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I Understanding the product topology

  1. Jan 29, 2017 #1
    I am having some trouble visualising the following problem and I hope someone will be able to help me:

    Let (X, dx) and (Y,dy) be metric spaces and consider their product topology X x Y (T1) and the topology T2 induced by the metric d((x1,y1),(x2,y2)) = max(dx(x1,x2),dy(y1,y2)) so the maximum of their respective metrics.

    I have to show that the topologies are the same. I understand that I have to show that the basis opens of T1 should be unions of basis opens of T2 and vice versa.

    T1 => T2 is not giving me a hard time: If (x,y) is in U x V, with U open in X and V open in Y, there exist open balls respectively for x in U and for y in V. The minimum of these radii give a ball for (x,y) in U x V. I know that this is the way and I know how to write it down mathematically correct but I am not sure how to conclude that U x V is in T2 now.

    T2 => T1 is giving me more struggles. I can create an open ball around (x0,y0) but then I can't visualise the path my solution should follow.

    If someone could give me some advice, I would be very grateful!
  2. jcsd
  3. Jan 29, 2017 #2
    What do open balls in ##(X\times Y,d)## look like? For example, what do the open balls look like in ##\mathbb{R}^2##?
  4. Jan 29, 2017 #3
    Thanks for replying! In R2 I could just visualise them as circles around a point (x,y) right?
  5. Jan 29, 2017 #4
    No, not with this metric.
  6. Jan 29, 2017 #5
    I really don't see it. Of course the metric maps R x R ---> R, for example the distance between (1,2) and (2,4) would be 2 but I don't know how to "draw" it. This is the part where I need a hint.
  7. Jan 29, 2017 #6
    Oh I think I should think about the set of all elements of R2 that have fixed distance from (x0,y0) and draw that right?
  8. Jan 29, 2017 #7
    What are the points a distance of 1 away from (0,0)?
  9. Jan 29, 2017 #8
    {(a,1), (1,b), (c,-1), (-1,d)} with a,b,c,d smaller or equal to 1?
    ABsolute value!
  10. Jan 29, 2017 #9
    So what figure does that form if you draw it?
  11. Jan 29, 2017 #10
    Two line segments of length 2, a "cross"
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