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Showing connectedness

  1. Mar 21, 2008 #1
    1. The problem statement, all variables and given/known data

    Pathwise connectedness implies connectedness.
    Show that the converse is false by considering the following subspace of R^2:
    X = { (t, sin(1/t)) : t is nonzero } union { (0, t) : -1 <= t <= 1 }

    2. Relevant equations

    Definitions:

    A topological space X is called pathwise connected if for any two distinct points x and y of X, there exists a continuous map f: [0,1] -> X with f(0) = x and f(1) = y.

    A topological space X is disconnected if there exists two nonempty disjoint clopen subsets U, V of X such that (U union V) = X.

    A topological space X is disconnected if it is not connected.

    3. The attempt at a solution

    So we have to show that
    X = { (t, sin(1/t)) : t is nonzero } union { (0, t) : -1 <= t <= 1 }
    is connected but is not pathwise connected.

    To show X is not pathwise connected, choose x = (0,0) and say y = (1,sin1).
    Then there is no continuous map f : [0,1] -> X with f(0) = (0,0) and f(1) = (1,sin1) because lim_(t -> 0) sin(1/t) does not exist so sin(1/t) is not continuous at t=0.
    Therefore X is not pathwise connected.

    To show X is connected, suppose X is disconnected (to try to get a contradiction),
    so there exists two nonempty disjoint clopen subsets U, V of X such that (U union V) = X.
    Now, let
    A = { (t, sin(1/t)) : t > 0}
    B = { (t, sin(1/t)) : t < 0}
    C = { (0, t) : -1 <= t <= 1 }
    then A,B,C are all connected sets and also X = A union B union C.
    Take x in U, and y in V.
    Then x and y cannot be both in A, or both in B, or both in C (A,B,C are all connected)
    so without loss of generality assume either x is in A and y is in C, or x is in A and y is in B.

    And then I'm stuck not knowing what to do next.
    Can anyone give me some help?
     
  2. jcsd
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