existence of choice function of S can be proved without axiom of choice. I tried to prove ...
1
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2answers
101 views

Proof using only field axioms

Prove if $x, y ∈ R$ and $xy > 0$ then either $x > 0$ and $y > 0$, or, $x < 0$ and $y < 0$ using only the field axioms. These include the Field axioms for addition, multiplication, ...
0
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0answers
108 views

What kind of Properties are Allowed in the Schema of Comprehension?

Studying an introduction to set theory, we were presented with several axioms as the Axiom Schema of Comprehension and the Axiom of Infinity. This last aximom allows the definition of the inductive ...
1
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1answer
96 views

What book on Set Theory is best to understand motivation for axiomatization?

I am Master of Science in ICT, and I had always been in loved in math. On University we haven't been doing any Foundational Mathematics, the closest being Automata Theory and mention of Church-Turing ...