# 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. ### I Basic Probability Theory Question about Lebesgue measure

Mathematics uses Lebesgue measure for probability theory. However it is well known that it comes with a flaw that is not all sets are measurable. Is there a reason why the choice is also preferred in physics?
2. ### B Confusion about division by zero in sets

So the confusion here is that division by zero is often said to be undefined. So whereas, the point (0,0) certainly appears in the set of values where x=y, does the point (0,0) appear in the set of values where 1=y/x. Why or why not? In other words are the set of points where x=y the same as...
3. ### Physics based programming problem sets

Was wondering if anyone knew of any good resources with programming challenges akin to the website "Project Euler" (about page found here). To be more specific, I'm looking for something consisting of problems centred around physics topics that would require some level of problem solving and...
4. ### Trying to reconcile function composition problems with sets & formulas

I know how to solve each of those problems. For the set one, I look at the output of the S and try to match it with the input of T and then take the pair (input_of_S, output_of_T), and I do that for each pair. As for the formula one, I just plug in x = g(y). My confusion lies in trying to...

8. ### I Proof about pre-images of functions

The problem reads: ##f:M \rightarrow N##, and ##L \subseteq M## and ##P \subseteq N##. Then prove that ##L \subseteq f^{-1}(f(L))## and ##f(f^{-1}(P)) \subseteq P##. My co-students and I can't find a way to prove this. I hope, someone here will be able to help us out. It would be very...
9. ### Is it possible to have a vector space with restricted scalars?

I don't understand the solution: that for (1, ..., 1) the additive inverse is (-1, ..., -1), so the condition is not satisfied (and it is not a subspace). Which condition is not met? Thank you.
10. ### I Thinking about equality of infinite sets

I am reading an abstract algebra textbook and enjoying it. I am working through preliminaries some more to refine my knowledge on proofs with sets before really digging in. I understand that if $$X \subseteq Y$$ and $$Y \subseteq X$$ Then $$X = Y$$ This makes sense to me. However, the...
11. ### I GR: Practical Problem Sets w/ Solutions

I have been learning gr on YouTube for the last few months. Most of the videos and the book I have focus on high level understanding. I can do all of the tensor calculus proofs. However simple questions like how you set up a velocity vector or measure proper time in schwarzschild are beyond me...
12. ### I The number of intersection graphs of ##n## convex sets in the plane

Let ##S## be a set of n geometric objects in the plane. The intersection graph of ##S## is a graph on ##n## vertices that correspond to the objects in ##S##. Two vertices are connected by an edge if and only if the corresponding objects intersect. Show that the number of intersection graphs of...
13. ### I What does "the sequence of functions has limit in R" mean?

Suppose f1,f2... is a sequence of functions from a set X to R. This is the set T={x in X: f1(x),... has a limit in R}. I am confused about what is the meaning of the condition in the set. Is the limit a function or a number value? Why?
14. ### I Geometric Point of View of sets

A set is nothing more than a collection. To determine whether or not an object belongs to the set , we test it against one or more conditions. If it satisfies these conditions then it belongs to the set, otherwise it doesn't. The geometric point of view of sets- a set can be viewed as being...
15. ### I Are De Morgan's laws for sets necessary in this proof?

Good evening! Have a look at the following part of a proof: Mentor note: Fixed the LaTeX I don't understand the use of implications. Isn't ##x\in C_M(A\cup B)\iff x\notin(A\cup B)##? To me, all of these predicates are equivalent.

34. ### Prove a theorem about a vector space and convex sets

Summary:: Be the set X of vectors {x1,...,xn} belong to the vector space E. If this set X is convex, prove that all the convex combination of X yet belong to X. Where convex combination are the expression t1*x1 + t2*x2 + ... + tn*xn where t1,...,tn >= 0 and t1 + ... + tn = 1 I tried to suppose...
35. ### MHB Understanding the Intersection of Inductive Sets & the Limits of λ Cardinality

By ZFC, the minimal set satisfying the requirements of the axiom of infinity, is the intersection of all inductive sets. In case that the axiom of infinity is expressed as ∃I (Ø ∈ I ∧ ∀x (x ∈ I ⇒ x ⋃ {x} ∈ I)) the intersection of all inductive sets (let's call it K) is defined as set K = {x...
36. ### MHB Counting the elements in sets

Question: How many elements are in each set? For the first set, I think it's 8995 because the set is the union of {1,2,3,4,5},{1,2,3,4,5,6},...{1,2,3,...9000}. So 9000 - 5 = 8995. For the second set, I'm not too sure about counting the elements in the set. Since 1<x≤i, I can't think of any x...
37. ### MHB Combinations / Sets of objects

Hi, I am looking for a solution that generates combinations of objects from a series of objects in a set. For example, {Apple, Pear, Orange} should bring back Apple Pear Orange Apple, Pear Apple, Pear, Orange, Apple, Orange ... Items in the series should not repeat (i.e. Apple, Orange /...
38. T

### I Can We Discover a Number Set More General Than Reals with Similar Properties?

Hello there.We know that we have sets of numbers like the real numbers, complex numbers, quaternions, octonions.Could we find a set of numbers more general than that of real numbers that has basic properties of the real numbers like commutativity, order, addition, multiplication, division and...
39. ### I Direction of logical implication in bijectively related sets

I have a hypothesis of which I wonder if it's sound. Perhaps you guys can advise me: Suppose ##x_n\Rightarrow a_n## (logical implication) for some set X and set A. I think we have to assume a bijection. Then, if ##a_m = False##, ##x_m## should be ##False##, right? So, in case of a bijection...
40. ### Union of sets and indexed sets question from Vellerman

Homework Statement:: x Relevant Equations:: x I stumbled upon the following example in the book - " How to prove it, A structured approach " ( 2nd edition) , Vellerman. Homework Statement:: He then asks to describe the set: ## \bigcup_{s \in S} L_{s} \, \backslash \, \bigcup_{s \in S}...
41. ### I Unclear steps in a Zorich proof (Measurable sets and smooth mappings)

From Zorich, Mathematical Analysis II, sec. 11.5.2: where as one can read from the statement, the sets could also be unbounded. I do not report here the proof of the fact a), beacuse I have no doubt about it and one can, without the presence of dark steps in the reasoning, assume a) as...
42. ### MHB The Set of Borel Sets .... Axler Pages 28-29 .... ....

I am reading Sheldon Axler's book: Measure, Integration & Real Analysis ... and I am focused on Chapter 2: Measures ... I need help in order to fully understand the set of Borel sets ... ... The relevant text reads as follows: My questions related to the above text are as follows:QUESTION 1...
43. ### I The Set of Borel Sets .... Axler Pages 28-29 .... ....

I am reading Sheldon Axler's book: Measure, Integration & Real Analysis ... and I am focused on Chapter 2: Measures ... I need help in order to fully understand the set of Borel sets ... ... The relevant text reads as follows: My questions related to the above text are as follows:QUESTION...
44. ### I Looking for a formula (how many possible connections between 2 sets of objects?)

I am looking for a formula that will them me how many possible connections between 2 sets of objects. Ex: 12 on left and 4 on right, how many possible states can it be in? state one: top on left connected to top on right state two: top on left connected to 2 on right state three: top on left...
45. ### MHB The Union of Two Open Sets is Open

Let x ∈ A1 ∪ A2 then x ∈ A1 or x ∈ A2 If x ∈ A1, as A1 is open, there exists an r > 0 such that B(x,r) ⊂ A1⊂ A1 ∪ A2 and thus B(x,r) is an open set. Therefore A1 ∪ A2 is an open set. How does this prove that A1 ∪ A2 is an open set. It just proved that A1 ∪ A2 contains an open set; not that...
46. ### MHB ZFC and the Axiom of Power Sets ....

I am reading Micheal Searcoid's book: Elements of Abstract Analysis ( Springer Undergraduate Mathematics Series) ... I am currently focused on Searcoid's treatment of ZFC in Chapter 1: Sets ... I need help in order to fully understand the Axiom of Power Sets and Definition 1.1.1 ... The...
47. ### Are There Any Alternative Brass Weight Sets for Lab Equipment?

For the past 8 years I have used weight sets like the one below and they have been very versatile. But, just before Christmas, most of them were stolen. The person was nice enough to leave me some of the 100g and below masses. I've finally gotten permission to get new sets and now I can't find...
48. ### Is the Monotone Class Theorem Applicable to the Borel Sets?

Proof: Let ##A, B \in \mathcal{O}## and ##x \in A \cap B##. Then there exists ##\varepsilon_A, \varepsilon_B > 0## such that ##B_{\varepsilon_A}(x) \subset A## and ##B_{\varepsilon_B}(x) \subset B##. Let ##\varepsilon = \min\lbrace\varepsilon_A, \varepsilon_B\rbrace##. Then ##B_\varepsilon(x)...
49. ### MHB Open Sets in a Discrete Metric Space .... ....

In a discrete metric space open balls are either singleton sets or the whole space ... Is the situation the same for open sets or can there be sets of two, three ... elements ... ? If there can be two, three ... elements ... how would we prove that they exist ... ? Essentially, given the...
50. ### MHB Open Sets in R .... .... Willard, Example 2.7 (a) .... ....

I am reading Stephen Willard: General Topology ... ... and am currently focused on Chapter 1: Set Theory and Metric Spaces and am currently focused on Section 2: Metric Spaces ... ... I need help in order to fully understand Example 2.7(a) ... .. The relevant text reads as follows:My...