What is Sets: Definition and 1000 Discussions

In mathematics, two sets are said to be disjoint sets if they have no element in common. Equivalently, two disjoint sets are sets whose intersection is the empty set.
For example, {1, 2, 3} and {4, 5, 6} are disjoint sets, while {1, 2, 3} and {3, 4, 5} are not disjoint. A collection of more than two sets is called disjoint if any two distinct sets of the collection are disjoint.

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  1. grassstrip1

    Linearly Independent Sets and Spans in R4

    Hey everyone I just had a quick thought that was bothering me. For a set to be a basis it must be linearly independent and span the vector space. I've seen cases however of only two vectors forming a basis for R4 I don't see how two vectors could span 4 space or am I missing something. Thanks!
  2. evinda

    MHB How to Efficiently Merge a Sorted List and a Pre-Order Sorted Tree?

    Hi! (Smile) We are given two sets $S_1$ and $S_2$. We consider that $S_1$ is implemented, using a sorted list, and $S_2$ is implemented, using a pre-order sorted tree. I have to write a pseudocode, that implements the operation Merge() of the sets $S_1$ and $S_2$. The time complexity of the...
  3. evinda

    MHB Proving Sentences about Sets

    Hello! (Wave) I want to prove the following sentences: $A \subset B \wedge B \subset C \rightarrow A \subset C$ $A \subset B \wedge B \subset A \rightarrow A=B$ $\varnothing \subset A$ $\varnothing=\{ x: x \neq x \}$ That's what I have tried: $A \subset B \leftrightarrow \forall x(x...
  4. evinda

    MHB Exploring Algebraic Sets: Finding Irreducibility and Prime Ideals

    Hello! (Smile) Notice that in $\mathbb{C}[X,Y,Z]$: $$V(Y-X^2,Z-X^3)=\{ (t,t^2,t^3)/ t \in \mathbb{C}\}$$ In addition, show that: $$I(V(Y-X^2,Z-X^3))=<Y-X^2,-X^3>$$ Finally, prove that the ideal $<Y-X^2,Z-X^3>$ is a prime ideal of $\mathbb{C}[X,Y,Z]$. Conclude that the algebraic set...
  5. S

    A misunderstanding of compact sets

    I am trying to understand the definition of compact sets (as given by Rudin) and am having a hard time with one issue. If a finite collection of open sets "covers" a set, then the set is said to be compact. The set of all reals is not compact. But we have for example: C1 = (-∞, 0) C2 = (0, +∞)...
  6. sweet springs

    Emuberable and Demumerable sets

    Hi. I have a question about numbers of basis in quamutum mechanics space. Hamiltonian of harmonic oscillator is observable and have countably infinite sets |En>s Together with position or momentum basis identity equation is, |state>=\int|x><x|state>dx=\int|p><p|state>dp=\Sigma_n\...
  7. evinda

    MHB Why doesn't it come from a cartesian product of sets?

    Hello! (Wave) There is the following sentence in my notes: Let $A$ be a set. We define the set $I_A=\{ <a,a>, a \in A \}$. $$A \times A=\{ <a_1,a_2>: a_1 \in A \wedge a_2 \in A \}$$ Then $I_A$ is a relation, but does not come from a cartesian product of sets. Could you explain me the last...
  8. emanoelvianna

    Natural deduction sets (Rules of nature deduction)

    Hello fine. I'm studying logic and great difficulties to understand its principles, and should prove some theories involving the laws of identity of sets of mathematics using the method of natural deduction, they are: a) A ∪ ∅ = A b) A ∩ ∅ = ∅ I am trying as follows, but I can not solve...
  9. D

    Comparing Lagrangians to find sets of values

    Homework Statement Hey guys, so this is a follow up from my previous post. So I have this Lagrangian for two coupled fields: \begin{split}...
  10. D

    Analyze Size of Pre-Image Sets Under Functions

    I am kind of curious what topics to read to understand this concept more. Suppose I want to find a function f: A \rightarrow B, where if you look at the pre-image of any point b \in B, the size of the pre-image of b will be quite large. Essentially, I want to find functions that map from very...
  11. Math Amateur

    MHB Ordered Index Sets for Direct Sums & Products of Modules: Explained by B&K

    I am reading An Introduction to Rings and Modules With K-Theory in View by A.J. Berrick and M.E. Keating (B&K). In Chapter2: Direct Sums and Short Exact Sequences in Sections 2.1.14 and 2.1.15 B&K deal with ordered index sets in the context of direct sums and products of modules. Although it...
  12. evinda

    MHB The set of all sets does not exist.

    Hey! (Wave) Theorem (Russell's paradox is not a paradox in axiomatic set theory) The set of all sets does not exist. Proof We suppose that the set of all sets exist, let $V$. So, for each set $x$, $x \in V$. We define the type $\phi: \text{ a set does not belong to itself, so } x \notin x$...
  13. evinda

    MHB Prove Identities of Sets Hello! (Wave)

    Hello! (Wave) Let $U$ be a set and $A,B$ subsets of $U$. I want to prove the following identities: $A \cap A^c= \varnothing, A \cup A^c=U$ $(A^c)^c=A$ $(A \cap B)^c=A^c \cup B^c$ $(A \cup B)^c=A^c \cap B^c$ $A \setminus B=A \cap B^c$ That's what I have tried: Let $x \in A \cap...
  14. W

    Why do problem sets take me so long?

    I'm taking Multivariable Calculus at Rice. This is for fun, I'm not required to take it and I'm not a science major. Problem sets take me a really long time, I wonder if it's normal. We are using Vector Calculus by Marsden. The problems sets are like about 15 problems due every week. They take...
  15. evinda

    MHB How can I show that the sets are pairwise disjoint?

    Hello! (Wave) Could you give me a hint how I can show that the sets: $$\{ \varnothing, \{ \varnothing \} \} , \ \ \ \{ \varnothing, \{ \varnothing, \{ \varnothing \} \} \}, \ \ \ \{ \{ \varnothing \}, \{ \varnothing, \{ \varnothing \} \} \}$$ are pairwise disjoint=are not equal ? :confused:
  16. rayne1

    MHB Expressing a set as the difference between two sets.

    Let a, b, c, and d be real numbers with a < b < c < d. Express the set [a, b]U[c, d] as the difference of two sets. I know that [a,b]U[c,d] is a union and what a difference of two sets is, but I don't quite understand this question.
  17. R

    Understanding Sets & Images: A Beginner's Guide to Set Theory

    Could someone please explain how the image of a set A' ⊆ A is the set: f(A') = {b | b = f(a) for some a ∈ A'}. And how can the complement of A be a subset of A? Forgive my ignorance here, I'm a beginning student of set theory. Edit: Maybe I should rephrase my question: Could you explain what...
  18. K

    Disproving A=B with Counter Example: Sets A, B & C

    Hi , Can anyone please give me an idea to disprove the following with counter example: A , B & C be sets. If A X C = B X C , the A = B . I tried giving random numbers in venn diagram but didn't work. And, using subset way to prove equal but still couldn't solve it.
  19. C

    How do you calculate the power set of a set of sets?

    How are you supposed to go about putting together the power set of a set of sets such as X = {{1},{1,2}} What is the power set of X then? And what's the rule for calculating cardinality for the power set of a set that consists of elements which are sets such as the above? Because the set X...
  20. tom.stoer

    No uniform distribution on infinite sets

    What exactly prevents us from ruling out a uniform distribution on infinite sets? To be more precise, why are distributions and limits like \int_{-\infty}^{+\infty}dx\,\lim_{\sigma\to\infty}f_{\mu,\sigma}(x) = 1 \int_{-\infty}^{+\infty}dx\,\lim_{\Lambda\to\infty}\frac{1}{\Lambda} \chi_{[a,a+L]}...
  21. P

    Relations, power sets and the empty/null set.

    Homework Statement Suppose R is a relation on A, and define a relation S on P (A) as follows: S = {(X, Y ) ∈ P (A) × P (A) | ∀x ∈ X∃y ∈ Y (xRy)}. For each part, give either a proof or a counterexample to justify your answer. (a) If R is reflexive, must S be reflexive? (b) If R is symmetric...
  22. S

    Understanding Sets within Sets: Empty Set & Subsets

    Ok. I don't understand sets within sets. No one has been able to explain it to me simply. So you can have sets inside of sets. That I get. But what happens when you get to something tricky like the empty set? For example, I know that ø ε {ø} is true. I don't understand how. How do I read...
  23. J

    Elementary Sets and their Measures

    Homework Statement I am struggling with what seems like a very simple problem from Terrence Tao's Introduction to Measure Theory book (which is available for free online by the way). What I am trying to prove is the following: Give an alternate proof of Lemma 1.1.2(ii) by showing that any two...
  24. P

    Setting a family of sets equal to the empty/null set?

    Homework Statement Suppose A is a set, and for every family of sets F, if ∪F = A then A ∈ F. Prove that A has exactly one element. (Hint: For both the existence and uniqueness parts of the proof, try proof by contradiction.) Homework Equations The Attempt at a Solution Let A be...
  25. B

    Proving Two Sets Are Equivalent

    To prove that two sets are in fact the same, do I actually have to prove that the two are subsets of each other; or could I prove that they are equivalent by some other means, such as invoking the definitions of the sets? For instance, the I am trying to show that the binary set operator...
  26. mesa

    LaTeX How do I make a Complex 'C' in Latex for sets?

    Hello all, I was wondering how to make that nice looking Complex 'C' when writing a set in Latex. For example, $$\{k\in C\mid k>0\}$$ looks okay but can be better.
  27. M

    Topology on a set ##X## (find interior, closure and boundary of sets)

    Homework Statement . Let ##X## be a nonempty set and let ##x_0 \in X##. (a) ##\{U \in \mathcal P(X) : x_0 \in U\} \cup \{\emptyset\}## is a topology on ##X##. (b) ##\{U \in \mathcal P(X) : x_0 \not \in U\} \cup \{X\}## is a topology on ##X##. Describe the interior, the closure and the...
  28. Math Amateur

    MHB Infinite Direct Sums and Indexed Sets

    I am reading Chapter 2: Vector Spaces over \mathbb{Q}, \mathbb{R} \text{ and } \mathbb{C} of Anthony W. Knapp's book, Basic Algebra. I need some help with some issues regarding infinite direct sums and products and indexed families of sets ... ... On page 62, Knapp introduces direct...
  29. Math Amateur

    MHB Notes & Texts on Sets, Relations and Functions

    On a post involving the proof of the Fourth Isomorphism Theorem for vector spaces (in which I was immeasurably helped by Deveno) I have become aware that my knowledge of sets and functions was not all it should be when it comes to things like inverse images, left and right inverses and the like...
  30. E

    Separating hyperplane theorem for non-disjoint sets

    Consider the sets ##X:= \{x\in\mathbb R^2: \enspace ||x-(-1,0)||_2 \leq 1\}## (a ball) and ##Y:=co\{(0,-1), (0,1), (1,0)\}## (a triangle). Both ##X## and ##Y## are compact and convex, but they aren't disjoint: ##X\cap Y = \{(0,0)\}##. Since they aren't disjoint, the most common separating...
  31. D

    Multidimensional fitting of two sets of data

    Hello, my problem is the following: A lasers gives out a bunch of data points which are reflected off a metal surface and recorded by a camera attached to the side of the laser. The image the camera receives is however distorted. In order to calibrate the camera I need to find a function...
  32. J

    Sets and Counting: Drug Relief Study Results

    Homework Statement A study was done to determine the efficacy of three different drugs – A, B, and C – in relieving headache pain. Over the period covered by the study, 50 subjects were given the chance to use all three drugs. The following results were obtained: 21 reported relief from drug...
  33. S

    Set theory: proofs regarding power sets

    Let X be an arbitrary set and P(X) the set of all its subsets, prove that if ∀ A,B ∈ P(X) the sets A∩B,A∪B are also ∈ P(X). I really don't know how to get started on this proof but I tried to start with something like this: ∀ m,n ∈ A,B ⇒ m,n ∈ X ⇒ Is this the right way to start on this proof...
  34. A

    Orthonormal Sets - Find a projection matrix - Linear Algebra

    Homework Statement Let A be the 4x2 matrix |1/2 -1/2| |1/2 -1/2| |1/2 1/2| |1/2 1/2| Find the projection matrix P that projects vectors in R4 onto R(A) Homework Equations projSx = (x * u)u where S is a vector subspace and x is a vectorThe Attempt at a Solution v1 = (1/2, 1/2...
  35. J

    Proving Equality of Sets in Algebra

    Homework Statement Prove A \times (B \cap C) = (A \times B) \cap (A \times C) The Attempt at a Solution Let x \in A and y \in B \cap C \rightarrow y \in B \wedge y \in C now \exists (x,y) \in A \times (B \cap C) so (x,y) \in A \times B \wedge (x,y) \in A \times C thus...
  36. C

    MHB Well Ordering, finite sets

    Hi, I want to show that there exists a well ordering for every finite set. (I know if you add axiom of choice you can prove this theorem for infinite sets too but I think the finite sets do not need axiom of choice to become well ordered)
  37. Mandelbroth

    Category Theory: Inverse Limit in Sets

    I think this looks like a homework problem, so I'll just put it here. Homework Statement Demonstrate that, for any index category ##\mathscr{J}## and any diagram ##\mathcal{F}:\mathscr{J}\to\mathbf{Sets}##, $$\varprojlim_{\mathscr{J}}A_j=\left\{a\in \prod_{j\in \operatorname{obj}(...
  38. A

    Nested sequence of closed sets and convergence in a topological space.

    Homework Statement Let ##X## be a topological space. Let ##A_1 \supseteq A_2 \supseteq A_3...## be a sequence of closed subsets of ##X##. Suppose that ##a_i \in Ai## for all ##i## and that ##a_i \rightarrow b##. Prove that ##b \in \cap A_i##. Homework Equations The Attempt at a Solution...
  39. E

    Proof involving numerical equivalence of sets

    Homework Statement Show that for a set A\subsetN, which is numerically equivalent to N=Z+, and the set B = A \cup{0}, it holds that A and B are numerically equivalent, i.e., that A \approxB Hint: Recall the definition of A≈B and use the fact that A is numerically equivalent to N. Note...
  40. A

    Open Covers and Compact Sets

    Homework Statement I'm trying to understand what compact sets are but I am having some trouble because I am having trouble understanding what open covers are. If someone could reword the following definitions to make them more understandable that would be great. Homework Equations...
  41. K

    I'm sorry, I'm not sure I understand your question. Can you clarify?

    Set Theory -- Uncountable Sets Homework Statement Prove or disprove. There is no set A such that ##2^A## is denumberable. The Attempt at a Solution A set is denumerable if ##|A| = |N|## My book shows that the statement is true. If A is denumerable, then since ##|2^A| > |A|, 2^A ##...
  42. K

    Cardinalities of Infinite Sets: Proving Bijective Relationship (0,1) and (0,2)

    Homework Statement Explain why ## (0,1 ) ## and ## (0,2)## I have proved that a function ## f: (0,1 ) -> (0,2) ##defined by f(x) =2x is bijective.Homework Equations The Attempt at a Solution I could state that due to the sets relationship being bijective they have the same cardinality, but...
  43. H

    Can AUB can be said as a relation or not between A and B sets?

    My question is just to ask whether the operations like:- AUB is a relation or not? in our book it is written that the relations of two sets should be subset of the cartesian product of two sets but i think that relations are those which connects two sets and that can be AUB(A union B)...
  44. S

    Metric Spaces - Distance Between sets and it's closures

    I was trying to prove: d(A,B) = d( \overline{A}, \overline{B} ) I "proved" it using the following lemmas: Lemma 1: d(A,B) = \inf \{ d(x,B) \}_{x \in A} = \inf \{ d(A,y) \}_{y \in B} (By definition we have: d(A,B) = \inf \{ d(x,y) \}_{x \in A, y \in B} ) Lemma 2: d(x_{0},A) = d(x_{0}...
  45. kaliprasad

    MHB Proving Co-Prime Numbers in Sets of Five

    show that in a set of any 5 consecutive numbers there is at least one number that is co-prime to all the rest 4 (for example (2,3,4,5,6- 5 is co-prime to 2,3,4,6)
  46. C

    How to compare these two sets of data?

    Hi I have two sets of data, y as a function of x, for two different experiments. Y is the dependant variable, an instrument reading, x is the independant variable, a concentration of something in a solution. Plotting the data in excel gives me 2 curves. The experiments were...
  47. A

    Continuous Function- Open Sets

    Homework Statement I'm trying to do a problem, and in order to do it I need to find a function f:R→R which is continuous on all of R, where A\subseteqR is open but f(A) is not. Can anyone give an example of a function that satisfies these properties? I think once I have an example I'll...
  48. Ken G

    What sets the fusion rate in a main-sequence star?

    A few years back I started a thread to make the point that there is a common misconception about main-sequence stars that their fusion rate sets their luminosity, in the sense that to know what the luminosity of the star will be, you need to know what the fusion rate is. In particular, you...
  49. C

    MHB Proof of Sets X,Y: X⊆Y <=> P(X)⊆P(Y)

    If I have finite sets X,Y, and need to prove that X ⊆ Y <=> P(X) ⊆ P(Y), where P() denotes the power set of a set. I started out saying that for infinite sets X,Y, x⊆X, and y⊆Y. Given that X⊆Y, we want to show that P(B)⊆P(Y). x⊆X, so through transitivity, x⊆Y (is this correct?). From here, I...
  50. D

    Cartesian product of (possible infinite) family of sets

    Hi all. I'm having trouble understanding the cartesian product of a (possible infinite) family of sets. Lets say \mathcal{F} = \{A_i\}_{i \in I} is a family of sets. According to wikipedia, the cartesian product of this family is the set \prod_{i \in I} A_i = \{ f : I \to \bigcup_{i...
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