Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!


  1. Sep 8, 2010 #1
    OK, for metric spaces there are apparantly 3 different possibilities for the distance function in M where M is the usual Euclidean Plane:

    (A) D(u,v) = sqrt((x1-x2)2 + (y1-y2)2)
    (B) D(u,v) = max(|x1-x2|,|y1-y2|)
    (C) D(u,v) = |x1-x2| + |y1-y2|
    which somehow correspond to the picture I have attached.
    A corresponds to the circle, B to the square and C to the diamond(this is supposed to be a square diamond but i created the image in paint, sorry)
    Now, I understand (A) but I cannot seem to understand why (B) and (C) end up looking this way. and to be honest, I dont understand B and C at all.

    Attached Files:

    Last edited: Sep 8, 2010
  2. jcsd
  3. Sep 8, 2010 #2
    These are the unit balls with respect to each metric. In other words they mark the points which have the distance "1" to the origin (x_2, y_2) = (0,0). So the first one is a circle equation. The second one has the max of any coordinates, therefore max of (1,1) is 1 which is on the square. So figure out the diamond...

    And you have definitely much more choices than 3. These are the most common three.
  4. Sep 9, 2010 #3


    User Avatar
    Science Advisor

    To elaborate on trambolin's last sentence: these three are instances of a special class of metrics defined for every real p>=1:


    (A) corresponds to p=2
    (C) corresponds to p=1
    (B) is the extension for [itex]p=\infty[/itex]
    Besides these p-metrics there are lots of other metrics on R^2.
    Last edited: Sep 9, 2010
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook