This tag is for elementary questions on set theory, spanning topics usually found in introductory courses in set theory, in addition to review sections of graduate textbooks in the same field. Topics include intersections and unions, de Morgan's laws, Venn diagrams, relations, functions, ...

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1
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3answers
22 views

A\(B∩C)=(A\B)∪(A\C); only one way seem to work

I encounter the following problem: A\ (B∩C)=(A\B)∪(A\C) So I need to prove: A\ (B∩C)⊆(A\B)∪(A\C) (A\B)∪(A\C)⊆A\ (B∩C) First one is fairly easy to me; what I don't understand is 2. My approach ...
0
votes
1answer
74 views

Why does $\bigcap_{m = 1}^\infty ( \bigcup_{n = m}^\infty A_n)$ mean limsup of sequence of set?

Why does $\bigcap_{m = 1}^\infty ( \bigcup_{n = m}^\infty A_n)$ mean the limit superior of sequence of set? I'm not getting it. ${A_n}$ is a sequence of set in $S$. I do know what limsup means ...
-4
votes
0answers
20 views

Sets and ordering sets [on hold]

Let $n=10^{10^{1000000000000}}$. It then holds $n<{2n \choose n}$<$2^{2n}$
-1
votes
1answer
16 views

A,B,X,Y are sets.. when is $X^Y \leq A^B$?

Let A,B,X,Y be sets with $X \leq A$ and $Y \leq B$. Prove that, apart from some exceptional cases, $X^Y \leq A^B$. What are the exceptional cases?
1
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2answers
22 views

Intersection of elements of a union

I am working on a proof where I would like the following identity to hold. $$\bigcap_{w \in \bigcup_{i \in \mathbb{N}} W_i}w = \bigcup_{i \in \mathbb{N}} \bigcap_{w \in W_i} w,$$ where $w$ are sets ...
2
votes
2answers
75 views

Proving $(0,1)$ and $[0,1]$ have the same cardinality [duplicate]

Prove $(0,1)$ and $[0,1]$ have the same cardinality. I've seen questions similar to this but I'm still having trouble. I know that for $2$ sets to have the same cardinality there must exist a ...
1
vote
1answer
11 views

Characteristic function of a disjoint union

I just could not convince myself about what $\chi_{\cup_{A_n}}$ is worth, if $(A_n)$ is a sequence of pairwise disjoint sets? Is it equal to the series $\sum_{n=1}^{\infty}\chi_{A_n}$? We need to ...
17
votes
4answers
7k views

lim sup and lim inf of sequence of sets.

I was wondering if someone would be so kind to provide a very simple explanation of lim sup and lim inf of s sequence of sets. For a sequence of subsets $A_n$ of a set $X$, the $\limsup A_n= ...
17
votes
5answers
2k views

How many different sizes of infinity are there?

It's pretty straightforward to say that there is an infinite number of different sizes of infinity, but then I thought, "What size of infinity is that?" My thoughts are that the number of unique ...
4
votes
2answers
67 views

Let $X\subset \mathbb{R}$ Lebesgue measurable, $|X|<|\mathbb{R}|$, is it true that $X$ is null?

Let $X\subset \mathbb{R}$ Lebesgue measurable, $|X|<2^{\aleph_0}$, is it true that $X$ is null? Of course I am not assuming the Continuum Hypothesis. EDIT: It might be helpful to know that all ...
0
votes
0answers
21 views

What is the Cardinality of all symmetric density function pairs on reals?

$X=$(total number of all pairs of probability density functions $(f_0,f_1)$ on the real numbers) and let $Y=$(total number of all symmetric probability density functions $(f_0,f_1)$ on the real ...
-2
votes
1answer
54 views

Applications of set theory

We know that science and specially mathematics are based on the set theory. But, I would like to know some direct applications of set theory for computer science and engineering. For example, is there ...
-1
votes
1answer
26 views

Doubt with Intervals and Inequalities

This doubt has been bothering me for ages. I would be truly grateful for any help. Problem 1: $\dfrac{2}{|x-4|}>1$ Express the solutions using intervals Solution: $x\in(2,4)\cup(4,6)$ ...
1
vote
1answer
43 views

Proving $k^{m+l} = k^m k^l$ by constructing a bijective function F : $ ^MK \times ^LK \to ^{L\bigcup M}K $

For cardinals k which is cardinal of K and l which is cardinal of L and m which is cardinal of M. W.T.S [ $ k^{m+l} $ = $ k^m k^l $] by constructing a bijective function F : $ ^MK \times ^LK \to ...
2
votes
4answers
73 views

Split the set of real numbers into $2$-element sets

How can we split $\mathbb{R}$ into disjoint sets, each consisting of $2$ elements? I have found a similar (though much more general) question here. But I am unable to deduce an answer to my specific ...
1
vote
1answer
52 views

a problem in Stein's book 'Real analysis', relate to continuum hypothesis.

The question is from chapter 2, problem 5 in Stein's book 'Real analysis': 5.There is an ordering $≺$ of $\mathbb R$ with the property that for each $y\in\mathbb R$ the set $\{x\in\mathbb R : x ≺ ...
1
vote
1answer
18 views

Inverse of a set of ordered pairs.

An exam ask me the following question. Let $r=\{(x,y) \ | \ x \in [-1,1] \ \text{and} \ y=x^2\}$, is the following statement true? $$r^{-1}=\{(x,y) \ | \ x \in [0,1] \ \text{and} \ y=\pm\sqrt{|x|} ...
-1
votes
0answers
17 views

The elements in set $\Bbb Z$ under ordinary addition has infinite order [on hold]

For elements, $g$, in a set with ordinary addition operation defined on the set, the order of any element in the set, denoted $|g|$, is the smallest positive integer, $n$, such that $n\times g=0$. For ...
1
vote
5answers
57 views

Is the Cartesian product of two uncountable sets uncountable? [duplicate]

Is Cartesian product of two uncountable sets uncountable? Suppose we have a set of real numbers $R$, Can't it be shown that $R$ is uncountable by Cantor's diagonalization method, so it follows that ...
0
votes
3answers
40 views

Is a relation from a set $A$ to a set $B$ always a proper subset of $A\times B$?

Is a relation from a set $A$ to a set $B$ always a proper subset of $A\times B$? Or, is it possible that the relation covers the entire set $A\times B$?
0
votes
2answers
25 views

Equivalence: Injective function from natural numbers to a set $X$, and injective but not surjective function from $X$ to $X$

How do I go about proving the equivalence of these statements? (1) There is an injective function $f: \mathbb{N} \rightarrow X$ (2) There is an injective but not surjective function $g:X \rightarrow ...
1
vote
2answers
25 views

Lexicographic order on $\alpha^\beta$ is well-ordered

Suppose that $\alpha$ and $\beta$ are ordinals, and define the following order on $\alpha^\beta$, the set of all functions $\beta\to\alpha$ with finite support: $$f\,R\,g\iff\exists ...
6
votes
1answer
143 views

Confusion between an element and its preimage

Let $X$ be a set and $\sim$ is an equivalence relation on $X$, so that the quotient set $X/_\sim=\bigcup_{x\in X}{[x]}$ with $[x]=[y]$ if and only if $x\sim y$. Consider the quotient map $f:X\to ...
6
votes
6answers
228 views

Is the set of all pairs of natural numbers countable? [duplicate]

Say that $\Bbb N \times \Bbb N$ is the set of all pairs $(n_1, n_2)$ of natural numbers. Is it countable? My hypothesis is yes it is countable because sets are countable. But I am unable to come up ...
-2
votes
2answers
60 views

Proving that $A\cup\emptyset=A$ and $A\cap\emptyset=\emptyset$ [on hold]

I need to prove that $A\cup\emptyset=A$, and $A\cap\emptyset=\emptyset$. It's seem like it's obvious, yet how can I prove it mathematically?
1
vote
2answers
138 views

Is the set of all pairs of real numbers uncountable?

My hypothesis is that $\mathbb{R \times R}$, the set of all pairs $(r_1, r_2)$, of real numbers is uncountable. I understand that the set of all pairs of natural numbers is countable. But could ...
0
votes
2answers
45 views

Closure of $\Bbb R$ [on hold]

Is the closure of $\Bbb R$ equal to $\Bbb R$ itself or the extended real numbers $\bar{\Bbb R}$? Thanks for any comment.
0
votes
1answer
25 views

Uncountable “relatively independent” subset of finite dimensional vector spaces over an uncountable field

Let $V$ be a $n$ dimensional vector space over an uncountable field ; then does there always exist an uncountable subset $S$ of $V$ such that any $n$ vectors of $S$ are linearly independent ? ( I can ...
0
votes
1answer
40 views

Can an element of a power set $2^A$ be a subset of $2^A$?

This question is continued from a previous thread I started, but it had more than one question so I had to move the other question here. For this example consider an injective map $f: A \to 2^A$ then ...
1
vote
1answer
14 views

Is it overkill to define the closure of a set $A,A\subseteq B$ by the union of the range of the recursive function $h(0)=A, h(n^+) = h(n)\cup f[h(n)]$

$f:B\to B,A\subseteq B$. Is it overkill to define the closure of a set $A,A\subseteq B$ over $f$ by the union of the range of the recursive function $h(0)=A, h(n^+) = h(n)\cup f[h(n)]$? I ...
0
votes
1answer
51 views

If $A$ is a non-empty set and $2^A$ is the power set of $A$. Is $2^A \subseteq A$?

I'm aware that if there exists an injective map $f: A \to 2^A$ then for each element $a\in A$ $\exists$ $f(a)\subseteq A$. But does this also mean $f(a)\subseteq 2^A$? I ask this because when ...
5
votes
1answer
98 views

Is this enough to explain why set theory work in real analysis?

Sorry for starting a lot of topics in set theory; I think this will be my last. I just want to know what are the basics I need to know about set theory to mathematical analysis. Is what I have ...
0
votes
5answers
82 views

A better proof for the set of irrational number not closed under ordinary multiplication.

A positive irrational number $$q$$ is by definition a real number than cannot be expressed as a ratio of $2$ integers. To show that the set of irrational number is not closed under ordinary ...
0
votes
1answer
29 views

Show that the set $\mathbb{Q}^+$ is a group under ordinary multiplication

To be a group, a set with a binary operation has to satisfy all four of the group axioms. My problem is with closure as each time I am unsure if my proof suffices. The set of positive rational numbers ...
1
vote
2answers
55 views

Is the fact that these sets can not exist a consequence of Russels paradox?

Some time ago I asked why a given collection of objects could not be a set(something to do with abstract algebra). I got three answers, one was close to the Russel paradox, two other explanations ...
2
votes
0answers
39 views

On the properties of an interesting set on the real line…

Let $K$ be the set of all real numbers of the decimal form $$ 0.\;e_1\;\underbrace{0}_{1!\text{ times}}\;e_2\;\underbrace{00}_{2!\text{ times}}\;e_3\;\underbrace{000000}_{3!\text{ ...
0
votes
0answers
30 views

Which of the following sets have the cardinality the same as $R$ [duplicate]

Which of the following sets has the same cardinality as that of $\mathbb{R}$? $W$=The set of constant functions on $\mathbb{R}$ $X$=The set of polynomial functions on $\mathbb{R}$ $Y$=The set of ...
3
votes
3answers
91 views

If $A$ and $B$ are sets, then either $A \in B$ or $A\notin B$

Given that $A$ and $B$ are two sets, is the following proposition a tautology: $A\in B \vee A\notin B$. I do not know any set theory beyond the naive one.
1
vote
1answer
57 views

Pushout in $\mathsf{Set}$ where one of the maps is injective

From I.M. James' book General Topology and Homotopy Theory: Suppose we have a cotriad $$X \xleftarrow{\xi}W \xrightarrow{\eta} Y.$$ ... we might expect the pushout of the cotriad to be a ...
0
votes
2answers
40 views

$A \subset B \implies f^{-1}(A) \subset f^{-1}(B)$

Prove: $A \subset B \implies f^{-1}(A) \subset f^{-1}(B)$ I am busy setting up a proof for Real Analysis, and have come to a point where I need to use the above statement. Intuitively, I ...
2
votes
0answers
768 views

Maximum and minimum values of intersection of sets

I know how to do this problem, but my question is more on the proving the inequality and the extreme values. So here is the problem: Of the 24 students in a class, 18 like to play basketball and 12 ...
1
vote
2answers
53 views

What kind of set-theory is sufficient to understand mathematical analysis?(book recommendation))

I am looking for books with set theory and logic that is sufficient to understand mathematical analysis. I guess another question might be if there even exists such a book. There are basically two ...
-2
votes
2answers
33 views

Cardinality of Two Sets

Show that two sets $(0,1)$ and $(a, \infty) $ have the same cardinality. There are proofs all over the Internet, but I do not understand why. I cannot make head or tail of it. Can someone please ...
0
votes
1answer
60 views

Proving set identities

I am attempting to work on some proofs for my math assignment, but I'll be honest in that I am really struggling to understand them. I read through the power point given by my teacher; however, even ...
2
votes
2answers
119 views

(Revisited$_2$) Injectivity Relies on The Existence of an Onto Function Mapping Back to Its Preimage

QUEST: For any sets $X$ and $Y$, there exists an injective function $f:X\rightarrow Y$ if and only if there exists a surjective function $g:Y\rightarrow X$. QUESTION$_1$: How do you people ...
0
votes
1answer
26 views

simplifying set theory expression

I'm trying to simplify a set theory expression, it relates to a programming problem... I have a set of staff members who are grouped into various teams (Team A, Team B, Team C etc ...up to Team H). ...
1
vote
2answers
22 views

Set Theory - Simplify expression

Can the following be simplified? It's been a long time since I did set theory and I don't remember my simplification rules. This is probably totally easy... can I simplify this any further? $(A \cap ...
1
vote
2answers
58 views

Given nonempty sets S and T, does there exist a set R that is disjoint from S with |R|=|T|?

Let $S$ and $T$ be nonempty sets. I would like to show that there exists a set $R$ such that $S\cap R=\emptyset$ and $\left\vert T\right\vert = \left\vert R\right\vert$. Here is my work so far. Let ...
0
votes
3answers
156 views

Understanding the use of the Cartesian Product in the proof of $|\mathbb R\times \mathbb R|=|\mathbb R|$

Where the Cartesian Product of two sets $\mathbb A$ and $\mathbb B$ is such that $\mathbb A\times \mathbb B=\{{ (a,b)|a \in \mathbb{A}, b \in \mathbb{B}\}}$ In trying to understand the proof that ...
-4
votes
0answers
21 views

If the function f and function g is one to one, show that the composition g of f is one to one? [on hold]

If the function $f$ and function $g$ is one to one, show that the composition $g$ of $f$ is one to one? How do I show that the composition of two function both of which are one to one produces a ...