1. M

    Topological space and continuous function

    Let $X$ be a toological space. Suppose that $f$ and $g$ are continuous maps $X\to \mathbb{R}$. Define the function $(f+g)(x)$ by $(f+g)(x)=f(x) + g(x)$. Show that $$(f+g)^{-1}((a,b))=\bigcup_{s,t\in \mathbb{R}}f^{-1}((s,t))\cap g^{-1}((a-s,b-t))$$ and deduce that $f+g$ is continuous. Here's...
  2. H

    connected sets in the topological space of real n-matrices Mn(R)

    Hi, I need help with proving the following two properties: 1)The set of matrices in Mn(R) with det(A)=1 is connected. 2)The set of matrices in Mn(R) with positive determinant,and the set of matrices with negative determinant are connected,and are the two connected components of the invertible...
  3. J

    Topological space

    Are power sets always a discrete topology? One video lecture I watched introduced a topology as 2^X for some set X. (I know this forum is more for discussion not help but this is extracurricular work so I figured no need to take up space in the forum where people have questions for...
  4. J

    Topological space: Condition 3

    \for A,B \in \mathfrak{T}, A \cap B \in\mathfrak{T} What is a good way to prove this true? One video I saw said it can be proved by induction but I haven't done an induction proof on a set before. I thought maybe an element proof would be easier to work out.
  5. S

    finding a specific topological group with specific conditions

    Hi; I have a question, it sounds difficult. The question is the following: Let X be a topological group such that the binary operation defined on it is *. For any two points a and b in X define a new operation by a(*)b=b^-1*a*b, [(*) is a new operation on X inherited from *]. By this (*), we...
  6. R

    A Property of Boundary of a Set in a Topological Space

    Dear Colleagues, I want a help for proving the following property: Bd[Bd{Bd(A)}]=Bd[Bd(A)], where Bd denotes the boundary and A is subset of a toplogical space X. Actually I proved that Bd[Bd{Bd(A)}] is a subset of Bd[Bd(A)] but I could not prove the converse. Regards, Raed.
  7. Z

    Topological Space, Finite Intersections

    My book gives the following as an exercise: Let (X,T) be any topological space. Verify that the intersection of any finite number of members of T is a member of T. [Hint: to prove this result use "mathematical induction."] Doesn't this follow directly from the definition of a topology? If...
  8. L

    Category theory of topological groups

    How does one show (directly) that the category of topological groups is protomodular?
  9. B

    Topological Space, Continuous, G delta

    Let X and Y be topological spaces, and let f:X->Y. Let C denote the set of points at which f is continuous. What axioms (of separation) must X and/or Y satisfy such that C is a G delta set?
  10. R

    Definition of convex polytope (I think it's topological problem)

    I know, in general, a convex polytope is an intersection of halfspaces described by a system of "inequalities". But what if these inequalities are replaced by equations, namely, a system of equations, just like solving it using algebra? Geometrically is it also a convex polytope by definition...
  11. H

    A topological puzzle

    Find two sets, P and Q, such that: 1) Both P and Q are contained within the square in R^2 with vertices (1, 1), (1, -1), (-1, -1) and (-1, 1). (In other words all (x, y) in P or Q must satisfy -1\le x\le 1, -1\le y\le 1.) 2) P contains the diagonally opposite points (1, 1) and (-1, -1)...
  12. H

    showing that a topological space is t4

    Hi there If I can prove that any point in a topology is open (for example the Sorgenfrey line) then it logically follows that the topological space concerned is T4 am I right? Are there other usual ways to show whether a topological space is T4? Regards
  13. L

    A question about multiresolution analysis (from a topological point of view)

    Hi, I have a problem understanding something This is a snapshot of a book I am reading Point no. 2 concerns me, because it looks to me like it contradicts itself, with "this or this" The first part says \sum_{j}V_j = {L^2(R)} which, to me, looks completely equivavalent to \lim_{j...
  14. Siron

    Countability in topological spaces

    Hi, I need to prove the following: Consider the topological space \mathbb{R} with the euclidian topology. Define an equivalence relation on \mathbb{R} as follows xRy \Leftrightarrow x=y \ \mbox{or} \ \{x,y\} \subset \mathbb{N}. Now, consider the quotient topology on \mathbb{R}/ R then this...
  15. B

    In the topological space of R, Show that joins do NOT distribute over meets?

    I am studying topology for computer science, using a book titled "topology via logic" and the question was an exercise from it. How would you prove that? It seems that the opposite is true. Isn't it?
  16. M

    Compactness of topological space

    Hey. My assignment says the following: Let X be a topolical space with \tau as its topology. Let \infty be a point not in X. Let X^* = X \cup \{ \infty \} . Let \tau^* = \tau \cup \{ U \in X | X^* \setminus U is a closed, compact subset of X \}. (1) Prove that \tau^* is a topology on...
  17. M

    Proof of topological space

    I'm new to topology and have just started looking at whether spaces are topologies or not, I've come across a question where i have to show a space is a topology on N and am not sure how to approach it, T = {Tn :n ∊N} ∪ {∅} where Tn ={m ∊N:m ≥ n} I am supposed to prove T is a topology on N...
  18. H

    Topological Metric Space with 3 elements

    Can you create a topological metric space from three points (the set X) in the plane? Obviously, d is defined. A ball at each point is the point itself (satisfies def of neighborhood of a point). Union of all the balls is X. Intersection of any two balls is empty (contains no member of X) so...
  19. S

    property of a topological group??

    Hello; Show that if G is a first countable topological group, then there is a sequence of symmetric neighborhoods (U_{n})_{n} of the neutral element e of G such that (1){ {U_{n}:n\in Z }} is a local base at e in G (2) U_{n+1}^{3}\subset U_{n}, for every n I understand number 1 is due to...
  20. D

    A is a subset of a topological space X

    I am having trouble understanding topology. I have read the sections 3 times and hasn't helped. Could someone explain this question, the methodology to answering it, and how it is done? Let X be a topological space; let A be a subset of X. Suppose that for each x\in A there is an open...